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Final Electrical Specifications
LTC2920-1/LTC2920-2 Single/Dual Power Supply Margining Controller
March 2003
FEATURES
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DESCRIPTIO
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Margin Voltage Precision <0.4% 400:1 Current Programming Range Symmetric/Asymmetric High and Low Voltage Margining Single Control Pin per Supply--High, Float, Low Single Current Setting Resistor per Supply Wide VCC Compliance 2.2V < VCC < 6V Wide Output Compliance 0.6V < VMARGIN < (VCC - 0.6V) Single in 5-Pin ThinSOTTM (LTC2920-1) Dual in 8-Pin MSOP (LTC2920-2)
The LTC(R)2920 allows power supplies and power supply module output voltages to be precisely adjusted both up and down for automated PCB testing. The power supply output voltage is changed by sourcing or sinking current into the feedback node or voltage adjust pin of the power supply. This allows a system to test the correct operation of electrical components at the upper and/or lower power supply voltage limits specified for a given design (Power Supply "Margining"). The LTC2920 uses a single resistor to set the voltage margining current. The margining current is adjustable over a 400:1 range. Precision margin currents can be supplied to within 0.6V of ground or VCC. The LTC2920-1 is a single margining controller. The LTC2920-2 has two independently controllable margining channels. Each channel has its own control pin and current setting resistor. The LTC2920-2 can be used to symmetrically margin two power supplies, or asymmetrically margin a single power supply. Both the LTC2920-1 and LTC2920-2 feature a trimmed onboard voltage reference. Typical power supply margining accuracy is better than 0.4%.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
APPLICATIO S
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Automated PCB Production Testing Automated Preventative Maintenance Testing DC/DC Converter Module Margining
TYPICAL APPLICATIO
1 +VIN +VOUT 5
3.3V Quarter Brick with 5% Voltage Margining
3.3V AT 4A 150 2F IN1 IM1 LTC2920-1 RS1 GND 6 7
2920-1/2 TA01
5% SYSTEM CONTROLLER THREE-STATE RSET1 10k 1% -5% LOGIC HI IN1 LOGIC FLOAT LOGIC LOW 1ms/DIV
+
33F
POWER ONE I5S013ZE-A
+
0.1F NOM +VOUT
TRIM
-VOUT -48V 2 -VIN
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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2920-1/2 TA01a
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LTC2920-1/LTC2920-2
ABSOLUTE
(Note 1)
AXI U RATI GS
Operating Temperature Range LTC2920-1C/LTC2920-2C ....................... 0C to 70C LTC2920-1I/LTC2920-2I .................... - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
Supply Voltage (VCC) ................................- 0.3V to 6.5V Input Voltages (IN1, IN2, RS1, RS2)................. - 0.3V to (VCC + 0.3V) Output Voltages (IM1, IM2) ........... - 0.3V to (VCC + 0.3V)
PACKAGE/ORDER I FOR ATIO
TOP VIEW VCC 1 GND 2 IM1 3 4 RS1 5 IN1
ORDER PART NUMBER LTC2920-1CS5 LTC2920-1IS5 S5 PART MARKING LTD7 LTD8
TOP VIEW RS2 IN2 IN1 RS1 1 2 3 4 8 7 6 5 VCC IM2 GND IM1
S5 PACKAGE 5-LEAD PLASTIC SOT-23
TJMAX = 125C, JA = 250C/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.
SYMBOL Supplies VCC ICC(SOURCE) ICC(Q) TSDJ RSETL RSETH CRS Supply Operating Range Supply Current while Sourcing Max IIM Quiescent Supply Current Thermal Shutdown Temperature Current Setting Resistor Low Range Current Setting Resistor High Range Maximum External Capacitance on RS1, RS2 Low Range IMARGIN Current-- Sourcing or Sinking High Range IMARGIN Current-- Sourcing or Sinking IM1, IM2 Output Voltage Compliance Total Capacitance to VCC and GND RSET1, RSET2 Tied to GND, IN1, IN2 > VIH or IN1, IN2 < VIL, (Note 4) RSET1, RSET2 Tied to VCC, IN1, IN2 > VIH or IN1, IN2 < VIL, (Note 4) (Note 3)
q q q
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS (Note 2) RSET1 = RSET2 = 15k, IN1 = IN2 < VIL RSET1 = RSET2 = 200k, IN1 = IN2 VIL (Note 5)
q q q q q
Current Setting RS1, RS2 6 15 200 200 20 k k pF
Current Margining Outputs IM1, IM2 IIMLOW IIMHIGH VM 5 0.15 0.55 167 2 VCC - 0.55 A mA V
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ORDER PART NUMBER LTC2920-2CMS8 LTC2920-2IMS8 MS8 PART MARKING LTB6 LTA4
MS8 PACKAGE 8-LEAD PLASTIC MSOP TJMAX = 125C, JA = 200C/W
MIN 2.3
TYP
MAX 6 6
UNITS V mA mA C
0.23 145
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LTC2920-1/LTC2920-2
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C.
SYMBOL IIMACCURACY PARAMETER Low Range Current Accuracy CONDITIONS 100A IM 167A, (Note 6) C-Grade I-Grade 30A IM < 100A, (Note 6) C-Grade I-Grade 5A IM < 30A, (Note 6) C-Grade I-Grade High Range Current Accuracy 1.5mA IM 2mA, (Note 7) C-GradeC I-Grade 600A IM 1.5mA, (Note 7) C-Grade I-Grade 150A IM 600A, (Note 7) C-Grade I-Grade IOZ CIM IM1, IM2 Leakage Current Equivalent Capacitance At IM1, IM2 VIN = VOFF, (Note 5) VIN = VIL, High Range, (Note 5) VIN = VIL, Low Range, (Note 5) VCC < 2.5V VCC 2.5V
q q q q q q q q q q q q q q q q q
ELECTRICAL CHARACTERISTICS
MIN
TYP 3 3 5 5 5 5 3 3 5 5 5 5 10 2 30
MAX 7.5 13 11 15 20 25 7.5 11 11 15 15 20 100
UNITS % % % % % % % % % % % % nA pF nF pF V V
Control Inputs IN1, IN2 VIH VIL VOFF VOZ RIN IFLT Control Voltage for IM Current Sinking Control Voltage for IM Current Sourcing Control Voltage for IM Current Off Control Voltage when Left Floating IN1, IN2 Input Resistance Maximum Allowed Leakage at IN1, IN2 for IM Current Off IM1, IM2 Turn-On Time IM1, IM2 Turn-Off Time IM1 Rise Time IM1 Fall Time VIN Transitions from VOFF to VIH or VIL VIN Transitions from VIH or VIL to VOFF
IM 5% to 95%, (Note 5) IM 95% to 5%, (Note 5) q q
2.1 2.4 0.6 1.1 1.2 5 -10 12 20 10 1.4
V V V k A
Switching Characteristics VIN(DELAYON) VIN(DELAYOFF) IM(ON) IM(OFF)
q q
15 15 5 0.3
100 100
s s s s
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: VCC must always be above the maximum of IM1 and IM2 less 0.2V. See Preventing Potential Power Supply Overvoltages in the Applications Information section. Note 3: VM compliance is the voltage range within which IM1 and IM2 are guaranteed to be sourcing or sinking current. IM accuracy will vary within this range.
Note 4: Consult LTC Marketing for parts specified with wider IM current limits. Note 5: Determined by design, not production tested. Note 6: 1 - (IM - RS) * 100%; VCC 4V: 0.58 VM (VCC - 1.1); VCC > 4V: 0.58 VM (VCC - 1.4) Note 7: 1 - (IM * RS / 30) * 100%; 0.79 VM (VCC - 0.6)
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LTC2920-1/LTC2920-2
PI FU CTIO S
VCC (Pin 1/Pin 8): Power Supply Input. All internal circuits are powered from this pin. VCC should be connected to a low noise power supply voltage between 2.2V and 6V and should be bypassed with at least a 0.1F capacitor to the GND pin in close proximity to the LTC2920. Current sourced out of the IM pins comes from the VCC pin. Note that VCC must come up no later than the time the controlled power supply turns on or damage to the load may result. See Preventing Potential Power Supply Overvoltages in the Applications Information section for power sequencing considerations. In certain applications, it may be necessary to further isolate VCC by adding a resistor in series with its power source. See VCC Power Filtering in the Applications Information section. GND (Pin 2/Pin 6): Ground. All internal circuits are returned to the GND pin. Connect this ground pin to the ground of the power supply(s) being margined. Current sunk into the IM pins of the LTC2920 is returned to ground through this pin. RS1 (Pin 4/Pin 4): IM1 Current Set Input. The RS1 pin is used to set the margining current which is sourced or sunk from the IM1 pin. The RS1 pin must be connected to either VCC or ground with an external resistor RSET with a value between 6k and 200k. Connecting RSET to ground sets the current at the IM1 pin with a multiplier of 1. Connecting RSET to VCC sets the current at the IM1 pin with a multiplier of 30. If RSET is connected to ground, 1V will appear at the RS1 pin. If RSET is connected to VCC, (VCC - 1V) will appear at the RS1 pin. In either case, the current through RSET will be 1V/RSET.
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(S5 Package/MS8 Package)
IM1 (Pin 3/Pin 5): IM1 Current Output. This pin should be connected to the power supply feedback pin or voltage adjust pin. (See the Applications Information section for further details.) Current is either sourced out of or sunk into this pin. The direction of the current is controlled by the IN1 pin. The amount of current flowing into or out of the IM1 pin is controlled by the RS1 pin. IN1 (Pin 5/Pin 3): IM1 Control Pin. This pin is a 3-level input pin which controls the IM1 pin. If the IN1 pin is pulled above 2V, current is sunk into the IM1 pin. If the IN1 pin is pulled below 0.6V, current is sourced from the IM1 pin. If the IN1 pin is left floating, or held between 1.1V and 1.4V, the IM1 pin is a high impedance output. Internally, the IN1 pin is connected to a 1.2V voltage source by an internal ~10k resistor. The LTC2920 has an internal RC circuit to suppress noise entering from this pin. LTC2920-2 Only RS2 (NA/Pin 1): IM2 Current Set Input. Sets the current for IM2. See RS1. IM2 (NA/Pin 7): IM2 Current Output. This pin is the second margin current output for the LTC2920. See IM1. IN2 (NA/Pin 2): IM2 Control Pin. This pin controls the current at the IM2 pin. See IN1.
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LTC2920-1/LTC2920-2 TYPICAL PERFOR A CE CHARACTERISTICS
ICC vs IMARGIN High Range Sourcing Current
5.0 4.5 4.0 3.5
ICC (mA)
2 CHANNELS
ERROR (%)
ICC (A)
3.0 2.5 2.0 1.5 1.0 0.5 0 0 0.5 1.5 IMARGIN (mA) 1 2 2.5
2920-1/2 G01
1 CHANNEL
IMARGIN Error vs VMARGIN
5.0 4.5 4.0 3.5
ERROR (%)
VCC = 2.5V HIGH RANGE
ERROR (%)
ERROR (%)
3.0 2.5 2.0 1.5 1.0 0.5 0 0 0.5 1 1.5 VMARGIN (V)
IMARGIN Rise Time
HIGH RANGE SOURCE 100% VIN(DELAYON) ENDS 0% LOW RANGE RSET = 20k 100%
HIGH RANGE 1s/DIV
UW
(mA) 0.15 0.3 0.5 1 2 2 2.5
2920-1/2 G04
ICC vs IMARGIN Low Range Sourcing Current
1800 1600 2 CHANNELS 1400 1200 1000 800 600 400 200 0 0 20 40 60 80 100 120 140 160 180 IMARGIN (A)
2920-1/2 G02
IMARGIN Error vs VMARGIN
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 VMARGIN (V) 4 4.5 5 VCC = 5V HIGH RANGE (mA) 0.15 0.3 0.5 1 2
1 CHANNEL
2920-1/2 G03
IMARGIN Error vs VMARGIN
6 5 4 3 2 1 VCC = 5V LOW RANGE 0 0 0.5 1 1.5 2 2.5 3 3.5 VMARGIN (V) 4 4.5 5 0 50 100 166.7 (A) 5 20 4 3 2 1 6 5
IMARGIN Error vs VMARGIN
(A) 5 20 50 100 166.7
VCC = 2.5V LOW RANGE 0 0.5 1 1.5 VMARGIN (V) 2 2.5
2920-1/2 G06
2920-1/2 G05
IMARGIN Fall Time
SOURCE VIN(DELAYOFF) ENDS LOW RANGE HIGH RANGE 0% LOW RANGE HIGH RANGE 100% SINK
2920-1/2 G07
RSET = 20k
100% SINK 100ns/DIV
2920-1/2 G08
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LTC2920-1/LTC2920-2
FU CTIO AL BLOCK DIAGRA
UVLO
THERMAL SHUTDOWN
IN1
INPUT DETECTION CURRENT SETTING VCC LOW RANGE
CONNECT TO VCC FOR HIGH RANGE OR TO GND FOR LOW RANGE
RSET1
RS1
IRNG
RANGE DETECTION
IN2 RS2
LTC2920-2 ONLY
APPLICATIO S I FOR ATIO
OVERVIEW POWER SUPPLY VOLTAGE MARGINING
In high reliability PCB manufacturing and test, it is desirable to test the correct operation of electrical components at the upper and/or lower power supply voltage limits allowed for a given design (known as "power supply margining"). Doing so can greatly improve the lifetime reliability of a system. The LTC2920 provides a means of power supply voltage margin testing which is: * Flexible * Easy to design * Requires very little PCB board space Symmetric/Asymmetric Power Supply Margining Any one LTC2920 channel requires only a single external resistor to symmetrically margin both above and below the nominal power supply voltage. The LTC2920-2 can be used to symmetrically margin two different power supplies. In cases where the design calls for margining one
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VOLTAGE REFERENCES SOURCE OFF SINK IPROGRAM OUTPUT CONTROL HIGH RANGE VMOK IM1 VM COMPLIANCE IM2
2920-1/2 BD
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voltage above the nominal power supply voltage and a different voltage below the nominal, the LTC2920-2 can be used. One channel is used for margining above the nominal power supply voltage, and the other channel is used to margin below the nominal voltage. VOLTAGE MARGINING POWER SUPPLIES USING A FEEDBACK PIN One common power supply architecture supported by the LTC2920 is a power supply with a feedback pin and two feedback resistors. Even complicated switching power supplies can be typically modeled as a simple amplifier with a reference voltage and a two resistor feedback network. (Figure 1.)
RF IFB
RG
-
VPSOUT
+
VREF
+ -
2920-1/2 F01
Figure 1
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LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
VPSOUT = VREF * [1+ (RF/RG)]
Knowing the value of the resistors RF and RG, and the voltage of VREF, VPSOUT can be calculated by: Since the op amp keeps its inverting terminal equal to the noninverting terminal, the voltage at the inverting terminal between RF and RG is VREF. Knowing the current flowing in the feedback resistor network, VPSOUT can be also calculated by: VPSOUT = VREF + (IFB * RF) This is the voltage on one side of RF, plus the voltage across RF. This equation is helpful in understanding how the LTC2920 changes the power supply output voltage. Figure 2 shows the simplified model with the LTC2920 added. Again in this circuit, the op amp will keep the voltage at its inverting input at VREF. If we add or subtract current at this node, the delta current will always be added or subtracted from IFB, and never IRG. ("IMARGIN" is used rather than a signed IMARGIN value to emphasize the fact that current is added or subtracted at the feedback pin.) Because of this, the voltage across RF will be: VRF = (IFBNOM IMARGIN) * RF or VRF = (IFBNOM * RF) (IMARGIN * RF) and finally VPSOUT = VREF + (IFBNOM * RF) (IMARGIN * RF) Note that the delta voltage VMARGIN depends only on IMARGIN and RF, not RG or VREF.
IMARGIN IM LTC2920 RS RSET RG IRG IFB RF
-
VPSOUT
+
VREF
+ -
2920-1/2 F02
Figure 2
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POWER SUPPLY MODULE VOLTAGE MARGINING Another method of accomplishing voltage margining is useful for power supply modules with voltage adjust pins. Typically, the power supply manufacturer will design the power supply to be adjusted up or down, using external resistors connected to the trim pin. The values of these resistors are usually calculated by the design engineer using two different equations supplied by the manufacturer. There is usually one equation for trimming the voltage up, and another equation for trimming the voltage down. In most cases, the power supply module is treated like a "black box" and very little information is given on how the trimming is accomplished from an internal circuit standpoint. Such power supply modules can be margined by calculating the two resistors, and alternately connecting each to VCC or ground with analog switches or relays. Figure 3 shows how the LTC2920 can be used in these applications. Using the LTC2920 for these applications can save a significant amount of PCB real estate and cost.
POWER MODULE SENSE + VIN+ VO+ TRIM VIN- VO- SENSE -
2920-1/2 F03
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RSYSTEM IMARGIN
VPSOUT LTC2920 IM VO- RS RSET
Figure 3
POWER SUPPLY MODULE DESIGN CONSIDERATIONS There are usually practical limits to VO+. For instance, VO+ usually has upper and lower voltage limits specified by the power module manufacturer. A common value is 10% above and 20% below the rated output voltage of the power supply module. This limit includes VMARGIN plus any voltage drop across RSYSTEM. See the manufacturer's power supply module specifications for details.
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LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
SELECTING THE RSET RESISTOR
Selecting RSET with an Existing Power Supply Containing a Feedback Pin and Two Feedback Resistors Calculating the value of the current setting resistor, RSET, for a power supply with a feedback pin is straight forward. When the LTC2920 is being added to an existing power supply design, the power supply feedback resistors RF and RG have already been selected. By knowing RF, the power supply output voltage, VPSOUT, and the amount to margin, %change, RSET can be calculated.
IMARGIN IM LTC2920 RS RSET RG IFB RF
-
VPSOUT
+
VREF
+ -
2920-1/2 F04
Figure 4
First, the margining voltage VPSOUT can be calculated by knowing the percentage of the power supply voltage VPSOUT change we desire. VPSOUT = %Change * VPSOUT Example: If a 3.3V power supply is to be margined by 5%, then: VPSOUT = 0.05 * 3.3V = 0.165V
IMARGIN = 16.5A IM LTC2920 RS RSET = 60.6k RG = 5.76k IFB = 210A RF = 10k
-
VPSOUT = 3.3V
+
VREF = 1.2V
+ -
2920-1/2 F05
Figure 5
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Since VPSOUT will appear on RF as noted in the Overview section, our margin current IMARGIN can be calculated by: IMARGIN = VPSOUT/RF Example: If VPSOUT = 0.165V and RF = 10k: IMARGIN = 0.165/10k = 16.5A If IMARGIN is between 5A and 167A, use the LTC2920's low current range. RSET is then calculated by: RSET = 1V/IMARGIN = 1V/16.5A = 60.6k In this case, RSET would be connected between the RS pin and ground. If IMARGIN is between 150A and 2mA, use the LTC2920's high current range. RSET is then calculated by: RSET = 1V/(IMARGIN/30) or simply: RSET = 30V/IMARGIN
VCC RSET = 90k IMARGIN = 330A IM LTC2920 RS RG = 286 IFB = 4.2mA RF = 500
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VPSOUT = 3.3V
+
VREF = 1.2V
+ -
2920-1/2 F06
Figure 6
Example: If the value of the feedback resistor RF is 500 in the example above then: VPSOUT = 0.05 * 3.3V = 0.165V IMARGIN = 0.165V/500 = 330A RSET = 30V/IMARGIN = 30V/330A = 90.1k In this case, RSET would be connected between the RS pin and VCC. If IMARGIN is less than 5A, or greater than 2mA, it will be necessary to adjust both power supply feedback resistors RF and RG. Again, this is usually a simple process. It is easy to calculate the magnitude of the change by dividing the IMARGIN current calculated above by the desired new
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LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
IMARGIN current. Select a new IMARGIN current that is within one of the two LTC2920's IMARGIN ranges, then calculate the scaling factor: IFACTOR = IMARGIN(OLD)/IMARGIN(NEW) The new feedback resistors would then be: RF(NEW) = RF(OLD) * IFACTOR RG(NEW) = RG(OLD) * IFACTOR And RSET can then be calculated as descibed above. WARNING In some cases, adjusting the feedback resistors on a switching supply might require recompensating the power supply. Please refer to the applications information supplied with the power supply for further information.
POWER MODULE SENSE + VIN+ VO+ TRIM VIN- VO- SENSE
-
2920-1/2 F07
TRIM DOWN RESISTANCE ()
VPSOUT IMARGIN IM VO- LTC2920 RS RSET
Figure 7. Using a Power Module Trim Pin for Voltage Margining
Selecting the RSET Resistor Using Voltage Trim Pins with `Brick' Type Power Supply Modules `Brick' power supply modules often have a trim pin which can be used for voltage margining. Figure 7 shows a typical connection using the LTC2920 for voltage margining a power supply module. The amount of current necessary to adjust the output voltage of the power supply module is not normally given directly by the manufacturer. However, by using information that is supplied by the manufacturer, a measurement can be made to determine a simple equation that is useful for power supply module voltage margining. Typically, the manufacturer will supply two different equations for selecting trim resistors: one for trimming the output voltage up and a different one for trimming the output voltage down. Trim resistors are nominally placed
TRIM CURRENT (A)
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between the trim pin and the power supply positive voltage output or the trim pin and the negative power supply output (ground). The polarity of the voltage trim and trim resistor configuration are chosen by the manufacturer. The equations describing the resistor values versus the desired output voltage changes are typically not linear. Fortunately, the relationship between trim pin current and output voltage change is typically linear. The current trim equation is usually the same (in magnitude) for changing the output voltage up or down. Once the equation for trim current is determined, it is much easier to use than trim resistors. To illustrate this, Figure 8 shows a typical resistor trim down curve for a power module. Figure 9 shows a typical current trim down curve for the same power module.
1M 100k 10k 1k 100 10 1 0 0.1 0.2 0.3 TRIM VOLTAGE (V) 0.4 0.5
2920-1/2 F08
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Figure 8. Typical Trim Voltage vs Trim Resistor Curve
300 250 200 150 100 50 0 0 0.1 0.2 0.3 TRIM VOLTAGE (V) 0.4 0.5
2920-1/2 F09
Figure 9. Typical Trim Voltage vs Trim Current Curve
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LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
Even though the manufacturer does not directly supply the equation for the trim current, a simple measurement can be made to calculate an equation for VTRIM as a function of ITRIM. To do this, select the trim resistor configuration which places the trim resistor between the trim pin and ground (see Figure 10). With the trim resistor connected to ground, note the direction of the power module output voltage change. This is the direction that the power module output voltage will change when the LTC2920 IN control pin is HIGH, above VIH. Remember that the direction of the voltage trim for this configuration can vary among power modules, even among power modules from the same manufacturer. Calculate a resistor value from the manufacturer's equation, or select it from a chart (if a chart is supplied by the manufacturer). Pick a value near the middle of the trim resistor range. Obtain and measure the selected resistor with an ohmmeter or use a precision 0.1% resistor. Knowing the correct value of this resistance is critical to obtaining good results. Make provisions to connect and disconnect this test resistor between the trim pin and the power supply module's negative output pin. (Figure 10.) Carefully follow all other manufacturer's application notes regarding power supply input voltage, minimum and maximum output voltages, sense pin connections (if any), minimum and maximum current loads, etc. Failure to do so may permanently damage the power supply module! Apply the specified input voltage to the power supply module. Measure the power supply output voltage VPS and the VT voltages before and after connecting the trim resistor. Subtract the untrimmed (VPSNOM) and trimmed (VPSTRIM) power supply output voltages to obtain the trim voltage (VDELTA): VDELTA = VPSNOM - VPSTRIM and the trim current: ITRIM = VTRIM/RTRIM Calculate the linear current trim constant KTRIM: KTRIM = VDELTA/ITRIM
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For any desired VMARGIN: ITRIM = VMARGIN/KTRIM RSET can now be calculated for the LTC2920. For 5A ITRIM 167A: RSET = 1V/ITRIM Connect RSET between the RS pin and the LTC2920 ground pin. For 167A < ITRIM 2mA: RSET = 1V/(ITRIM/30) Connect RSET between the RS pin and the LTC2920 VCC pin. If ITRIM falls outside of this range, the LTC2920 cannot be used for this application. The LTC2920 can source or sink current only when the voltage at the IM pin is between 0.6 and (VCC - 0.6) volts. In order to be sure that the LTC2920 will operate correctly in this application, ensure that the VT node will stay within these limits. To do this, calculate the effective output resistance of the power supply module's trim output pin, RVT (refer to Figure 10). Using the measurements taken above, the open circuit voltage is: VREF = VTNOM To calculate RVT, subtract the untrimmed VTNOM and trimmed VTTRIM voltages measured above: VTDELTA = VTNOM - VTTRIM The effective TRIM pin source resistance can then be calculated by: RVT = VTDELTA/ITRIM The voltage at the LTC2920 IMARGIN pin for any ITRIM can now calculated for both voltage margin directions. Refering to Figure 10: VTSINK = VREF - (RVT * ITRIM) VTSOURCE = VREF + (RVT * ITRIM) Note: be sure to use these equations to verify that VTSINK and VTSOURCE are within LTC2920 VM voltages specified in
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APPLICATIO S I FOR ATIO
the IMACCURACY specification. If VT does not fall within this range, the LTC2920 cannot be used for this application.
SENSE + VIN+ RVT VO+ TRIM VT RTRIM VO- SENSE -
2920-1/2 F10
VPS
VREF VIN-
+ -
ITRIM VO-
Figure 10. Power Module ITRIM Model
Accuracy of Power Supply Voltages when Margining The accuracy of margined power supply voltages depends on several factors. Figure 11 shows the magnitude of the errors discussed in detail below as a function of power supply margining percentage. In a typical feedback model (Figure 12), the delta voltage is a function of the margin current, IMARGIN, and the feedback resistor, RF. VMARGIN = IMARGIN * RF Errors in VMARGIN are directly proportional to errors in IMARGIN and errors in RF. A 5% error in IMARGIN will cause a 5% error in VMARGIN. In this example, a 3.3V power supply is margined by 2.5%, or 0.0825V to 3.3825V. With a 5% VMARGIN error, the actual margin voltage is 0.0866V and the actual power supply voltage is 3.3866V. The error in the expected voltage is then: Error = 1 - (3.3866/3.3825) * 100 = 0.12% Similarly, a 1% inaccuracy in the RSET resistor would cause only 0.024% error in the expected power supply margined voltage. In effect, IMARGIN errors caused by the RSET resistor or the LTC2920 are attenuated by the voltage margining percentage. The accuracy of the RF resistor introduces two errors in the margined supply voltage. The first is the error in VMARGIN (IMARGIN * RF). This error is similar in magnitude to the errors described above and is generally quite small (0.024%
POWER SUPPLY MARGINED VOLTAGE ERROR 1 - ACTUAL VOLTAGE/EXPECTED VOLTAGE * 100 (%)
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for this example). The second error is the power supply initial set point accuracy. In this example the RF resistor has a 1% accuracy error causing a 0.6% initial set point error in the power supply. Because the margined power supply voltage is the change in the voltage, VMARGIN, from the power supply initial set point voltage, this error shows up in the margined power supply voltage. When these two errors are combined, the error is: Error = 1 - (3.4043/3.3825) * 100 = 0.65% The error caused by a 1% inaccuracy in RG will be similar since the dominate error source is the power supply initial set point voltage. Errors caused by RF and RG can be a major contributor to voltage margin errors. Using 0.1% resistors for both RF and RG is often the best choice for improving both voltage margin accuracy and power supply initial accuracy.
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 2 3 4 5 6 POWER SUPPLY VOLTAGE MARGINING (%)
2920-1/2 F11
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1% FEEDBACK RESISTOR INACCURACY 1% RSET RESISTOR INACCURACY 5% LTC2920 IMARGIN INACCURACY
Figure 11. Sources of Power Supply Margined Voltage Errors
IMARGIN = 50A IM LTC2920 RS RSET = 20k RG = 944k IFB = 1.27mA RF = 1.65k
-
VPSOUT = 3.3V
+
VREF = 1.2V
+ -
2920-1/2 F12
Figure 12. Power Supply Voltage Margin Model
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11
LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
PREVENTING POTENTIAL POWER SUPPLY OVERVOLTAGES
Care must be take when selecting the power source for the LTC2920. If VCC on the LTC2920 is not powered, and the power supply being margined is on, undesired IM fault current can flow into the IM pin of the LTC2920. This can cause the margined power supply to create an overvoltage condition causing serious damage to power supply and its load. The best solution is to connect the LTC2920 to a power source that is guaranteed to be on when the power supply being margined is on. Often this is the input or output voltage of the power supply being margined. See the design guidelines below for the best solution for your application. Be sure to follow all other LTC2920 design specifications. At a minimum, the voltage at the VCC pin of the LTC2920 must be maintained above 0.2V below the highest voltage present at the IM1 and IM2 pins. This will keep the IM fault current below 5A. The voltage at the IM1 and IM2 pins is normally the voltage at the feedback node of the power supply. See the power supply manufacturer's data sheet for this voltage. PREVENTING IM FAULT CURRENT IN THE LTC2920-1 SINGLE Connecting VCC to the Power Supply VIN or VOUT of the Supply Being Margined Connecting the LTC2920-1 VCC to VIN or VOUT is the best choice and should be used when conditions permit. It requires no external components and provides the best protection from power supply overvoltage. If the power supply being margined has a VIN voltage that is within the LTC2920's VCC range, connect the LTC2920-1 VCC pin to the power supplies VIN (Figure 13). If the power supply being margined has a VOUT voltage that is within the LTC2920's VCC range, connect the LTC2920-1 VCC pin the power supplies VOUT (Figure 14). Make sure the power supply voltage is within the LTC2920's VCC specification when the power supply is being margined!
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VIN 2.3V TO 6V VCC VO VOUT VCC LTC2920-1 FB IM GND 0.1F
2920-1/2 F13
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Figure 13. Connect LTC2920-1 to VIN
VIN VIN VO VOUT VOUT 2.3V TO 6V VCC LTC2920-1 FB IM GND 0.1F
2920-1/2 F14
Figure 14. Connect LTC2920-1 to VOUT
Connecting VCC to Power Sources Other than the Supply Being Margined If it is not practical to power the LTC2920-1 from the VIN or VOUT of the power supply being margined, connect the VCC pin of the LTC2920-1 using a Schottky diode (Figure 15). This solution works with power supply feedback voltages of less than 1.5V and IMARGIN currents >30A. Be sure to account for the diode drop across all temperatures to ensure the LTC2920-1 VCC and VMARGIN specifications are met.
VPOWER BAT54C SCHOTTKY DIODE VIN VO VCC LTC2920-1 FB <1.5V IM GND 0.1F
VIN VOUT
2920-1/2 F15
Figure 15. Diode Connected VCC
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LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
PREVENTING IM FAULT CURRENT IN THE LTC2920-2 DUAL Connecting VCC to a Common VIN Connecting the LTC2920-2 VCC to VIN is the best choice and should be used when conditions permit. It requires no external components and provides the best protection from power supply overvoltage (Figure 16).
VIN VIN FB OUT
VIN FB
OUT
IM2
VCC
LTC2920-2 IM1 GND
2920-1/2 F16
Figure 16. Connecting VCC to VIN
Connecting VCC to a Diode OR'd VINs or VOUTs If the margined power supplies derive their VIN from different sources, or if a common VIN cannot supply power to the LTC2920-2, power the LTC2920-2 using a diode OR'd connection (Figure 17). Note that in this example, Power Supply 2 has only a 1.8V output. Power Supply 1 will supply the LTC2920-2 under normal operation conditions. If Power Supply 1 fails, or if it is sequenced up after
POWER SUPPLY 1 OUT FB VOUT1 3.3V
POWER SUPPLY 2 OUT FB VOUT2 1.8V
IM2
VCC
LTC2920-2 IM1 BAT54C GND
2920-1/2 F17
Figure 17. Dual Diode Connected VCC
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Power Supply 2, Power Supply 2 supplies enough voltage to keep the LTC2920 from sinking fault current into the IM1 and IM2 pins. The LTC2920-2 will not operate normally under these conditions but it will not cause overvoltage to occur. Connecting VCC to Power Sources Other than the Supplies Being Margined If it is not practical to power the LTC2920-2 from the VINs and/or VOUTs of the power supplies being margined, connect the VCC pin of the LTC2920-2 using a Schottky diode (Figure 18). This solution works with power supply feedback voltages less that 1.5V and IMARGIN currents >30A. Be sure to account for the diode drop across all temperatures to ensure the LTC2920-2 VCC and VMARGIN specifications are met. VCC Power Supply Filtering If the LTC2920 is both powered by and margins a power supply that is marginally stable, oscillations can occur. In these cases, it may be necessary to provide an additional filtering resistor between the LTC2920 and the power supply being margined (see Figure 19). The oscillation is most likely to occur when the LTC2920 is sourcing current from the IMARGING pin. The RBYP resistor in combination with the CBYP capacitor form a lowpass filter. The value of the filter resistor RBYP can be calculated by deciding how much voltage drop across the resistor the application can tolerate and how much current the LTC2920 will sink under worst-case conditions. In the LTC2920 low current range, a safe value for the LTC2920 ICC current is the maximum LTC2920 quiescent current plus 4 times the IMARGIN current. In the high current range, a safe value for the LTC2920 ICC current is the maximum LTC2920 quiescent current plus 1.2 times the IMARGIN current. Example: If the IMARGIN current is 100A, then: ICCMAX = IQ + (4 * IMARGIN) = 1mA + (4 * 100A ) = 1.4mA
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LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
In this example, the power supply voltage is 3.3V. Dropping 0.5V across RBYP will provide a VCC at the LTC2920 of 2.8V. This is well above the LTC2920's minimum VCC voltage. The value of the RBYP resistor can then be calculated by: RBYP = VRB/ICCMAX = 0.5V/1.4mA = 360 With CBYP = 0.1F, this will provide a pole at 2870Hz. If additional filtering is necessary, the value of CBYP can be increased. In this example, if CBYP is increased from 0.1F to 1F, the pole would now be at 287Hz.
POWER SUPPLY 1 OUT FB
POWER SUPPLY 2 OUT FB
IM2
VCC BAT54C SCHOTTKY DIODE
POWER
LTC2920-2 IM1
2920-1/2 F18
Figure 18. Diode Connected to VCC
Controlling IMARGIN Turn On and Turn Off Times Designers of power supply voltage margining circuits often need to ensure that power supply voltages do not overshoot or undershoot (the desired margining voltage) when the margining current is enabled or disabled. The LTC2920 IMARGIN current sourced or sinked at the IM pin(s) is reasonably well behaved (see the Typical PerforRBYP 360 CBYP 0.1F VPSOUT = 3.3V VCC LTC2920 RS RSET IM GND IMARGIN = 100A
RF
-
RG
+ + -
VREF = 1.2V
2920-1/2 F19
Figure 19. VCC Power Filtering
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mance Characteristics curves). The differences in speed between the various curves is caused by the relative impedance differences within the LTC2920. If slower turn on and turn off times are desired, a resistorcapacitor network can be used at the IM pin(s). Referring to Figure 20, Slowing Down VMARGIN, a capacitor (CS) and a resistor (RS) have been added to the power supply model described in previous applications sections. To choose RS, the voltage at the feedback pin of the power supply must be known. Refer to the power supply manufacturer's data sheet for this voltage. The voltage at the IM pin must be within specified limits of the LTC2920, including the voltage drop across RS. In the example below, the power supply feedback pin voltage is 1.21V, IMARGIN is 100A and VCC is 3.3V. To maintain LTC2920 current accuracy, the voltage at the IM pin must be between 0.58V and (VCC - 1) or 2.3V (in the low current range). A reasonable value for the voltage drop across RS is 0.5V. The value of RS is then: RS = IMARGIN/VRS = 0.5V/100A = 5k Assuming the desired RC time constant is 1ms, CS is calculated by: CS = TRC/RS = 1ms/5k = 0.2F Note: When CS and RS are used, an additional pole and a zero are added to the power supply feedback loop. It is beyond the scope of this data sheet to predict the behavior of all power supplies but, in general, as long as the smaller of the two feedback resistors is no larger than 2 * RS, the effect on the power supply stability should be minimal. The larger RS is with respect to the two feedback resistors, the less effect it will have.
3.3V VCC LTC2920 IM GND IMARGIN 5k RS 5k CS 0.2F 1.5k
2920-1/2 F20
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-
+ + -
VREF 1.21V
Figure 20. Slowing Down VMARGIN
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LTC2920-1/LTC2920-2
PACKAGE DESCRIPTIO
0.62 MAX
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF 0.09 - 0.20 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193
5.23 (.206) MIN
0.42 0.04 (.0165 .0015) TYP
RECOMMENDED SOLDER PAD LAYOUT DETAIL "A" 0 - 6 TYP 4.90 0.15 (1.93 .006) 3.00 0.102 (.118 .004) NOTE 4
GAUGE PLANE 0.53 0.015 (.021 .006) DETAIL "A" 0.18 (.077) SEATING PLANE 0.22 - 0.38 (.009 - .015) TYP 0.13 0.076 (.005 .003)
MSOP (MS8) 0802
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
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S5 Package 5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE 0.30 - 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 - 0.90 0.01 - 0.10 1.90 BSC
S5 TSOT-23 0302
MS8 Package 8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
0.889 0.127 (.035 .005)
3.2 - 3.45 (.126 - .136)
0.65 (.0256) BSC
3.00 0.102 (.118 .004) (NOTE 3)
8
7 65
0.52 (.206) REF
0.254 (.010)
1 1.10 (.043) MAX
23
4 0.86 (.034) REF
0.65 (.0256) BSC
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LTC2920-1/LTC2920-2
TYPICAL APPLICATIO S
12V Supply with 5% Margining
L1 10H D1 RB 1k CB 0.1F SYSTEM CONTROLLER THREE-STATE 4 RSET 188.3k GND VOUT 12V 300mA MARGIN 5%
VIN 5V C1 2.2F
5 VIN LT1930 4
SHDN
SHDN GND 2
C1: TAIYO YUDEN X5R LMK212BJ225MG C2: TAIYO YUDEN X5R EMK316BJ475ML D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CR43-100 *OPTIONAL
COSC 68pF
COSC CSS 0.1F RUN/SS CC 150pF RC 10k SGND 100pF VOSENSE SENSE - ITH
VIN TG SW LTC1435A M1 Si4412DY
51pF
INTVCC BOOST
BG PGND SENSE +
1000pF
RELATED PARTS
PART NUMBER LTC1426 LTC1427-50 LTC1428-50 LTC1663 LTC2900-1/LTC2900-2 LTC2901-1/LTC2901-2 LTC2902-1/LTC2902-2 DESCRIPTION Micropower Dual 6-Bit PWM DAC SMBus Micopower 10-Bit IOUT DAC in SO-8 Micropower 8-Bit IOUT DAC in SO-8 Micropower 10-Bit VOUT DAC Quad Voltage Monitors in MSOP Quad Voltage Monitors with Watchdog Quad Voltage Monitors with RST Disable COMMENTS 10A/50A Sourcing, Pulse Mode or SPI Input Pulse Mode or Pushbutton Input 50A Sourcing, -15V to (VCC - 1.3V) Compliance 50A Sinking, Pulse Mode or SPI Input 2-Wire Interface, Rail-to-Rail Output, SOT-23 or MSOP 16 User-Selectable Combinations, 1.5% Threshold Accuracy 16 User-Selectable Combinations, Adjustable Timers 16 Selectable Combinations, RST Disable for Margining
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LTC1329-10/LTC1329-50 Micropower 8-Bit IOUT DAC in SO-8
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507 q www.linear.com
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1 SW 3
R1 113k
VCC C3* 10pF LTC2920-1 C2 4.7F 3 IM1 GND 2 IIN1 RS1
1
5
FB
R2 13.3k
RS 113k
CS 0.01F
2920-1/2 TA02
3.3V Supply with 0.165V (5%) Voltage Margining
VIN 4.5V TO 28V
+
CIN 22F 35V x2
DB CMDSH-3 CB 0.1F
RSENSE 0.025
VCC RB 500 CBYP 0.1F
L1 4.7H
VOUT 3.3V 4.5A
R1 3.57k
+
+
4.7F M2 Si4412DY D1 MBRS140T3 R2 2k
COUT 100F 6.3V x2
VCC LTC2920 IN SYSTEM CONTROLLER THREE-STATE
IM GND
RS 21.5k GND
2920-1/2 TA03
LT/TP 0303 1.5K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2003

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