View LTC3803IS6-3_4343254.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

LTC3803-3 Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT
FEATURES

DESCRIPTIO
VIN and VOUT Limited Only by External Components Adjustable Slope Compensation Internal Soft-Start -40C to 125C Operating Temperature Range Constant Frequency 300kHz Operation 1.5% Reference Accuracy Current Mode Operation for Excellent Line and Load Transient Response No Minimum Load Requirement Low Quiescent Current: 240A Low Profile (1mm) SOT-23 Package
The LTC(R)3803-3 is a constant frequency current mode flyback controller optimized for driving 6V-rated N-channel MOSFETs in high input voltage applications. Constant frequency operation is maintained down to very light loads, resulting in less low frequency noise generation over a wide range of load currents. Slope compensation can be programmed with an external resistor. The LTC3803-3 provides 1.5% output voltage accuracy and consumes only 240A of quiescent current. Groundreferenced current sensing allows LTC3803-3-based converters to accept input supplies beyond the LTC3803-3's absolute maximum VCC. A micropower hysteretic start-up feature allows efficient operation at high input voltages. For simplicity, the LTC3803-3 can also be powered from a high VIN through a resistor, due to its internal 9.4V shunt regulator. An internal undervoltage lockout shuts down the LTC3803-3 when the input voltage falls below 4.4V, guaranteeing at least 4.4V of gate drive to the external MOSFET. The LTC3803-3 is available in a low profile (1mm) 6-lead SOT-23 (ThinSOTTM) package.
APPLICATIO S

Telecom Power Supplies 42V and 12V Automotive Power Supplies Auxiliary/Housekeeping Power Supplies Power Over Ethernet Powered Devices
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
VIN 36V TO 72V
5V Output Nonisolated Telecom Housekeeping Power Supply
UPS840 T1 10k 1F 10V X5R 82k 470pF 1F 100V X5R
100
VOUT 5V 2A MAX EFFICIENCY (%)
95 90 85 80 75 70 65 60 55
* *
FDC2512
300F* 6.3V X5R
VCC ITH/RUN NGATE LTC3803-3 GND VFB 20k 105k
38033 TA01
4.7k 68m
SENSE
150pF 200V 220
50 250
T1: COOPER CTX02-15242 *THREE 100F UNITS IN PARALLEL
U
Efficiency vs Load Current
VOUT = 5V VIN = 36V VIN = 48V VIN = 60V VIN = 72V 500 750 1000 1250 1500 1750 2000 LOAD CURRENT (mA)
38033 TA02
U
U
38033fa
1
LTC3803-3
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW ITH/RUN 1 GND 2 VFB 3 6 NGATE 5 VCC 4 SENSE
VCC to GND Low Impedance Source .......................... - 0.3V to 8V Current Fed ...................................... 25mA into VCC* NGATE Voltage ......................................... - 0.3V to VCC VFB, ITH/RUN Voltages ..............................- 0.3V to 3.5V SENSE Voltage ........................................... - 0.3V to 1V NGATE Peak Output Current (<10s) ........................ 1A Operating Temperature Range (Note 2) LTC3803E-3 ....................................... - 40C to 85C LTC3803I-3 ...................................... - 40C to 125C Junction Temperature (Note 3) ............................ 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
*LTC3803-3 internal clamp circuit self regulates VCC voltage to 9.5V.
S6 PACKAGE 6-LEAD PLASTIC TSOT-23 TJMAX = 125C, JA = 230C/W
ORDER PART NUMBER LTC3803ES6-3 LTC3803IS6-3
S6 PART MARKING LTCJS LTCJT
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL VTURNON VTURNOFF VHYST VCLAMP1mA VCLAMP25mA VMARGIN ICC PARAMETER VCC Turn On Voltage VCC Turn Off Voltage VCC Hysteresis (VTURNON - VTURN0FF) VCC Shunt Regulator Voltage VCC Shunt Regulator Voltage VCLAMP1mA - VTURNON Margin Input DC Supply Current Normal Operation Start-Up Shutdown Threshold (at ITH/RUN)
The indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 8V, unless otherwise noted. (Note 2)
CONDITIONS
MIN 7.6 4.6 4.4 1 8.3 8.4 0.05

TYP 8.7 5.7 5.7 3 9.4 9.5 0.6 240 40
MAX 9.2 7 7 10.3 10.5
UNITS V V V V V V V
LTC3803E-3 LTC3803I-3 ICC = 1mA, VITH/RUN = 0V ICC = 25mA, VITH/RUN = 0V (Note 4) VITH/RUN = 1.3V VCC = VTURNON - 100mV VCC = VTURNON - 100mV LTC3803E-3 LTC3803I-3 VITH/RUN = 0V 0C TA 85C LTC3803E-3: -40C TA 85C LTC3803I-3: -40C TA 125C RSL = 0 (Note 6) LTC3803E-3 LTC3803I-3 ITH/RUN Pin Load = 5A (Note 5) VTURNOFF < VCC < VCLAMP (Note 5) (Note 5) VITH/RUN = 1.3V
350 90 0.45 0.45 0.4 0.812 0.812 0.820 115 115 500 50 330
VITHSHDN

0.15 0.10 0.2 0.788 0.780 0.780 90 85 200
0.28 0.28 0.3 0.800 0.800 0.800 100 100 333 0.05 10
IITHSTART VFB
Start-Up Current Source Regulated Feedback Voltage (Note 5)

VIMAX
Peak Current Sense Voltage
gm VO(LINE) IFB fOSC
Error Amplifier Transconductance Output Voltage Line Regulation VFB Input Current Oscillator Frequency
270
300
38033fa
2
U
W
U
U
WW
W
A A V V A V V V V V A/V mV/V nA kHz
LTC3803-3
ELECTRICAL CHARACTERISTICS
SYMBOL DCON(MIN) DCON(MAX) tRISE tFALL ISLMAX tSFST PARAMETER Minimum Switch On Duty Cycle Maximum Switch On Duty Cycle Gate Drive Rise Time Gate Drive Fall Time Peak Slope Compensation Output Current Soft-Start Time
The indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 8V, unless otherwise noted. (Note 2)
CONDITIONS VITH/RUN = 1.3V, VFB = 0.8V VITH/RUN = 1.3V, VFB = 0.8V CLOAD = 3000pF CLOAD = 3000pF (Note 7) (Note 7) 70 MIN TYP 8 80 40 40 5 1.4 MAX 9.6 90 UNITS % % ns ns A ms
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3803E-3 is guaranteed to meet specifications from 0C to 85C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3803I-3 is guaranteed to meet performance specifications over the -40C to 125C operating temperature range. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * 230C/W).
Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: The LTC3803-3 is tested in a feedback loop that servos VFB to the output of the error amplifier while maintaing ITH/RUN at the midpoint of the current limit range. Note 6: Peak current sense voltage is reduced dependent on duty cycle and an optional external resistor in series with the SENSE pin (RSL). For details, refer to the programmable slope compensation feature in the Applications Information section. Note 7: Guaranteed by design.
38033fa
3
LTC3803-3 TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage vs Temperature
812 808
801.0
VCC = 8V
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
804 800 796 792 788 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G01
VFB VOLTAGE (mV)
6 6.5 8 7.5 8.5 VCC SUPPLY VOLTAGE (V) 7 9 9.5
Oscillator Frequency vs Temperature
330 VCC = 8V OSCILLATOR FREQUENCY (kHz) 330
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
320 310 300 290 280 270 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G04
VCC Undervoltage Lockout Thresholds vs Temperature
10.0 9.5
VCC UNDERVOLTAGE LOCKOUT (V)
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0
VTURNON
9.7
VCC (V)
9.6 9.5 9.4 9.3
ICC = 25mA
SUPPLY CURRENT (A)
VTURNOFF
4.5 -50 -30 -10 10 30 50 80 90 110 130 TEMPERATURE (C)
38033 G07
4
UW
TA = 25C unless otherwise noted. Reference Voltage vs VCC Shunt Regulator Current
804 803 802 801 800 799 798 797 796 0 5 10 15 ICC (mA) 20 25
38033 G03
Reference Voltage vs Supply Voltage
800.8 800.6 800.4 800.2 800.0 799.8 799.6 799.4 799.2 799.0 VCC VCLAMP1mA
38033 F02
Oscillator Frequency vs Supply Voltage
330 320 310 300 290 280 270 6 6.5 8.5 7 7.5 8 VCC SUPPLY VOLTAGE (V) 9
Oscillator Frequency vs VCC Shunt Regulator Current
320 310 300 290 280 270
0
5
10
15 20 ICC (mA)
25
30
35
38033 G05
38033 G06
VCC Shunt Regulator Voltage vs Temperature
10.0 9.9 9.8
300 275 250 225 200 175 350 325
ICC Supply Current vs Temperature
VCC = 8V VITH/RUN = 1.3V
ICC = 1mA
9.2 9.1 9.0 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G08
150 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G09
38033fa
LTC3803-3 TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up ICC Supply Current vs Temperature
70 VCC = VTURNON - 0.1V
SHUTDOWN THRESHOLD (mV)
START-UP SUPPLY CURRENT (A)
60 50 40 30 20 10 0 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G10
ITH/RUN PIN CURRENT SOURCE (nA)
Peak Current Sense Voltage vs Temperature
120 115 VCC = 8V
SENSE PIN VOLTAGE (mV)
SOFT-START TIME (ms)
110 105 100 95 90 85 80 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G13
UW
TA = 25C unless otherwise noted. ITH/RUN Start-Up Current Source vs Temperature
700 600 500 400 300 200 100 0 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G12
ITH/RUN Shutdown Threshold vs Temperature
450 400 350 300 250 200 150 100 -50 -30 -10 10 30 50 70 90 110 130 TEMPERATURE (C)
38033 G11
VCC = VTURNON + 0.1V VITH/RUN = 0V
Soft-Start Time vs Temperature
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38033 G14
38033fa
5
LTC3803-3
PI FU CTIO S
ITH/RUN (Pin 1): This pin performs two functions. It serves as the error amplifier compensation point as well as the run/shutdown control input. Nominal voltage range is 0.7V to 1.9V. Forcing this pin below 0.28V causes the LTC3803-3 to shut down. In shutdown mode, the NGATE pin is held low. GND (Pin 2): Ground Pin. VFB (Pin 3): Receives the feedback voltage from an external resistive divider across the output. SENSE (Pin 4): This pin performs two functions. It monitors switch current by reading the voltage across an external current sense resistor to ground. It also injects a current ramp that develops slope compensation voltage across an optional external programming resistor. VCC (Pin 5): Supply Pin. Must be closely decoupled to GND (Pin 2). NGATE (Pin 6): Gate Drive for the External N-Channel MOSFET. This pin swings from 0V to VCC.
BLOCK DIAGRA
800mV REFERENCE
+
3 VFB
-
GND 2
20mV 1.2V
1
ITH/RUN
38033 BD
6
+
-
W
U
U
U
5 VCC
0.3A 0.28V VCC SHUNT REGULATOR
+ -
SHUTDOWN COMPARATOR VCC < VTURNON UNDERVOLTAGE LOCKOUT
SOFTSTART CLAMP CURRENT COMPARATOR R Q S
SHUTDOWN
ERROR AMPLIFIER
VCC SWITCHING LOGIC AND BLANKING CIRCUIT GATE DRIVER NGATE
6
300kHz OSCILLATOR
SLOPE COMP CURRENT RAMP
SENSE
4
38033fa
LTC3803-3
OPERATIO
The LTC3803-3 is a constant frequency current mode controller for flyback and DC/DC boost converter applications in a tiny ThinSOT package. The LTC3803-3 is designed so that none of its pins need to come in contact with the input or output voltages of the power supply circuit of which it is a part, allowing the conversion of voltages well beyond the LTC3803-3's absolute maximum ratings. Main Control Loop Due to space limitations, the basics of current mode DC/DC conversion will not be discussed here; instead, the reader is referred to the detailed treatment in Application Note 19, or in texts such as Abraham Pressman's Switching Power Supply Design. Please refer to the Block Diagram and the Typical Application on the front page of this data sheet. An external resistive voltage divider presents a fraction of the output voltage to the VFB pin. The divider must be designed so that when the output is at the desired voltage, the VFB pin voltage will equal the 800mV from the internal reference. If the load current increases, the output voltage will decrease slightly, causing the VFB pin voltage to fall below 800mV. The error amplifier responds by feeding current into the ITH/RUN pin. If the load current decreases, the VFB voltage will rise above 800mV and the error amplifier will sink current away from the ITH/RUN pin.
U
The voltage at the ITH/RUN pin commands the pulse-width modulator formed by the oscillator, current comparator and RS latch. Specifically, the voltage at the ITH/RUN pin sets the current comparator's trip threshold. The current comparator monitors the voltage across a current sense resistor in series with the source terminal of the external MOSFET. The LTC3803-3 turns on the external power MOSFET when the internal free-running 300kHz oscillator sets the RS latch. It turns off the MOSFET when the current comparator resets the latch or when 80% duty cycle is reached, whichever happens first. In this way, the peak current levels through the flyback transformer's primary and secondary are controlled by the ITH/RUN voltage. Since the ITH/RUN voltage is increased by the error amplifier whenever the output voltage is below nominal, and decreased whenever output voltage exceeds nominal, the voltage regulation loop is closed. For example, whenever the load current increases, output voltage will decrease slightly, and sensing this, the error amplifier raises the ITH/RUN voltage by sourcing current into the ITH/RUN pin, raising the current comparator threshold, thus increasing the peak currents through the transformer primary and secondary. This delivers more current to the load, bringing the output voltage back up. The ITH/RUN pin serves as the compensation point for the control loop. Typically, an external series RC network is connected from ITH/RUN to ground and is chosen for optimal response to load and line transients. The impedance of this RC network converts the output current of the error amplifier to the ITH/RUN voltage which sets the current comparator threshold and commands considerable influence over the dynamics of the voltage regulation loop.
38033fa
7
LTC3803-3
OPERATIO
Start-Up/Shutdown The LTC3803-3 has two shutdown mechanisms to disable and enable operation: an undervoltage lockout on the VCC supply pin voltage, and a forced shutdown whenever external circuitry drives the ITH/RUN pin low. The LTC3803-3 transitions into and out of shutdown according to the state diagram (Figure 1).
VCC < VTURNOFF (NOMINALLY 5.7V)
Figure 1. Start-Up/Shutdown State Diagram
The undervoltage lockout (UVLO) mechanism prevents the LTC3803-3 from trying to drive a MOSFET with insufficient VGS. The voltage at the VCC pin must exceed VTURNON (nominally 8.7V) at least momentarily to enable
8
U
LTC3803-3 operation. The VCC voltage is then allowed to fall to VTURNOFF (nominally 5.7V) before undervoltage lockout disables the LTC3803-3. This wide UVLO hysteresis range supports the use of a bias winding on the flyback transformer to power the LTC3803-3--see the section Powering the LTC3803-3. The ITH/RUN pin can be driven below VSHDN (nominally 0.28V) to force the LTC3803-3 into shutdown. An internal 0.3A current source always tries to pull this pin towards VCC. When the ITH/RUN pin voltage is allowed to exceed VSHDN, and VCC exceeds VTURNON, the LTC3803-3 begins to operate and an internal clamp immediately pulls the ITH/RUN pin up to about 0.7V. In operation, the ITH/RUN pin voltage will vary from roughly 0.7V to 1.9V to represent current comparator thresholds from zero to maximum. Internal Soft-Start
LTC3803-3 ENABLED
38033 F01
LTC3803-3 SHUT DOWN
> VITHSHDN V VITH/RUN < VITHSHDN ITH/RUN AND VCC > VTURNON (NOMINALLY 0.28V) (NOMINALLY 8.7V)
An internal soft-start feature is enabled whenever the LTC3803-3 comes out of shutdown. Specifically, the ITH/RUN voltage is clamped and is prevented from reaching maximum until roughly 1.4ms has passed. This allows the input and output currents of LTC3803-3based power supplies to rise in a smooth and controlled manner on start-up.
38033fa
LTC3803-3
OPERATIO
Powering the LTC3803-3 In the simplest case, the LTC3803-3 can be powered from a high voltage supply through a resistor. A built-in shunt regulator from the VCC pin to GND will draw as much current as needed through this resistor to regulate the VCC voltage to around 9.5V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is always active, even when the LTC3803-3 is in shutdown, since it serves the vital function of protecting the VCC pin from seeing too much voltage. For higher efficiency or for wide VIN range applications, flyback controllers are typically powered through a separate bias winding on the flyback transformer. The LTC3803-3 has a wide UVLO hysteresis (1V min) and small VCC supply current draw (<90A when VCC < VTURNON) that is needed to support such bootstrapped hysteretic start-up schemes.
U
The VCC pin must be bypassed to ground immediately adjacent to the IC pins with a minimum of a 1F ceramic or tantalum capacitor. Proper supply bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. Adjustable Slope Compensation The LTC3803-3 injects a 5A peak current ramp out through its SENSE pin which can be used for slope compensation in designs that require it. This current ramp is approximately linear and begins at zero current at 8% duty cycle, reaching peak current at 80% duty cycle. Additional details are provided in the Applications Information section.
38033fa
9
LTC3803-3
APPLICATIO S I FOR ATIO
Many LTC3803-3 application circuits can be derived from the topology shown in Figure 2. The LTC3803-3 itself imposes no limits on allowed power output, input voltage VIN or desired regulated output voltage VOUT; these are all determined by the ratings on the external power components. The key factors are: Q1's maximum drain-source voltage (BVDSS), on-resistance (RDS(ON)) and maximum drain current, T1's saturation flux level and winding insulation breakdown voltages, CIN and COUT's maximum working voltage, ESR, and maximum ripple current ratings, and D1 and RSENSE's power ratings.
T1 LBIAS D2 VIN
*
CIN
D1 VOUT LSEC COUT
R3 CVCC 1 CC 2
RSTART 5 VCC ITH/RUN NGATE LTC3803-3 GND VFB R1 3 R2 SENSE
*
LPRI
*
6 4 RSL RSENSE
38033 F02
Q1
Figure 2. Typical LTC3803-3 Application Circuit
SELECTING FEEDBACK RESISTOR DIVIDER VALUES The regulated output voltage is determined by the resistor divider across VOUT (R1 and R2 in Figure 2). The ratio of R2 to R1 needed to produce a desired VOUT can be calculated: R2 = VOUT - 0.8 V * R1 0.8 V
Choose resistance values for R1 and R2 to be as large as possible in order to minimize any efficiency loss due to the static current drawn from VOUT, but just small enough so that when VOUT is in regulation, the error caused by the nonzero input current to the VFB pin is less than 1%. A good rule of thumb is to choose R1 to be 80k or less.
10
U
TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of applying the LTC3803-3 successfully. In addition to the usual list of caveats dealing with high frequency power transformer design, the following should prove useful. Turns Ratios Due to the use of the external feedback resistor divider ratio to set output voltage, the user has relative freedom in selecting transformer turns ratio to suit a given application. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which yield more freedom in setting total turns and mutual inductance. Simple integer turns ratios also facilitate the use of "off-the-shelf" configurable transformers such as the Coiltronics VERSA-PACTM series in applications with high input to output voltage ratios. For example, if a 6-winding VERSA-PAC is used with three windings in series on the primary and three windings in parallel on the secondary, a 3:1 turns ratio will be achieved. Turns ratio can be chosen on the basis of desired duty cycle. However, remember that the input supply voltage plus the secondary-to-primary referred version of the flyback pulse (including leakage spike) must not exceed the allowed external MOSFET breakdown rating. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the output switch (Q1) turn-off. This is increasingly prominent at higher load currents, where more stored energy must be dissipated. In some cases a "snubber" circuit will be required to avoid overvoltage breakdown at the MOSFET's drain node. Application Note 19 is a good reference on snubber design. A bifilar or similar winding technique is a good way to minimize troublesome leakage inductances. However, remember that this will limit the primary-to-secondary breakdown voltage, so bifilar winding is not always practical.
38033fa
W
UU
LTC3803-3
APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS The external current sense resistor (RSENSE in Figure 2) allows the user to optimize the current limit behavior for the particular application. As the current sense resistor is varied from several ohms down to tens of milliohms, peak switch current goes from a fraction of an ampere to several amperes. Care must be taken to ensure proper circuit operation, especially with small current sense resistor values. For example, a peak switch current of 5A requires a sense resistor of 0.020. Note that the instantaneous peak power in the sense resistor is 0.5W and it must be rated accordingly. The LTC3803-3 has only a single sense line to this resistor. Therefore, any parasitic resistance in the ground side connection of the sense resistor will increase its apparent value. In the case of a 0.020 sense resistor, one milliohm of parasitic resistance will cause a 5% reduction in peak switch current. So the resistance of printed circuit copper traces and vias cannot necessarily be ignored. PROGRAMMABLE SLOPE COMPENSATION The LTC3803-3 injects a ramping current through its SENSE pin into an external slope compensation resistor (RSL in Figure 2). This current ramp starts at zero right after the NGATE pin has been high for the LTC3803-3's minimum duty cycle of 8%. The current rises linearly towards a peak of 5A at the maximum duty cycle of 80%, shutting off once the NGATE pin goes low. A series resistor (RSL) connecting the SENSE pin to the current sense resistor (RSENSE) thus develops a ramping voltage drop. From the perspective of the SENSE pin, this ramping voltage adds to the voltage across the sense resistor, effectively reducing the current comparator threshold in proportion to duty cycle. This stabilizes the control loop against subharmonic oscillation. The amount of reduction
U
in the current comparator threshold (VSENSE) can be calculated using the following equation:
VSENSE = Duty Cycle - 8% * 5A * RSL 80%
W
UU
Note: LTC3803-3 enforces 8% < Duty Cycle < 80%. A good starting value for RSL is 5.9k, which gives a 30mV drop in current comparator threshold at 80% duty cycle. Designs not needing slope compensation may replace RSL with a short circuit. INTERNAL WIDE HYSTERESIS UNDERVOLTAGE LOCKOUT The LTC3803-3 is designed to implement DC/DC converters operating from input voltages of typically 48V or more. The standard operating topology employs a third transformer winding (LBIAS in Figure 2) on the primary side that provides power for the LTC3803-3 via its VCC pin. However, this arrangement is not inherently self-starting. Start-up is affected by the use of an external "tricklecharge" resistor (RSTART in Figure 2) and the presence of an internal wide hysteresis undervoltage lockout circuit that monitors VCC pin voltage. Operation is as follows: "Trickle charge" resistor RSTART is connected to VIN and supplies a small current, typically on the order of 100A, to charge CVCC. After some time, the voltage on CVCC reaches the VCC turn-on threshold. The LTC3803-3 then turns on abruptly and draws its normal supply current. The NGATE pin begins switching and the external MOSFET (Q1) begins to deliver power. The voltage on CVCC begins to decline as the LTC3803-3 draws its normal supply current, which exceeds that delivered by RSTART. After some time, typically tens of milliseconds, the output voltage approaches its desired value. By this time, the third transformer winding is providing virtually all the supply current required by the LTC3803-3.
38033fa
11
LTC3803-3
APPLICATIO S I FOR ATIO
One potential design pitfall is undersizing the value of capacitor CVCC. In this case, the normal supply current drawn by the LTC3803-3 will discharge CVCC too rapidly; before the third winding drive becomes effective, the VCC turn-off threshold will be reached. The LTC3803-3 turns off, and the VCC node begins to charge via RSTART back up to the VCC turn-on threshold. Depending on the particular situation, this may result in either several on-off cycles before proper operation is reached or permanent relaxation oscillation at the VCC node. Component selection is as follows: Resistor RSTART should be made small enough to yield a worst-case minimum charging current greater than the maximum rated LTC3803-3 start-up current, to ensure there is enough current to charge CVCC to the VCC turn-on threshold. It should be made large enough to yield a worstcase maximum charging current less than the minimum rated LTC3803-3 supply current, so that in operation, most of the LTC3803-3's supply current is delivered through the third winding. This results in the highest possible efficiency. Capacitor CVCC should then be made large enough to avoid the relaxation oscillation behavior described above. This is complicated to determine theoretically as it depends on the particulars of the secondary circuit and load behavior. Empirical testing is recommended. The third transformer winding should be designed so that its output voltage, after accounting for the D2's forward voltage drop, exceeds the maximum VCC turn-off threshold. Also, the third winding's nominal output voltage should be at least 0.5V below the minimum rated VCC clamp voltage to avoid running up against the LTC3803-3's VCC shunt regulator, needlessly wasting power. VCC SHUNT REGULATOR In applications including a third transformer winding, the internal VCC shunt regulator serves to protect the LTC3803-3 from overvoltage transients as the third winding is powering up.
12
U
In applications where a third transformer winding is undesirable or unavailable, the shunt regulator allows the LTC3803-3 to be powered through a single dropping resistor from VIN to VCC, in conjunction with a bypass capacitor, CVCC, that closely decouples VCC to GND (see Figure 3). This simplicity comes at the expense of reduced efficiency due to the static power dissipation in the RVCC dropping resistor. The shunt regulator can draw up to 25mA through the VCC pin to GND to drop enough voltage across RVCC to regulate VCC to around 9.5V. For applications where VIN is low enough such that the static power dissipation in RVCC is acceptable, using the VCC shunt regulator is the simplest way to power the LTC3803-3.
VIN RVCC LTC3803-3 VCC CVCC GND
38033 F03
W
UU
Figure 3. Powering the LTC3803-3 Via the Internal Shunt Regulator
EXTERNAL PREREGULATOR The circuit in Figure 4 shows a third way to power the LTC3803-3. An external series preregulator consisting of series pass transistor Q1, Zener diode D1, and bias resistor RB brings VCC to at least 7.6V nominal, well above the maximum rated VCC turn-off threshold of 6.8V. Resistor RSTART momentarily charges the VCC node up to the VCC turn-on threshold, enabling the LTC3803-3.
VIN
RB
Q1
RSTART
LTC3803-3 VCC
D1 8.2V
CVCC
GND
38033 F04
Figure 4. Powering the LTC3803-3 with an External Preregulator
38033fa
LTC3803-3
TYPICAL APPLICATIO S
2W Isolated Housekeeping Telecom Converter
PRIMARY SIDE 10V, 100mA OUTPUT BAS516 T1
VIN 36V TO 75V 9.2k 1nF BAS516 1k 220k
22k
806
U
*
2.2F
1F
BAS516
*
2.2F
*
FDC2512
SECONDARY SIDE 10V, 100mA OUTPUT
1 2 3
LTC3803-3 6 ITH/RUN NGATE GND VFB VCC SENSE 5 4 5.6k 1F PRIMARY GROUND
SECONDARY SIDE GROUND
T1: PULSE ENGINEERING PA0648 OR TYCO TTI8698 0.1
38033 TA03
38033fa
13
LTC3803-3
TYPICAL APPLICATIO S
4:1 Input Range 3.3V Output Isolated Flyback DC/DC Converter
VIN+ 18 V TO 72V VIN- 2.2F 220k MMBTA42 100k T1 PA1277NL VOUT+ 3.3V 3A
PDZ6.8B VCC 680
1
ITH/RUN
GATE
6
LTC3803-3 2 5 VIN GND 3 VFB SENSE 4 4.7k
EFFICIENCY (%)
14
U
* *
68 150pF 22 BAS516 10 PDS1040
100F 6.3V x3
GND BAS516
*
FDC2512 0.1F
VOUT+
BAT760 0.1F 0.040 VCC 6.8k PS2801-1 1 2 BAS516 0.1F 0.33F 270 1 2 3 6 2.2nF 5 4 22.1k
38033 TA05
BAS516
VIN OPTO LT4430 GND OC COMP FB
VOUT+ 56k 47pF
100k
Efficiency vs Load Current
84 82 80 78 76 74 72 70 VIN = 48V VIN = 24V 0 1 2 IOUT (A) 3 4
38033 TA05a
38033fa
LTC3803-3
PACKAGE DESCRIPTIO
0.62 MAX
0.95 REF
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
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
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.
U
S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE ID 0.95 BSC 0.30 - 0.45 6 PLCS (NOTE 3) 0.80 - 0.90 0.01 - 0.10 0.09 - 0.20 (NOTE 3) 1.90 BSC
S6 TSOT-23 0302
38033fa
15
LTC3803-3
TYPICAL APPLICATIO S
Efficiency vs Load
100 95 90 85 80 75 70 500 VIN = 36V VIN = 48V VIN = 60V VIN = 72V 750 1000 1250 1500 LOAD CURRENT (mA) 1750 2000 VOUT = 3.3V
90% Efficient Synchronous Flyback Converter
VIN 36V TO 72V 220k CIN
T1
* *
Q2 CO
VOUT* 3.3V 1.5A
1n
D1 33k 1 ITH/RUN GATE 6 560 4.7k
38033 TA04a
EFFICIENCY (%)
LTC3803-3 2 5 VCC GND 8.06k 25.5k* RFB VOUT T1: PULSE ENGINEERING PA1006 Q1: FAIRCHILD FDC2512 Q2: VISHAY Si9803 D1: PHILIPS BAS516 3 VFB = 0.8V SENSE 4
10F 10V
EFFICIENCY (%)
CIN: TDK 1F, 100V, X5R CO: TDK 100F, 6.3V, X5R RCS: VISHAY OR IRC, 80m *FOR 5V OUTPUT CHANGE RFB TO 42.2k
RELATED PARTS
PART NUMBER LT 1425 LT1725 LTC1871 LT1950 LT3420 LT3468/LT3468-1 LTC3803 LTC3803-5 LTC3806
(R)
DESCRIPTION Isolated Flyback Switching Regulator with No External Power Devices General Purpose Isolated Flyback Controller Wide Input Range, No RSENSETM Current Mode Flyback, Boost and SEPIC Controller Current Mode PWM Controller Photoflash Capacitor Charger with Automatic Refresh Photoflash Capacitor Charger in 5-Pin SOT-23 Constant Frequency Flyback Controller Constant Frequency Flyback Controller Synchronous Flyback Controller
Burst Mode is a registered trademark of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation.
38033fa
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
U
38083 TA04b
Q1
*
0.1F
100 95 90 85 80 75 70 500
Efficiency vs Load
VOUT = 5V*
RCS
VIN = 36V VIN = 48V VIN = 60V VIN = 72V 750 1000 1250 1500 LOAD CURRENT (mA) 1750 2000
38083 TA04c
COMMENTS No Optoisolator or "Third Winding" Required, Up to 6W Output No Optoisolator Required, VIN and VOUT Limited Only by External Power Components Adjustable Switching Frequency, Programmable Undervoltage Lockout, Optional Burst Mode(R) Operation at Light Load Controller for Forward Converters from 30W to 300W Specialized Flyback Charges High Voltage Photoflash Capacitors Quickly and Efficiently Minimal Component Count, Uses Small Transformers; VIN from 2.5V to 16V 200kHz Switching Frequency, Low Profile (1mm) ThinSOT Package 200kHz Switching Frequency, 4.8V Turn-On Voltage High Efficiency (89%); Multiple Output with Excellent Cross Regulation
LT 0407 * PRINTED IN THE USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006

▲Up To Search▲

Price & Availability of LTC3803IS6-3

	To Download LTC3803IS6-3 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .