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FEATURES
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LTC2925 Multiple Power Supply Tracking Controller with Power Good Timeout DESCRIPTIO
The LTC(R)2925 provides a simple solution to power supply tracking and sequencing requirements. By selecting a few resistors, the supplies can be configured to ramp-up and ramp-down together or with voltage offsets, time delays or different ramp rates. The LTC2925 controls the outputs of three independent supplies without inserting any pass element losses. For systems that require a fourth supply, or when a supply does not allow direct access to its feedback resistors, one supply can be controlled with a series FET. When the FET is used, an internal remote sense switch compensates for the voltage drop across the FET and current sense resistor, and an electronic circuit breaker provides protection from short-circuit conditions. The LTC2925 also includes a power good timeout feature that turns off the supplies if an external supply monitor fails to indicate that the supplies have entered regulation within an adjustable time-out period.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Flexible Power Supply Tracking Up and Down Power Supply Sequencing Supply Stability Is Not Affected Controls Three Supplies Without Series FETs Controls an Optional Fourth Supply With a Series FET Electronic Circuit Breaker Remote Sense Switch Compensates for Voltage Drop Across a Series FET Supply Shutdown Outputs FAULT Output Adjustable Power Good Timeout Available in Narrow 24-Lead SSOP and Tiny 24-Lead QFN Packages
APPLICATIO S
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VCORE and VI/O Supply Tracking Microprocessor, DSP and FPGA Supplies Servers Communication Systems
TYPICAL APPLICATIO S
0.015 3.3V VIN 0.1F 10 0.1F SUPPLY MONITOR 154k 100k SD1 REMOTE VIN 10k STATUS VIN LTC2925 10k FAULT RAMPBUF 1.65k TRACK1 13k 88.7k TRACK2 41.2k 86.6k TRACK3 100k GND SCTMR 0.47F SDTMR 0.082F PGTMR 0.82F 100k
2925 TA01
Si4412ADY MASTER
1V/DIV
VCC ON
SENSEP SENSEN
GATE
RAMP PGI
RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT 16.5k 3.3V
FB1
1.8V SLAVE1
35.7k
SD2 FB2
IN RUN/SS DC/DC FB = 0.8V OUT 88.7k 3.3V
2.5V SLAVE2
3.3V 2.5V 1.8V 1.5V 1V/DIV
41.2k
SD3 FB3
RUN/SS IN DC/DC FB = 0.8V OUT 86.6k
1.5V SLAVE3
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3.3V 2.5V 1.8V 1.5V
10ms/DIV
2925 TA02a
10ms/DIV
2925 TA02b
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1
LTC2925
ABSOLUTE
(Notes 1, 2)
AXI U RATI GS
GATE (Note 3) ........................................ -0.3 to 11.5V Average Current TRACK1, TRACK2, TRACK3 .................................. 5mA FB1, FB2, FB3 ....................................................... 5mA Operating Temperature Range LTC2925C ............................................... 0C to 70C LTC2925I ............................................. -40C to 85C Storage Temperature Range .................. -65C to 150C Lead Temperature (Soldering, 10 sec) MS Package ..................................................... 300C
Supply Voltage (VCC) ................................. -0.3V to 10V Input Voltages ON, PGI, SENSEP, SENSEN ..................... -0.3V to 10V TRACK1, TRACK2, TRACK3 .......... -0.3V to VCC + 0.3V SCTMR, SDTMR, PGTMR ............. -0.3V to VCC + 0.3V Output Voltages FAULT, SD1, SD2, SD3, FB1, FB2, FB3, STATUS ......................... -0.3V to 10V RAMPBUF, REMOTE ..................... -0.3V to VCC + 0.3V RAMP .............................................. -0.3V to VCC + 1V
PACKAGE/ORDER I FOR ATIO
TOP VIEW VCC SENSEP SENSEN ON SDTMR SD1 SD2 SD3 RAMPBUF 1 2 3 4 5 6 7 8 9 24 SCTMR 23 PGTMR 22 PGI 21 STATUS 20 FAULT 19 GATE 18 RAMP 17 REMOTE 16 FB1 15 TRACK1 14 TRACK2 13 FB2
SENSEN
SENSEP
PGTMR
SCTMR
ORDER PART NUMBER LTC2925CGN LTC2925IGN
VCC
PGI
GND 10 FB3 11 TRACK3 12
GN PART MARKING LTC2925CGN LTC2925IGN
GND
FB3
TRACK3
FB2
TRACK2
TRACK1
GN PACKAGE 24-LEAD PLASTIC SSOP TJMAX = 125C, JA = 85C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V unless otherwise noted.
SYMBOL VCC ICC PARAMETER Input Supply Range Input Supply Current IFBx = 0, ITRACKx = 0, IRAMPBUF = 0 IFBx = -1mA, ITRACKx = -1mA, IRAMPBUF = -3mA VCC(UVL) Input Supply Undervoltage Lockout VCC Rising CONDITIONS
q q q q
ELECTRICAL CHARACTERISTICS
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TOP VIEW
ORDER PART NUMBER LTC2925CUF LTC2925IUF
18 STATUS 17 FAULT 16 GATE 15 RAMP 14 REMOTE 13 FB1
24 23 22 21 20 19 ON 1 SDTMR 2 SD1 3 SD2 4 SD3 5 RAMPBUF 6 7 8 9 10 11 12 25
UF PART MARKING 2925 2925
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN EXPOSED PAD (PIN 25) INTERNALLY CONNECTED TO GND (PCB CONNECTION OPTIONAL) TJMAX = 125C, JA = 37C/W
MIN 2.9
TYP 1.5 10.5
MAX 5.5 3 15 2.7
UNITS V mA mA V
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2.3
2.5
LTC2925
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V unless otherwise noted.
SYMBOL VGATE IGATE PARAMETER External N-Channel Gate Drive (VGATE - VCC) GATE pin current CONDITIONS IGATE = -1A Gate On, VGATE = 0V, No Faults Gate Off, VGATE = 5V, No Faults Gate Off, VGATE = 5V, Short-Circuit or Power Good Timeout VON(TH) VON(HYST) VON(FC) ION(IN) ON Pin Threshold Voltage ON Pin Hysteresis ON Pin Fault Clear Threshold Voltage ON Pin Input Current VON = 1.2V, VCC = 5.5V VON rising
q q q q q q q q q q q q q q q q q q q q q q q q q q q q q
ELECTRICAL CHARACTERISTICS
MIN 5 -7 7 5 1.214 30 0.3 40 30 -1 -1 -30
TYP 25 5.5 -10 10 20 1.232 75 0.4 0 50 50 5 5 0 0.2 0.2 0.2 0 90 100 0 0
MAX 6 -13 13 50 1.250 150 0.5 100 60 70 10 10 30 0.4 0.4 0.4 1 150 200 5 5 0.82 0.82 10 2.5 30 -65 3 1.4 -13 1.4 -15 1.4 -14 1.4
UNITS mV V A A mA V mV V nA mV mV A A mV V V V A mV mV % % V V nA V A A V A V A V A V
VCC(UVL, HYST) Input Supply Undervoltage Lockout Hysteresis
VRS-SENSE(TH) Sense Resistor Over-Current Voltage Threshold 1V < VSENSEP < VCC 0V < VSENSEP < 1V (VSENSEP - VSENSEN) ISENSEN ISENSEP VOS VFAULT(OL) VSDx(OL) VSTATUS(OL) IRAMP VRAMPBUF(OL) VRAMPBUF(OH) IERROR(%) VTRACKx IFB(LEAK) VFB(CLAMP) RREMOTE ISCTMR(UP) ISCTMR(DN) VSCTMR(TH) ISDTMR(UP) VSDTMR(TH) IPGI(UP) VPGI(TH) IPGTMR(UP) VPGTMR(TH) SENSEN Pin Input Current SENSEP Pin Input Current Ramp Buffer Offset (VRAMPBUF - VRAMP) FAULT Output Low Voltage SDx Output Low Voltage STATUS Output Low Voltage RAMP Pin Input Current RAMPBUF Low Voltage RAMPBUF High Voltage (VCC - VRAMPBUF ) IFBx to ITRACKx Current Mismatch IERROR(%) = (IFBx - ITRACKx)/ITRACKx TRACK pin voltage IFB Leakage Current VFB Clamp Voltage REMOTE Feedback Switch Resistance Short-Circuit Timer Pullup Current Short-Circuit Timer Pulldown Current Short-Circuit Timer Threshold Voltage Shutdown Timer Pullup Current Shutdown Timer Threshold Voltage Power Good Input Pullup Current Power Good Input Threshold Voltage Power Good Timer Pullup Current Power Good Timer Threshold Voltage VPGTMR = 1V VPGI = 0V VSDTMR = 1V 0V < VSENSEN < VCC 0V < VSENSEP < VCC VRAMPBUF = VCC/2, IRAMPBUF = 0A IFAULT = 3mA ISDx = 1mA, VCC = 2.3 ISTATUS = 3mA 0V < RAMP < VCC, VCC = 5.5V IRAMPBUF = 3mA IRAMPBUF = -3mA ITRACKx = -10A ITRACKx = -1mA ITRACKx = -10A ITRACKx = -1mA VFB = 1.5V, VCC = 5.5V 1A < IFB < 1mA 2V < VREMOTE < VCC VSCTMR = 1V VSCTMR = 1V
0.78 0.78 1.6 -35 1 1.1 -7 1.1 -5 0.8 -8 1.1
0.8 0.8 2.1 15 -50 2 1.23 -10 1.23 -10 -10 1.23
q q q q q q
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: All currents into the device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified.
Note 3: The GATE pin is internally limited to a minimum of 11.5V. Driving this pin to voltages beyond the clamp may damage the part.
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LTC2925 TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25C. ICC vs VCC
1.5 ITRACKx = IFBx = 0mA IRAMPBUF = 0mA
1.4
VGATE (V)
ISDx = 5mA ISDx = 10A 0 1 2 3 VCC (V) 4 5
2925 G02
ICC (mA)
1.3
VSDx(OL) (V)
1.2
1.1
1.0 2.9
3.5
4.5 4 VCC (V)
PI FU CTIO S
GN/UF Packages
VCC (Pin 1/Pin 22): Positive Supply Input. The operating supply input range is 2.9V to 5.5V. An undervoltage lockout circuit resets the part when the supply is below 2.5V. VCC should be bypassed to GND with a 0.1F capacitor. SENSEP (Pin 2/Pin 23): Circuit Breaker Positive Sense Input. SENSEP and SENSEN measure the voltage across the sense resistor and trigger the circuit breaker function when the current exceeds the level programmed by the sense resistor for longer than a short circuit timer cycle (see SCTMR). If unused, tie SENSEN and SENSEP to VCC. SENSEN (Pin 3/Pin 24): Circuit Breaker Negative Sense Input. SENSEN connects to the low side of the current sense resistor. SENSEP and SENSEN monitor the current through the external FET by measuring the voltage across the sense resistor. The circuit breaker turns off the FET when the sense voltage exceeds 50mV for longer than a short circuit timer cycle (see SCTMR). If the short-circuit timer times out, the GATE pin will be pulled low immediately to protect the FET. If unused, tie SENSEN and SENSEP to VCC. ON (Pin 4/Pin 1): On Control Input. The ON pin has a threshold of 1.23V with 75mV of hysteresis. An active high will cause 10A to flow from the GATE pin, ramping up the supplies. An active low pulls 10A from the GATE pin,
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2925 G01
VSDx(OL) vs VCC
1.0
12
VGATE vs VCC
0.8
11
0.6
10
0.4
9
0.2
0
5.5
8 2 3 4 VCC (V) 5 6
2925 G03
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ramping the supplies down. Pulling the ON pin below 0.4V resets the electronic circuit breaker in the LTC2925. If a resistive divider connected to VCC drives the ON pin, the supplies will automatically start up when VCC is fully powered. SDTMR (Pin 5/Pin 2): Shutdown Timer. A capacitor from SDTMR to GND sets the delay time between the ON pin transitioning high (which releases the SDx pins) and the supplies beginning to ramp-up. Float SDTMR when it is unused. SD1, SD2, SD3 (Pins 6, 7, 8/Pins 3, 4, 5): Outputs for Slave Supply Shutdowns. The SDx pins are open-drain outputs that hold the shutdown (RUN/SS) pins of the slave supplies low until the ON pin is pulled above 1.23V. The SDx pins will be pulled low again when RAMP <100mV and ON <1.23V. If a slave supply is capable of operating with an input supply that is lower than the LTC2925's minimum operating voltage of 2.9V, the SDx pins can be used to hold off the slave supplies. Each SDx pin is capable of sinking greater than 1mA with supplies as low as 2.3V. RAMPBUF (Pin 9/Pin 6): Ramp Buffer Output. Provides a low impedance buffered version of the signal on the RAMP pin. This buffered output drives the resistive dividers that connect to the TRACKx pins. Limit the capacitance at the RAMPBUF pin to less than 100pF.
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LTC2925
PI FU CTIO S
GND (Pin 10/Pins 7, 25): Circuit Ground. TRACK1, TRACK2, TRACK3 (Pins 15, 14, 12/Pins 12, 11, 9): Tracking Control Input Pin. A resistive divider between RAMPBUF, TRACKx and GND determines the tracking profile of OUTx (see Applications Information). TRACKx pulls up to 0.8V and the current supplied at TRACKx is mirrored at FBx. The TRACKx pin is capable of supplying at least 1mA when VCC = 2.9V. It may be capable of supplying up to 10mA when the supply is at 5.5V, so care should be taken not to short this pin for extended periods. Limit the capacitance at the TRACKx pin to less than 25pF. Float the TRACKx pins if unused. FB1, FB2, FB3 (Pins 16, 13, 11/Pins 13, 10, 8): Feedback Control Output. FBx connects to the feedback node of slave supplies. Tracking is achieved by mirroring the current from TRACKx into FBx. If the appropriate resistive divider connects RAMPBUF and TRACKx, the FBx current will force OUTx to track RAMP. The LTC2925 is capable of controlling slave supplies with feedback voltages between 0V and 1.6V. To prevent damage to the slave supply, the FBx pin will not force the slave's feedback node above 2.5V. In addition, it will not actively sink current from this node even when the LTC2925 is unpowered. Float the FBx pins if unused. REMOTE (Pin 17/Pin 14): Remote Sense Switch. A 15 switch connects REMOTE to RAMP when the GATE is fully enhanced (GATE > RAMP + 4.9V). Otherwise, it presents a high-impedance. When the slave supplies track the master supply, REMOTE can be used to compensate for the voltage drop across the external sense resistor and Nchannel FET. A resistor between the output and the sense nodes of the master supply provides feedback before the external FET is fully enhanced. If an external FET is not used, float REMOTE. RAMP (Pin 18/Pin15): Ramp Buffer Input. When the RAMP pin is connected to the source of the external Nchannel FET, the slave supplies track the FET's source as it ramps up and down. Alternatively, when no external FET is used, the RAMP pin can be tied directly to the GATE pin. In this configuration, the supplies track the capacitor on the GATE pin as it is charged and discharged by the 10A current source controlled by the ON pin. When the GATE is fully enhanced (GATE > RAMP + 4.9V) the open-drain
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GN/UF Packages
STATUS pin goes high-impedance and the remote sense switch connects the RAMP pin to the REMOTE pin. GATE (Pin 19/Pin 16): Gate Drive for External N-Channel FET. When the ON pin is high, an internal 10A current source charges the gate of the external N-channel MOSFET. A capacitor connected from GATE to GND sets the ramp rate. It is a good practice to add a 10 resistor between this capacitor and the FET's gate to prevent high frequency FET oscillations. An internal charge pump guarantees that GATE will pull up to 5V above VCC ensuring that logic level N-channel FETs are fully enhanced. When the ON pin is pulled low, the GATE pin is pulled to GND with a 10A current source. Under a short-circuit condition, the electronic circuit breaker in the LTC2925 pulls the GATE low immediately with 20mA. Tie GATE to GND if unused. FAULT (Pin 20/Pin 17): Circuit Breaker and Power Good Timer Fault Output. FAULT is an open-drain output that pulls low when the electronic circuit breaker is activated or a power good timeout fault is detected. FAULT is reset by pulling ON below 0.4V. To allow retry, tie FAULT to ON. STATUS (Pin 21/Pin 18): Power Good Status Indicator. The STATUS pin is an open-drain output that pulls low until GATE has been fully charged at which time all supplies will have reached their final operating voltage. PGI (Pin 22/Pin 19): Power Good Timer Input. PGI connects to the RST pin of the downstream supply monitor. If PGI has not transitioned high within a power good timer cycle (see PGTMR), the FAULT pin will be pulled low and the supplies will be turned off by pulling the GATE pin low with 20mA. PGI is pulled up with 10A. An internal schottky diode allows PGI to be pulled safely above VCC. Float PGI when it is unused. PGTMR (Pin 23/Pin 20): Power Good Timer. A capacitor from PGTMR to GND sets the Power Good Timer duration. While ON > 1.23V, the PGTMR pin will pull up to VCC with 10A. Otherwise, it pulls to GND. If the voltage on the PGTMR pin exceeds 1.23V and PGI is still low, FAULT will be pulled low and the GATE will be pulled to ground with 20mA until the power good timer fault is cleared by pulling ON below 0.4V. If FAULT is tied back to ON the system will automatically retry after a FAULT. In this mode, verify that the slave supplies' current limits provide sufficient protection under short-circuit conditions. Float PGTMR when it is unused.
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LTC2925
PI FU CTIO S
SCTMR (Pin 24/Pin 21): Circuit Breaker Timer. A capacitor from SCTMR to GND programs the maximum time that a short circuit can be sustained before GATE is pulled low. When (SENSEP - SENSEN) > 50mV, SCTMR will pull up with 50A, otherwise it pulls down with 2A. When the voltage at SCTMR exceeds 1.23V, the GATE will be pulled to ground with 20mA and the FAULT pin will be pulled low.
FU CTIO AL BLOCK DIAGRA
(9) 12 (11) 14 (12) 15
TRACK3 TRACK2 TRACK1 VCC 50A
(21) 24
SCTMR
2A
1.2V (19) 22 PGI VCC 1.2V
+ -
GATE CHARGE PUMP
(20) 23
PGTMR
ONSIG 10A SCTMR 0.1V 0.4V (1) 4 ON
- + - +
S R Q
S R
Q VCC 10A SDTMR 5 (2)
1.2V
0.1V
VCC (6) 9 (14) 17 10 RAMPBUF REMOTE STATUS 2.6V 1x
- +
VCC
4.9V GND (7, 25)
GATE
2925 FBD
GATE > RAMP + 4.9V
10
6
+ -
- +
- +
- +
+ -
10A
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GN/UF Packages
The circuit breaker function is reset by pulling ON below 0.4V. The GATE pin will not rise again until SCTMR has been pulled below 100mV by the 2A current source. If FAULT is tied back to ON the system will automatically retry after a fault. Tie SCTMR to GND if the circuit breaker is not used.
Pin numbers in parentheses are for the UF package.
(22) 1 VCC VCC
FB3
11 (8) 13 (10) 16 (13)
+ -
0.8V
FB2 FB1
SENSEP > SENSEN + 50mV SENSEP 50mV SENSEN FAULT
2 (23) 3 (24) 20 (17)
19 (16)
10A 1.2V
SDx
RAMP
18 (15)
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LTC2925
APPLICATIO S I FOR ATIO
Power Supply Tracking and Sequencing The LTC2925 handles a variety of power-up profiles to satisfy the requirements of digital logic circuits including FPGAs, PLDs, DSPs and microprocessors. These requirements fall into one of the four general categories illustrated in Figures 1 to 4. Some applications require that the potential difference between two power supplies must never exceed a specified voltage. This requirement applies during power-up and power-down as well as during steady-state operation, often to prevent destructive latch-up in a dual supply IC. Typically, this is achieved by ramping the supplies up and down together (Figure 1). In other applications it is desirable to have the supplies ramp up and down with fixed voltage offsets between them (Figure 2) or to have them ramp up and down ratiometrically (Figure 3).
1V/DIV
10ms/DIV
2925 F01
Figure 1. Coincident Tracking
2V/DIV
10ms/DIV
2925 F03
Figure 3. Ratiometric Tracking
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Certain applications require one supply to come up after another. For example, a system clock may need to start before a block of logic. In this case, the supplies are sequenced as in Figure 4 where the 1.8V supply ramps up completely followed by the 2.5V supply and then the 1.5V supply. Operation The LTC2925 provides a simple solution to all of the power supply tracking and sequencing profiles shown in Figures 1 to 4. A single LTC2925 controls up to four supplies with three "slave" supplies that track a "master" signal. With just two resistors, a slave supply is configured to ramp up as a function of the master signal. This master signal can be a fourth supply that is ramped up through an external FET, whose ramp rate is set with a single capacitor, or it can be a signal generated by tying the GATE and RAMP pins to an external capacitor.
MASTER SLAVE1 SLAVE2 SLAVE3 1V/DIV MASTER SLAVE1 SLAVE2 SLAVE3 10ms/DIV
2925 F02
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Figure 2. Offset Tracking
MASTER SLAVE1 SLAVE2 SLAVE3 SLAVE1 2V/DIV SLAVE2 SLAVE3
10ms/DIV
2925 F04
Figure 4. Supply Sequencing
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LTC2925
APPLICATIO S I FOR ATIO
Tracking Cell The LTC2925's operation is based on the tracking cell shown in Figure 5, which uses a proprietary wide-range current mirror. The tracking cell shown in Figure 5 servos the TRACK pin at 0.8V. The current supplied by the TRACK pin is mirrored at the FB pin to establish a voltage at the output of the slave supply. The slave output voltage varies with the master signal, enabling the slave supply to be controlled as a function of the master signal with terms set by RTA and RTB. By selecting appropriate values of RTA and RTB, it is possible to generate any of the profiles in Figures 1 to 4.
VCC
+
MASTER RTB TRACK RTA RFA RFB
+ - -
0.8V DC/DC FB FB OUT SLAVE
2925 F05
Figure 5. Simplified Tracking Cell
Controlling the Ramp-Up and Ramp-Down Behavior The operation of the LTC2925 is most easily understood by referring to the simplified functional diagram in Figure 6. When the ON pin is low, the GATE pin is pulled to ground causing the master signal to remain low. Since the current through RTB1 is at its maximum when the master signal is low, the current from FB1 is also at its maximum. This current drives the slave's output to its minimum voltage. When the ON pin rises above 1.23V, the master signal rises and the slave supply tracks the master signal. The ramp rate is set by an external capacitor driven by a 10A current source from an internal charge pump. If no external FET is used, the ramp rate is set by tying the RAMP and GATE pins together at one terminal of the external capacitor (see the Ratiometric Tracking Example).
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In a properly designed system, when the master signal has reached its maximum voltage the current from the TRACK1 pin is zero. In this case, there is no current from the FB1 pin and the LTC2925 has no effect on the output voltage accuracy, transient response or stability of the slave supply. When the ON pin falls below VON(TH) - VON(HYST), typically 1.225V, the GATE pin pulls down with 10A and the master signal and the slave supplies will fall at the same rate as they rose previously. The ON pin can be controlled by a digital I/O pin or it can be used to monitor an input supply. By connecting a resistive divider from an input supply to the ON pin, the supplies will ramp up only after the monitored supply has reached a preset voltage.
VCC SENSEP - SENSEN > 50mV 50A SCTMR 50mV SENSEN 2A SENSEP
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1.2V RONB ON
-
10A GATE Q1 CGATE
+ -
RONA
1.2V
10A RAMPBUF RAMP VCC
1x
MASTER
+
RTB1 TRACK1 RTA1
0.8V
-
FB1 DC/DC R RFA1 FB1
2925 F06
SLAVE1
Figure 6. Simplified Functional Block Diagram
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LTC2925
APPLICATIO S I FOR ATIO
Optional External FET
Figure 7 illustrates how an optional external N-channel FET can ramp up a single supply that becomes the master signal. When used, the FET's gate is tied to the GATE pin and its source is tied to the RAMP pin. Under normal operation, the GATE pin sources or sinks 10A to ramp the FET's gate up or down at a rate set by the external capacitor connected to the GATE pin. The series FET easily controls any supply with an output voltage between 0V and VCC. See the Typical Applications section for examples.
VIN 0.1F 10 CGATE SUPPLY MONITOR RONB RONA REMOTE VIN 10k STATUS VIN LTC2925 10k FAULT RAMPBUF RTB1 TRACK1 RTA1 RTA2 RTB2 TRACK2 RTB3 TRACK3 RTA3 GND SCTMR CSCTMR SDTMR CSDTMR PGTMR CPGTMR RFA3
2925 F07
RSENSE
Q1 MASTER
VCC SENSEP SENSEN ON
GATE
RAMP PGI SD1 FB1
RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT RFB1 3.3V
RFA1
SD2 FB2
IN RUN/SS DC/DC FB = 0.8V OUT RFB2 3.3V
RFA2
SD3 FB3
RUN/SS IN DC/DC FB = 0.8V OUT RFB3
Figure 7. Typical Application With External FET
Electronic Circuit Breaker The LTC2925 features an electronic circuit breaker function that protects the optional series FET against short circuits. An external sense resistor is used to measure the current flowing in the FET. If the voltage across the sense resistor exceeds 50mV for more than a short-circuit timer cycle, the gate of the FET is pulled low with 20mA, turning it off.
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The short-circuit timer duration is configured by a capacitor tied between SCTMR and GND. SCTMR will pull up with 50A when SENSEP - SENSEN > 50mV. Otherwise, it pulls down with 2A. When the voltage at SCTMR exceeds 1.23V, the GATE will be pulled to ground with 20mA and the FAULT pin will be pulled low. Thus, the capacitor, CSCTMR, required to configure the short-circuit timer duration, tSCTMR is determined from: CSCTMR = 50A * tSCTMR 1.23V
1.8V SLAVE1 2.5V SLAVE2
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Because the slave supplies track the RAMP pin which is driven by the external FET, they are pulled low by the tracking circuit when a short-circuit fault occurs. Following a short-circuit fault, the FET is latched off and FAULT is pulled low until the fault is cleared by pulling the ON pin below 0.4V. Note that the supplies will not be allowed to ramp up again until SCTMR has been pulled below about 100mV by the 2A pull down current source. The electronic circuit breaker supports any supply voltage between 0V and VCC. Although it is normally used to monitor current through the optional series FET, it is capable of monitoring other currents, including the current from a slave supply. The Typical Applications section shows one such example. If the electronic circuit breaker is not used, tie SENSEP and SENSEN to VCC and SCTMR to GND. Power Good Timeout The power good timeout circuit turns off the supplies if an external supply monitor, connected to the PGI pin, fails to indicate that all supplies have entered regulation in time after power up begins. After power up is complete, it turns off the supplies if any supply exits regulation. The power good timer duration is configured by a capacitor tied between PGTMR and GND. PGTMR will pull up the CPGTMR capacitor with 10A starting when the ON pin is driven above 1.23V. Once the voltage at the PGTMR exceeds 1.23V, a fault will trip if the PGI pin is low. When the power good timeout circuit detects a fault, the GATE pin is
1.5V SLAVE3
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LTC2925
APPLICATIO S I FOR ATIO
pulled low, the supplies are latched off, and the FAULT pin is held low until the fault is cleared by taking the ON pin below 0.4V. The PGI pin, which is normally connected to the RST pin of an external supply monitor, is pulled up with 10A through a schottky diode allowing it to be pulled safely above VCC. Since, PGTMR pulls up with a 10A current source, the capacitor, CPGTMR, required to configure the power good timeout duration, tPGTMR, is determined from: CPGTMR = 10A * tPGTMR 1.23V
If the power good timeout circuit is unused, tie PGTMR low and float PGI. The Ramp Buffer The RAMPBUF pin provides a buffered version of the RAMP pin voltage that drives the resistive dividers on the TRACKx pins. When there is no external FET, it provides up to 3mA to drive the resistors even though the GATE pin only supplies 10A (Figure 8). The RAMPBUF pin also proves useVIN 0.1F CGATE
SUPPLY MONITOR RONB RONA REMOTE VIN VCC ON SENSEP SENSEN GATE RAMP PGI SD1 FB1 RST VIN RUN/SS IN DC/DC FB OUT RFB1 VIN LTC2925 SD2 FB2 IN RUN/SS DC/DC FB OUT RFB2 VIN SD3 FB3 TRACK3 RTA3 GND SCTMR SDTMR CSDTMR PGTMR CPGTMR RFA3
2925 F08
STATUS VIN
RFA1
FAULT RAMPBUF RTB1 TRACK1 RTA1 RTA2 RTB2 TRACK2 RTB3
RFA2
RUN/SS IN DC/DC FB OUT RFB3
Figure 8. Typical Application Without External FET
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ful in systems with an external FET. Since the track cell drives 0.8V on the TRACKx pins, if RTBx is connected directly to the FET's source, the TRACKx pin could potentially pull up the FET's source towards 0.8V when the FET is off. RAMPBUF blocks this path. Shutdown Outputs In some applications it might be necessary to control the shutdown or RUN/SS pins of the slave supplies. The LTC2925 may not be able to supply the rated 1mA of current from the FB1, FB2, and FB3 pins when VCC is below 2.9V. If the slave power supplies are capable of operating at low input voltages, use the open-drain SDx outputs to drive the SHDN or RUN/SS pins of the slave supplies (Figures 7 and 8). The SDx pins are released when the ON pin rises above 1.23V, VCC is above the 2.6V undervoltage lockout condition, and there are no faults latched. The shutdown timer begins at the same time, and the supplies begin to ramp up after the shutdown timer cycle completes. The duration of the timer cycle is configured by a capacitor tied between SDTMR and GND. The capacitor voltage is ramped up by a 10A current source and the SDTMR cycle completes when its voltage reaches 1.23V. Thus, the capacitor, CSDTMR, required for a given shutdown timer cycle, tSDTMR, is determined from: CSDTMR =
SLAVE1
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10A * tSDTMR 1.23V
The SDx pins pull low again when the ON pin is pulled below 1.23V and the RAMP pin is below about 100mV.
SLAVE2
SLAVE3
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LTC2925
APPLICATIO S I FOR ATIO
Status Output
The STATUS pin provides an indication that the supplies are finished ramping up. This pin is an open-drain output that pulls low until the GATE has been fully charged. Since the GATE pin drives the gate of the external FET, or the RAMP pin directly when no FET is used, the supplies are completely ramped up when the GATE pin is fully charged. The STATUS pin will go low again when the GATE pin is pulled low, either because of a short-circuit fault, a power good timeout fault, or because the ON pin has been pulled low. Fault Output The FAULT pin is an open-drain output that pulls low when the electronic circuit breaker is activated due to a shortcircuit or power good timeout fault. FAULT is reset by pulling ON below 0.4V. The supplies will not be allowed to ramp up again until the SCTMR, PGTMR, and SDTMR pins are below about 100mV, and the ON pin is pulled above 1.23V.
VIN 0.1F 10 RSENSE Q1
RONB RONA
VCC SENSEP SENSEN ON
REMOTE VIN 10k STATUS LTC2925
FAULT RAMPBUF RTB1 TRACK1 RTA1 RTA2 RTB2 TRACK2 RTB3 TRACK3 RTA3 GND SCTMR CSCTMR SDTMR CSDTMR PGTMR
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Retry on Fault The LTC2925 continuously attempts to ramp up the outputs after a fault if the FAULT pin is tied to the ON pin (Figure 9). If a short-circuit fault occurs in this configuration, the SCTMR pin ramps up the CSCTMR capacitor with 50A until it reaches 1.23V. Then, GATE is pulled low turning off the shorted FET. At the same time, the FAULT pin's open-drain output pulls ON low. The CSCTMR capacitor is pulled down with 2A until it reaches about 100mV. After the CSCTMR capacitor reaches 100mV, the shutdown timer begins and upon completing a shutdown timer cycle, the supplies start ramping up again. If there is no short-circuit this time, the supplies will come up normally. Otherwise, the retry cycle will repeat. If a longer off time is required between retry attempts, the CSDTMR capacitor value can be increased, providing a greater delay before the FET's GATE ramps up on each cycle. Note that tying FAULT to ON also causes the LTC2925 to retry on Power Good Timeout faults. In this mode, verify that the slave supplies' current limits provide sufficient protection under short-circuit conditions.
MASTER CGATE SUPPLY MONITOR GATE RAMP PGI SD1 FB1 RST VIN RUN/SS IN DC/DC FB OUT RFB1 VIN SD2 FB2 IN RUN/SS DC/DC FB OUT RFB2 VIN SD3 FB3 RUN/SS IN DC/DC FB OUT RFB3 SLAVE1 RFA1 SLAVE2 RFA2 SLAVE3 CPGTMR RFA3
2925 F09
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Figure 9. Retry on Fault
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LTC2925
APPLICATIO S I FOR ATIO
3-Step Design Procedure
The following 3-step design procedure allows one to choose the TRACK resistors, RTAx and RTBx, and the gate capacitor, CGATE, that give any of the tracking or sequencing profiles shown in Figures 1 to 4. A basic four supply application circuit is shown in Figure 10. 1. Set the ramp rate of the master signal. Solve for the value of CGATE, the capacitor on the GATE pin, based on the desired ramp rate (V/s) of the master supply, SM.
CGATE = IGATE where IGATE 10A SM (1)
If the external FET has a gate capacitance comparable to CGATE, then the external capacitor's value should be reduced to compensate for the FET's gate capacitance. If no external FET is used, tie the GATE and RAMP pins together, connect SENSEN and SENSEP to VCC, and connect SCTMR to GND. 2. Solve for the pair of resistors that provide the desired ramp rate of the slave supply, assuming no delay.
VIN 0.1F 10 CGATE SUPPLY MONITOR VCC SENSEP SENSEN ON RONA 100k REMOTE VIN 10k STATUS VIN LTC2925 10k FAULT RAMPBUF RTB1 TRACK1 RTA1 RTA2 RTB2 TRACK2 RTB3 TRACK3 RTA3 GND SCTMR SDTMR PGTMR RFA3
2925 F10
RSENSE
Q1 MASTER
RONB 154k
GATE
RAMP PGI SD1 FB1
RST VIN RUN/SS IN DC/DC FB OUT RFB1 VIN
RFA1
SD2 FB2
IN RUN/SS DC/DC FB OUT RFB2 VIN
RFA2
SD3 FB3
RUN/SS IN DC/DC FB OUT RFB3
Figure 10. Four Supply Application
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Choose a ramp rate for the slave supply, SS. If the slave supply ramps up coincident with the master supply or with a fixed voltage offset, then the ramp rate equals the master supply's ramp rate. Be sure to use a fast enough ramp rate for the slave supply so that it will finish ramping before the master supply has reached its final supply value. If not, the slave supply will be held below the intended regulation value by the master supply. Use the following formulas to determine the resistor values for the desired ramp rate, where RFB and RFA are the feedback resistors in the slave supply and VFB is the feedback reference voltage of the slave supply:
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RTB = RFB *
RTA =
SM SS
VTRACK V V + FB - TRACK RFA RTB
(2)
VFB RFB
(3)
where VTRACK 0.8V. Note that large ratios of slave ramp rate to master ramp rate, SS/SM, may result in negative values for RTA. If a sufficiently large delay is used in step 3, RTA will be positive, otherwise SS/SM must be reduced. 3. Choose RTA to obtain the desired delay. If no delay is required, such as in coincident and ratiometric tracking, then simply set RTA = RTA. If a delay is desired, as in offset tracking and supply sequencing, calculate RTA to determine the value of RTA where tD is the desired delay.
SLAVE1
RTA =
SLAVE2
VTRACK * RTB tD * SM
(4)
RTA = RTA || RTA the parallel combination of RTA and RTA
(5)
SLAVE3
As noted in step 2, small delays and large ratios of slave ramp rate to master ramp rate (usually only seen in sequencing) may result in solutions with negative values for RTA. In such cases, either the delay must be increased or the ratio of slave ramp rate to master ramp rate must be reduced.
LTC2925
APPLICATIO S I FOR ATIO
1V/DIV
10ms/DIV
Figure 11. Coincident Tracking from Figure 12
Coincident Tracking Example A typical four supply application is shown in Figure 12. The master signal is a 3.3V module. The slave 1 supply is a 1.8V switching power supply, the slave 2 supply is a 2.5V switching power supply, and the slave 3 supply is a 1.5V supply. All three slave supplies track coincidently with the 3.3V supply that is controlled with an external FET. The ramp rate of the supplies is 100V/s. The 3-step design procedure detailed previously can be used to determine component values. Only the slave 1 supply is considered here as the procedure is the same for the other supplies. 1. Set the ramp rate of the master signal. From Equation 1:
CGATE =
10A = 0.1F 100 V s
RONB 154k RONA 100k
2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: 100V s RTB = 16.5k * = 16.5k 100V s From Equation 3: RTA = 0.8 V 13k 1.235V 1.235V 0.8 V + - 16.5k 35.7k 16.5k
RTB1 16.5k RTA1 13k RTA2 41.2k VIN
3. Choose RTA to obtain the desired delay. Since no delay is desired, RTA = RTA In this example, all supplies remain low while the ON pin
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MASTER SLAVE2 SLAVE1 SLAVE3 1V/DIV
2925 F11a
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10ms/DIV
2925 F11b
is held below 1.23V. When the ON pin rises above 1.23V, 10A pulls up CGATE and the gate of the FET at 100V/s. As the gate of the FET rises, the source follows and pulls up the output to 3.3V at 100V/s. This output serves as the master signal and is buffered from the RAMP pin to the RAMPBUF pin. As this output and the RAMPBUF pin rise, the current from the TRACKx pins is reduced. Consequently, the voltages at the slave supplies' outputs increase, and the slave supplies track the master supply. When the ON pin is again pulled below 1.23V, 10A will pull down CGATE and the gate of the FET at 100V/s. If the loads on the outputs are sufficient, all outputs will track down coincidently at 100V/s.
0.015 3.3V VIN 0.1F 10 CGATE 0.1F SUPPLY MONITOR VCC SENSEP SENSEN ON GATE RAMP PGI SD1 REMOTE FB1 RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT RFA1 35.7k LTC2925 10k FAULT RAMPBUF TRACK1 TRACK2 TRACK3 RTA3 100k GND SCTMR CSCTMR 0.47F SDTMR CSDTMR 0.082F PGTMR CPGTMR 0.82F RFA3 100k RFB3 86.6k SD3 FB3 FB2 SD2 RFB1 16.5k 3.3V Q1 Si4412ADY MASTER
1.8V SLAVE1
10k STATUS VIN
IN RUN/SS DC/DC FB = 0.8V OUT RFA2 41.2k RFB2 88.7k
2.5V SLAVE2
RTB2 88.7k RTB3 86.6k
3.3V
RUN/SS IN DC/DC FB = 0.8V OUT
1.5V SLAVE3
2925 F12
Figure 12. Coincident Tracking Example
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LTC2925
APPLICATIO S I FOR ATIO
1V/DIV
10ms/DIV
Figure 13. Ratiometric Tracking from Figure 14
Ratiometric Tracking Example This example converts the coincident tracking example to the ratiometric tracking profile shown in Figure 13, using three supplies without an external FET. The ramp rate of the master signal remains unchanged (Step 1) and there is no delay in ratiometric tracking (Step 3), so only the result of step 2 in the 3-step design procedure needs to be considered. In this example, the ramp rate of the 1.8V slave 1 supply ramps up at 60V/s, the 2.5V slave 2 supply ramps up at 85V/s, and the 1.5V slave 3 supply ramps up at 50V/s. Always verify that the chosen ramp rate will allow the supplies to ramp-up completely before RAMPBUF reaches VCC. If the 1.8V supply were to ramp-up at 50V/s it would only reach 1.65V because the RAMPBUF signal would reach its final value of VCC = 3.3V before the slave supply reached 1.8V. 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: 100 V s RTB = 16.5k * 27.4k 60 V s
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SLAVE2 SLAVE1 SLAVE3 1V/DIV
2925 F13a
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10ms/DIV
2925 F13b
From Equation 3: RTA = 0.8 V 10k 1.235V 1.235V 0.8 V + - 16.5k 35.7k 27.5k
Step 3 is unnecessary because there is no delay, so RTA = RTA.
3.3V VIN 0.1F CGATE 0.1F SUPPLY MONITOR RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT RFA1 35.7k LTC2925 10k FAULT RAMPBUF TRACK1 TRACK2 TRACK3 RTA3 63.4k GND SCTMR CSCTMR 0.41F SDTMR CSDTMR 0.082F PGTMR CPGTMR 0.82F RFA3 100k RFB3 86.6k SD3 FB3 FB2 SD2 RFB1 16.5k 3.3V
RONB 154k RONA 100k
VCC ON
SENSEP SENSEN GATE
RAMP PGI SD1
REMOTE VIN 10k STATUS VIN
FB1
1.8V SLAVE1
IN RUN/SS DC/DC FB = 0.8V OUT RFA2 41.2k RFB2 88.7k
2.5V SLAVE2
RTB1 27.4k RTA1 10k RTA2 38.3k RTB2 100k RTB3 174k
3.3V
RUN/SS IN DC/DC FB = 0.8V OUT
1.5V SLAVE3
2925 F14
Figure 14. Ratiometric Tracking Example
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LTC2925
APPLICATIO S I FOR ATIO
1V/DIV
10ms/DIV
Figure 15. Offset Tracking from Figure 16
Offset Tracking Example Converting the circuit in the coincident tracking example to the offset tracking shown in Figure 15 is relatively simple. Here the 1.8V slave 1 supply ramps up 1V below the master. The ramp rate remains the same (100V/s), so there are no changes necessary to steps 1 and 2 of the 3-step design procedure. Only step 3 must be considered. Be sure to verify that the chosen voltage offsets will allow the slave supplies to ramp up completely. In this example, if the voltage offset were 2V, the slave supply would only ramp up to 3.3V - 2V = 1.3V. 3. Choose RTA to obtain the desired delay. First, convert the desired voltage offset, VOS, to a delay, tD, using the ramp rate:
V 1V tD = OS = = 10ms SS 100 V s
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2925 F15a
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2925 F15b
From Equation 4: RTA = 0.8 V * 16.5k = 13.2k 1ms * 100 V s
From Equation 5:
RTA = 13.1k || 13.2k 6.65k
0.015 3.3V VIN 0.1F 10 CGATE 0.1F SUPPLY MONITOR VCC SENSEP SENSEN ON RONA 100k REMOTE VIN GATE RAMP PGI SD1 FB1 RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT RFA1 35.7k LTC2925 10k FAULT RAMPBUF TRACK1 TRACK2 TRACK3 RTA3 31.6k GND SCTMR CSCTMR 0.41F SDTMR CSDTMR 0.082F PGTMR CPGTMR 0.82F RFA3 100k RFB3 86.6k SD3 FB3 FB2 SD2 RFB1 16.5k 3.3V Q1 Si4412ADY MASTER
RONB 154k
1.8V SLAVE1
(6)
VIN
10k STATUS
IN RUN/SS DC/DC FB = 0.8V OUT RFA2 41.2k RFB2 88.7k
2.5V SLAVE2
RTB1 16.5k RTA1 6.65k RTA2 31.6k RTB2 88.7k RTB3 86.6k
3.3V
RUN/SS IN DC/DC FB = 0.8V OUT
1.5V SLAVE3
2925 F16
Figure 16. Offset Tracking Example
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LTC2925
APPLICATIO S I FOR ATIO
1V/DIV
10ms/DIV
Figure 17. Supply Sequencing from Figure 18
Supply Sequencing Example In Figure 17, the three slave supplies are sequenced instead of tracking. As in the coincident tracking example, the 3.3V master supply ramps up at 100V/s through an external FET, so step 1 remains the same. The 1.8V slave 1 supply ramps up at 1000V/s beginning 10ms after the master signal starts to ramp up. The 2.5V slave 2 supply ramps up at 1000V/s beginning 20ms after the master signal begins to ramp up. The 1.5V slave 3 supply ramps up at 1000V/s beginning 25ms after the master signal begins to ramp up. Note that not every combination of ramp rates and delays is possible. Small delays and large ratios of slave ramp rate to master ramp rate may result in solutions that require negative resistors. In such cases, either the delay must be increased or the ratio of slave ramp rate to master ramp rate must be reduced. In this example, solving for the slave 1 supply yields: 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay.
From Equation 2: RTB = 16.5k * From Equation 3: 0.8 V RTA = -2.13k 1.235V 1.235V 0.8 V + - 16.5k 35.7k 1.65k 100 V s 1.65k 1000 V s
RTB1 1.65k RTA1 3.48k RTA2 4.87k
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MASTER SLAVE2 SLAVE1 SLAVE3 1V/DIV
2925 F17a
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10ms/DIV
2925 F17b
3. Choose RTA to obtain the desired delay. From Equation 4: RTA = 0.8 V * 1.65k = 1.32k 10ms * 100 V s
From Equation 5: RTA = -2.13k || 1.32k 3.48k
0.015 3.3V VIN 0.1F 10 CGATE 0.1F SUPPLY MONITOR VCC SENSEP SENSEN ON RONA 100k REMOTE VIN 10k STATUS VIN LTC2925 10k FAULT RAMPBUF TRACK1 TRACK2 TRACK3 RTA3 3.74k GND SCTMR CSCTMR 0.41F SDTMR CSDTMR 0.082F PGTMR CPGTMR 0.82F RFA3 100k RFB3 86.6k SD3 FB3 FB2 SD2 GATE RAMP PGI SD1 FB1 RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT RFA1 35.7k RFB1 16.5k 3.3V Q1 Si4412ADY MASTER
RONB 154k
1.8V SLAVE1
IN RUN/SS DC/DC FB = 0.8V OUT RFA2 41.2k RFB2 88.7k
2.5V SLAVE2
RTB2 8.87k RTB3 8.66k
3.3V
RUN/SS IN DC/DC FB = 0.8V OUT
1.5V SLAVE3
2925 F18
Figure 18. Supply Sequencing Example
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LTC2925
APPLICATIO S I FOR ATIO
Final Sanity Checks
The collection of equations below is useful for identifying unrealizable solutions. As stated in step 2, the slave supply must finish ramping before the master signal has reached its final voltage. This can be verified by the following equation:
R VTRACK 1+ TB < VMASTER RTA
Here, VTRACK = 0.8V. VMASTER is the final voltage of the master signal, either the supply voltage ramped up through the optional external FET or VCC when no FET is present. It is possible to choose resistor values that require the LTC2925 to supply more current than the Electrical Characteristics table guarantees. To avoid this condition, check that ITRACKx does not exceed 1mA and IRAMPBUF does not exceed 3mA. To confirm that ITRACKx < 1mA, the TRACKx pin(s) maximum guaranteed current, verify that:
VTRACK < 1mA RTA RTB
Finally, check that the RAMPBUF pin will not be forced to sink more than 3mA when it is at 0V or be forced to source more than 3mA when it is at VMASTER.
VTRACK V V + TRACK + TRACK < 3mA and RTA1 RTB1 RTA2 RTB2 RTA3 RTB3 VMASTER V V + MASTER + MASTER < 3mA RTA1 + RTB1 RTA2 + RTB2 RTA3 + RTB3
Caution with Boost Regulators and Linear Regulators Note that the LTC2925's tracking cell is not able to control the outputs of all types of power supplies. If it is necessary to control a supply, where the output is not controllable through its feedback node, the series FET can be used to control its output. For example, boost regulators commonly contain an inductor and diode between the input supply and the output supply providing a DC current path when the output voltage falls below the input voltage.
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Therefore, the LTC2925's tracking cell will not effectively drive the supply's output below the input. Special caution should be taken when considering the use of linear regulators. Three-terminal linear regulators have a reference voltage that is referred to the output supply rather than to ground. In this case, driving current into the regulator's feedback node will cause its output to rise rather than fall. Even linear regulators that have their reference voltage referred to ground, including low-dropout regulators (LDOs), may be problematic. Linear regulators commonly contain circuitry that prevents driving their outputs below their reference voltage. This may not be obvious from the datasheets, so lab testing is recommended whenever the LTC2925's tracking cell is used to control linear regulators. Load Requirements When the supplies are ramped down quickly, either the load or the supply itself must be capable of sinking enough current to support the ramp rate. For example, if there is a large output capacitance on the supply and a weak resistive load, supplies that do not sink current will have their falling ramp rate limited by the RC time constant of the load and the output capacitance. Figure 19 shows the case when the 2.5V supply does not track the 1.8V and 3.3V supplies near ground. Start-Up Delays Often power supplies do not start-up immediatley when their input supplies are applied. If the LTC2925 tries to ramp-up these power supplies as soon as the input supply is present, the start-up of the outputs may be delayed
MASTER SLAVE2 SLAVE1 1V/DIV 1ms/DIV
2925 F19
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Figure 19. Weak Resistive Load
2925f
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LTC2925
APPLICATIO S I FOR ATIO
defeating the tracking circuit (Figure 20). Often this delay is intentionally configured by a soft-start capacacitor. This can be remedied either by reducing the soft-start capacitor on the slave supply or by increasing the shutdown timer cycle configured by CSDTMR. Layout Considerations Be sure to place a 0.1F bypass capacitor as near as possible to the supply pin of the LTC2925. To minimize the noise on the slave supplies' outputs, keep the traces connecting the FBx pins of the LTC2925 and the feedback nodes of the slave supplies as short as possible. In addition, do not route those traces next to signals with fast transition times. In some circumstances it might be
MASTER
SLAVE1 1V/DIV SLAVE2
ON
1ms/DIV
2925 F20
Figure 20. Power Supply Start-Ups Delayed
VCC LTC2925 FB1 GND 0.1F
2925 F22
RSERIES MINIMIZE TRACE LENGTH
DC/DC FB OUT RFB
RFA
Figure 21. Layout Considerations Figure 22. Making PCB Connections to the Sense Resistor
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advantageous to add a resistor near the feedback node of the slave supply in series with the FBx pin of the LTC2925. This resistor must not exceed:
RSERIES = 1.6 V - VFB 1.6 V = - 1 (RFA || RFB) VFB IMAX
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This resistor is most effective if there is already a capacitor at the feedback node of the slave supply (often a compensation component). Increasing the capacitance on a slave supply's feedback node will further improve the noise immunity, but could affect the stability and transient response of the supply. For proper circuit breaker operation, Kelvin-sense PCB connections between the sense resistor and the LTC2925's SENSEP and SENSEN pins are strongly recommended. The drawing in Figure 22 illustrates the correct way of making connections between the LTC2925 and the sense resistor. PCB layout should be balanced and symmetrical to minimize wiring errors. In addition, the PCB layout for the sense resistor should include good thermal management techniques for optimal sense resistor power dissipation. The power rating of the sense resistor should accommodate steady-state fault current levels so that the component is not damaged before the circuit breaker trips.
CURRENT FLOW TO LOAD TRACK WIDTH W: 0.03' PER AMP ON 1 OZ COPPER IRC-TT SENSE RESISTOR LR251201R010F OR EQUIVALENT 0.01, 1%, 1W CURRENT FLOW TO LOAD
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2925 F23
TO SENSEP
TO SENSEN
2925f
LTC2925
PACKAGE DESCRIPTIO
.254 MIN
.0165 .0015 RECOMMENDED SOLDER PAD LAYOUT
.0075 - .0098 (0.19 - 0.25) .016 - .050 (0.406 - 1.270)
NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
4.50 0.05
2.45 0.05 (4 SIDES)
3.10 0.05
0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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GN Package 24-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.337 - .344* (8.560 - 8.738) 24 23 22 21 20 19 18 17 16 15 1413 .045 .005 .033 (0.838) REF .229 - .244 (5.817 - 6.198) .150 - .165 .150 - .157** (3.810 - 3.988) 1 .0250 BSC .015 .004 x 45 (0.38 0.10) 0 - 8 TYP 23 4 56 7 8 9 10 11 12 .0532 - .0688 (1.35 - 1.75) .004 - .0098 (0.102 - 0.249) .008 - .012 (0.203 - 0.305) TYP .0250 (0.635) BSC
GN24 (SSOP) 0204
UF Package 24-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1697)
BOTTOM VIEW--EXPOSED PAD 0.23 TYP (4 SIDES) 23 24 0.38 0.10 1 2 2.45 0.10 (4-SIDES)
4.00 0.10 (4 SIDES) 0.70 0.05 PIN 1 TOP MARK (NOTE 6)
0.75 0.05
R = 0.115 TYP
PACKAGE OUTLINE
(UF24) QFN 1103
0.200 REF 0.00 - 0.05
0.25 0.05 0.50 BSC
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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LTC2925
TYPICAL APPLICATIO S
External FET Controls 1V Supply
0.015 1V 0.1F 3.3V 10 Q1 Si4412ADY MASTER CGATE 0.1F SUPPLY MONITOR VCC ON RONA 100k REMOTE VIN 10k STATUS VIN LTC2925 10k FAULT RAMPBUF TRACK1 TRACK2 TRACK3 RTA3 348k GND SCTMR CSCTMR 0.41F SDTMR CSDTMR 0.082F PGTMR CPGTMR 0.82F RFA3 100k RFB3 86.6k SD3 FB3 FB2 SD2 SENSEP SENSEN GATE RAMP PGI SD1 FB1 RST 3.3V RUN/SS IN DC/DC FB = 1.235V OUT RFA1 35.7k RFB1 16.5k 3.3V
RONB 154k
RTB1 8.66k RTA1 48.7k RTA2 215k RTB2 32.4k RTB3 53.6k
RELATED PARTS
PART NUMBER LTC2900 LTC2901 LTC2902 LTC2920 LTC2921/LTC2922 LTC2923 DESCRIPTION Quad Voltage Monitor in MSOP and DFN Quad Voltage Monitor with Watchdog Quad Voltage Monitor with Adjustable Reset Power Supply Margining Controller Power Supply Tracker with Input Monitors Power Supply Sequencing/Tracking Controller COMMENTS 16 User Selectable Combinations, 1.5% Threshold Accuracy 16 User Selectable Combinations, Adjustable Timers 5%, 2.5%, 10% and 12.5% Selectable Supply Tolerances Single or Dual, Symmetric/Asymmetric High and Low Margining Includes Three (LTC2921) or Five (LTC2922) Remote Sense Switches Controls Two Supplies Without FETs, MSOP-10 and DFN-12 Packages
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
U
Electronic Circuit Breaker Monitors Slave Output
Q1 Si4412ADY VIN 0.1F 10 CGATE 0.1F SUPPLY MONITOR VCC ON RONA 100k REMOTE VIN 10k STATUS VIN SD2 10k
2.5V SLAVE2
MASTER
RONB 154k
GATE
RAMP PGI SD1 FB1
RST 3.3V RUN/SS IN DC/DC FB = 1.235VOUT RFA1 RFB1 35.7k 16.5k
1.8V SLAVE1
1.8V SLAVE1
3.3V
IN RUN/SS DC/DC FB = 0.8V OUT RFA2 41.2k RFB2 88.7k
RTB1 16.5k
3.3V
FAULT RAMPBUF TRACK1 TRACK2 TRACK3 RTA3 100k
FB2 LTC2925
IN RUN/SS DC/DC FB = 0.8V OUT RFA2 RFB2 41.2k 88.7k 3.3V
2.5V SLAVE2
RUN/SS IN DC/DC FB = 0.8V OUT
RTA1 13k
1.5V SLAVE3
RTB2 88.7k RTB3 86.6k
SD3 FB3
RTA2 41.2k
RUN/SS IN DC/DC FB = 0.8V OUT RFA3 RFB3 100k 86.6k
0.015 1.5V SLAVE3
2925 TA03a
SENSEP SENSEN GND SCTMR SDTMR PGTMR CPGTMR 0.82F
2925 TA03b
CSCTMR 0.41F
CSDTMR 0.082F
2925f LT/TP 0404 1K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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