Datasheet

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LT3837

3837fd

47pF

2.2nF

10Ω

22µF

22.1k

100k

BAS16

150k

12k

330Ω 15Ω

10Ω

100nH

8mΩ

Si7852DP

Si7336ADP

B0540W

1.37k

3.3nF

20k

1nF

1µF

0.1µF

2.2nF

BAT54

OSC

UVLO

SENSE

–

SFST

ENDLY

PGDLY

CMP

GND

LT3837

86.6k

•

10µF

• •

15k

10k

3837 TA01a

•

47µF × 3

220µF

OUTPUT 3.3V/10A

2N3906

FMMT718

FMMT618

FEATURES

APPLICATIONS

DESCRIPTION

Isolated No-Opto

Synchronous Flyback Controller

The LT

3837 is an isolated switching regulator controller

designed for medium power ﬂyback topologies. A typical

application is 10W to 60W with the part powered from a

DC supply.

The LT3837 is a current mode controller that regulates an

output voltage based on sensing the secondary voltage

via a transformer winding during ﬂyback. This allows for

tight output regulation without the use of an optoisolator,

improving dynamic response and reliability. Synchronous

rectiﬁcation increases converter efﬁciency and improves

output cross regulation in multiple output converters.

The LT3837 operates in forced continuous conduction

mode which improves cross regulation in multiple winding

applications. Switching frequency is user programmable

and can be externally synchronized. The part also has load

compensation, undervoltage lockout and soft-start circuitry.

The LT3837 is available in a thermally enhanced 16-pin

TSSOP package.

9V –18V to 3.3V at 10A Isolated Converter

Senses Output Voltage Directly from Primary Side

Winding—No Optoisolator Required

Synchronous Driver for High Efﬁciency

Supply Voltage Range 4.5V to 20V

Accurate Regulation Without User Trims

Programmable Switching Frequency from

50kHz to 250kHz

Synchronizable

Load Compensation

Undervoltage Lockout

Available in a Thermally Enhanced 16-Lead

TSSOP Package

Isolated Medium Power (10W to 60W) Supplies

Instrumentation Power Supplies

Isolated Medical Supplies

LOAD CURRENT (A)

EFFICIENCY (%)

3837 TA01b

8 10

18V

LOAD CURRENT (A)

3.00

OUTPUT (V)

3.10

3.30

3.40

3.50

3.60

3837 TA01c

3.20

8 10

18V

Efﬁciency vs Load Current

Regulation vs Load Current

TYPICAL APPLICATION

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

Protected by U.S. Patents including 6498466, 5841643.

Summary of content (28 pages)

PAGE 1
LT3837 Isolated No-Opto Synchronous Flyback Controller FEATURES n n n n n n n n n DESCRIPTION Senses Output Voltage Directly from Primary Side Winding—No Optoisolator Required Synchronous Driver for High Efficiency Supply Voltage Range 4.
PAGE 2
LT3837 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW VCC to GND................................................. –0.3V to 22V UVLO, SYNC Pin Voltage.............................–0.3V to VCC SENSE–, SENSE+ Pin Voltage.......................–0.5V, +0.5V FB Pin Current........................................................ ±2mA VC Pin Current........................................................ ±1mA Operating Junction Temperature Range (Notes 2, 3, 4)......................................
PAGE 3
LT3837 ELECTRICAL CHARACTERISTICS The l denotes specifications which apply over the full operating junction temperature range, otherwise specifications are TA = 25°C. VCC = 14V; PG, SG Open; VC = 1.4V, VSENSE = 0, RCMP = 1k, RTON = 90k, RPGDLY = 27.4k, RENDLY = 90k, unless otherwise specified. PARAMETER CONDITIONS MIN TYP MAX Feedback Amplifier Voltage Gain VC = 1.2V to 1.7V Soft-Start Charging Current VSFST = 1.5V 16 20 Soft-Start Discharge Current VSFST = 1.5V, VUVLO = 0V 0.7 1.
PAGE 4
LT3837 TYPICAL PERFORMANCE CHARACTERISTICS VCC Shutdown Current vs Temperature 10 VUVLO = 0 80 VCC Current vs Temperature VCC CURRENT (µA) 70 106 8 VCC = 14V IVCC (mA) 50 40 7 6 30 STATIC PART CURRENT 5 20 0 –50 –25 100 3 –50 –25 125 50 25 75 0 TEMPERATURE (°C) SENSE Fault Voltage vs Temperature 110 96 100 90 –50 125 108 210 205 200 195 190 1.240 COSC = 100pF 106 1.238 104 1.237 102 1.236 1.234 98 96 1.233 1.232 1.
PAGE 5
LT3837 TYPICAL PERFORMANCE CHARACTERISTICS Feedback Amplifier Source and Sink Current vs Temperature 1600 55 1550 1050 1500 gm (µmho) IVC (µA) 1650 SINK CURRENT VFB = 1.4V 60 1700 1100 SOURCE CURRENT VFB = 1.1V 65 Feedback Amplifier Voltage Gain vs Temperature AV (V/V) 70 Feedback Amplifier gm vs Temperature 1000 1450 1400 1350 1300 50 1250 950 1200 45 1150 40 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 900 –50 125 75 0 25 50 TEMPERATURE (°C) –25 100 UVLO vs Temperature 1.
PAGE 6
LT3837 TYPICAL PERFORMANCE CHARACTERISTICS PG Delay Time vs Temperature Enable Delay Time vs Temperature 300 320 250 300 RPGDLY = 27.4k 280 tED (ns) tPGDLY (ns) 200 150 RPGDLY = 16.9k 100 260 240 50 0 –50 RENDLY = 90k 220 –25 25 0 75 50 TEMPERATURE (°C) 100 125 3837 G19 200 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3837 G20 PIN FUNCTIONS SG (Pin 1): Synchronous gate drive output. This pin provides an output signal for a secondary-side synchronous switch.
PAGE 7
LT3837 PIN FUNCTIONS VC (Pin 9): Pin used for frequency compensation for the switcher control loop. It is the output of the feedback amplifier and the input to the current comparator. Switcher frequency compensation components are normally placed on this pin to ground. The voltage on this pin is proportional to the peak primary switch current. The feedback amplifier output is enabled during the synchronous switch on time.
PAGE 8
LT3837 BLOCK DIAGRAM CLAMPS VCC 0.7 1.25V INTERNAL REGULATOR DISABLE S Q R Q 9 COLLAPSE DETECT – UVLO CURRENT COMPARATOR TSD SFST 1V 6 OVERCURRENT FAULT – + UVLO VC + 3V 8 ERROR AMP + 10 + – – – 0.8V 1.3 – 1.
PAGE 9
LT3837 FLYBACK FEEDBACK AMPLIFIER T1 VFLBK FLYBACK LT3837 FEEDBACK AMP R1 FB 1V R2 + 8 • – VFB 1.237V VCC + 9 CVC VIN PRIMARY • SECONDARY • COUT ISOLATED OUTPUT MP – COLLAPSE DETECT MS R ENABLE S Q 3837 FFA TIMING DIAGRAM VIN VFLBK PRIMARY SIDE MOSFET DRAIN VOLTAGE VIN 0.
PAGE 10
LT3837 OPERATION The LT3837 is a current mode switcher controller IC designed specifically for use in an isolated flyback topology employing synchronous rectification. The LT3837 operation is similar to traditional current mode switchers. The major difference is that output voltage feedback is derived via sensing the output voltage through the transformer. This precludes the need of an optoisolator in isolated designs greatly improving dynamic response and reliability.
PAGE 11
LT3837 OPERATION Minimum Output Switch On-Time (tON(MIN)) The LT3837 affects output voltage regulation via flyback pulse action. If the output switch is not turned on, there is no flyback pulse and output voltage information is not available. This causes irregular loop response and start-up/latch-up problems. The solution is to require the primary switch to be on for an absolute minimum time per each oscillator cycle.
PAGE 12
LT3837 OPERATION VFLBK R1 • FB Q1 Q2 8 R2 Average primary side current is expressed in terms of output current as follows: T1 VFB VIN LOAD COMP I IIN = K1•IOUT • • MP + Q3 A1 – 14 RCMP So the effective change in VOUT target is: RCMPF + 50k SENSE 12 13 CCMP where: V K1= OUT VIN •Eff ∆VOUT = K1• ∆IOUT • RSENSE 3837 F01 Figure 1. Load Compensation Diagram Figure 1 shows the block diagram of the load compensation function.
PAGE 13
LT3837 APPLICATIONS INFORMATION Primary Winding Feedback Transformer Design The previous work was developed using a separate winding for voltage feedback. It is possible to use the primary winding as the feedback winding as well. This can simplify the design of the transformer. Transformer design/specification is the most critical part of a successful application of the LT3837. The following sections provide basic information about designing the transformer and potential tradeoffs.
PAGE 14
LT3837 APPLICATIONS INFORMATION ratio of between the two windings. Also keep the secondary MOSFET RDS(ON) small to improve cross regulation. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a spike after the primary side switch turn-off. This is increasingly prominent at higher load currents, where more stored energy is dissipated. Higher flyback voltage may break down the MOSFET switch if it has too low a BVDSS rating.
PAGE 15
LT3837 APPLICATIONS INFORMATION tight coupling usually increases primary-to-secondary capacitance and limits the primary-to-secondary breakdown voltage, so it isn’t always practical. Primary Inductance The transformer primary inductance, LP, is selected based on the peak-to-peak ripple current ratio (X) in the transformer relative to its maximum value. As a general rule, keep X in the range of 50% to 70% ripple current (i.e., X = 0.5 to 0.7).
PAGE 16
LT3837 APPLICATIONS INFORMATION Setting Feedback Resistive Divider Selecting the Load Compensation Resistor Use the equation developed in the Operation section for the feedback divider. The expression for RCMP was derived in the Operation section for primary winding sensing as: It is recommended that the Thevenin impedance of the resistors on the FB Pin is roughly 3k for bias current cancellation and other reasons. For the example using primary winding sensing if ESR = 0.002 and RDS(ON) = 0.
PAGE 17
LT3837 APPLICATIONS INFORMATION 5. Verify this result by connecting a resistor of this value from the RCMP pin to ground. 6. Disconnect the ground short to CCMP and connect the requisite 0.1µF filter capacitor to ground. Measure the output impedance RS(OUT) = ∆VOUT/∆IOUT with the new compensation in place. RS(OUT) should have decreased significantly. Fine tuning is accomplished experimentally by slightly altering RCMP.
PAGE 18
LT3837 APPLICATIONS INFORMATION Enable Delay Time (ENDLY) Enable delay time provides a programmable delay between turn-off of the primary gate drive node and the subsequent enabling of the feedback amplifier. As discussed earlier, this delay allows the feedback amplifier to ignore the leakage inductance voltage spike on the primary side. The worst-case leakage spike pulse width is at maximum load conditions. So set the enable delay time at these conditions.
PAGE 19
LT3837 APPLICATIONS INFORMATION dropping the voltage on UVLO. In this manner, hysteresis is produced. Referring to Figure 4, the voltage hysteresis at VIN is equal to the change in bias current times RA. The design procedure is to select the desired VIN referred voltage hysteresis, VUVHYS. Then: RA = VUVHYS IUVLO where: IUVLO = IUVLOL – IUVLOH is approximately 3.
PAGE 20
LT3837 APPLICATIONS INFORMATION Minimum Load Considerations At light loads, the LT3837 derived regulator goes into forced continuous conduction mode. The primary side switch always turns on for a short time as set by the tON(MIN) resistor. If this produces more power than the load requires, power will flow back into the primary during the “off” period when the synchronization switch is on. This does not produce any inherently adverse problems, though light load efficiency is reduced.
PAGE 21
LT3837 APPLICATIONS INFORMATION If the output load current is relatively constant, the feedback resistive divider is used to compensate for these losses. Otherwise, use the LT3837 load compensation circuitry (see Load Compensation). If multiple output windings are used, the flyback winding will have a signal that represents an amalgamation of all these windings impedances. Take care that you examine worst-case loading conditions when tweaking the voltages.
PAGE 22
LT3837 APPLICATIONS INFORMATION With CMILLER determined, calculate the primary-side power MOSFET power dissipation: PDPRI =IRMS(PRI)2 •RDS(ON) (1+ d ) + VIN(MAX) • where: PIN(MAX) DCIN •RDR • CMILLER •f VGATE(MAX) – VTH OSC RDR is the gate driver resistance approximately 10Ω VTH is the MOSFET gate threshold voltage fOSC is the operating frequency. (1 + d) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve. If you don’t have a curve, use d = 0.005/°C as an estimate.
PAGE 23
LT3837 APPLICATIONS INFORMATION Continuing our example, the output capacitor needs: 3.3V • (1– 52.4%) = 1.6mΩ 10A 10A = 1515µF COUT ≥ 1% • 3.3 • 200kHz IPRI PRIMARY CURRENT ESRCOUT ≤ 1% • SECONDARY CURRENT IPRI N RINGING DUE TO ESL ∆VCOUT OUTPUT VOLTAGE RIPPLE WAVEFORM These electrical characteristics require paralleling several low ESR capacitors possibly of mixed type. ∆VESR 3825 F07 Figure 7.
PAGE 24
LT3837 APPLICATIONS INFORMATION IC Thermal Considerations Take care to ensure that the LT3837 junction temperature does not exceed 125°C. Power is computed from the average supply current, the sum of quiescent supply current (ICC in the specifications) plus gate drive currents. The primary gate drive current is computed as: fOSC • QG where QG is the total gate charge at max VGS (obtained from the gate charge curve) and f is the switching frequency.
PAGE 25
LT3837 APPLICATIONS INFORMATION T1 VCC • VIN CVCC • GATE TURN-ON • LT3825 VCC PG GND CVIN MP GATE TURN-OFF OUT RSENSE VCC COUT CR LT3825 VCC SG • T2 GATE TURN-ON Q4 MS • GND GATE TURN-OFF Q3 3837 F09 Figure 9. High Current Paths TYPICAL APPLICATION 9V – 36V to 3.3V at 10A Isolated Converter 1/2 VIN 9V TO 36V 470pF 39Ω 10k MMBT3904 10µF 20Ω 47µF × 3 3/4 • 10/11/12 BAS70 1nF • 29.4k 1% 5 12k 4.7Ω Si7336ADP PG SENSE+ 100k 1.37k 1% GND SFST CCMP 47pF 0.1µF 3.
PAGE 26
LT3837 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev J) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 16 1514 13 12 11 6.60 ±0.10 4.50 ±0.10 0.48 (.019) REF 3.58 (.141) 2.94 (.116) 10 9 DETAIL B 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.51 (.020) REF DETAIL B IS THE PART OF THE LEAD FRAME FEATURE FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.
PAGE 27
LT3837 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 11/09 Change to Absolute Maximum Ratings Change to Electrical Characteristics D 12/12 2 2,3 Change to Pin Functions 6 Change to Block Diagram 8 Change to Flyback Feedback Amplifier 9 Text Change to Applications Information 23 Changes to Related Parts 28 Updated G20 graph 6 Updated package 26 3837fd Information furnished by Linear Technology Corporation is believed to be accurate and relia
PAGE 28
LT3837 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3825 Isolated No-Opto Synchronous Flyback Controller VIN 16V to 75V Limited by External Components, Up to 60W, Current Mode Control LT3573 Isolated Flyback Switching Regulator with 60V Integrated Switch 3V ≤ VIN ≤ 40V, No Opto-Isolator or "Third Winding" Required, MSOP-16E LT3757 Boost, Flyback, SEPIC and Inverting Controller 2.