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LT3825

3825fe

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Isolated No-Opto

Synchronous Flyback Controller

with Wide Input Supply Range

Isolated Medium Power (10W to 60W) Supplies

Isolated Telecom, Medical Converters

Instrumentation Power Supplies

Isolated Power over Ethernet Supplies

48V to 3.3V at 12A Isolated Supply

Efficiency

Senses Output Voltage Directly from Primary Side

Winding—No Opto-Isolator Required

Synchronous Driver for High Efficiency

Input Voltage Limited Only by External

Power Components

Accurate Output Regulation without User Trims

Switching Frequency from 50kHz to 250kHz

Synchronizable

Load Compensation

Programmable Undervoltage Lockout

Available in a Thermally Enhanced 16-Lead

TSSOP Package

The LT

3825 is an isolated switching regulator controller

designed for medium power flyback topologies. A typical

application is 10W to 60W with input voltage limited only

by external power path components. A third transformer

winding provides output voltage feedback.

The LT3825 is a current mode controller that regulates

output voltage based on sensing secondary voltage via a

transformer winding during flyback. This allows for tight

output regulation without the use of an opto-isolator,

improving dynamic response and reliability. Synchronous

rectification increases converter efficiency and improves

output cross regulation in multiple output converters.

The LT3825 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 circuity.

LOAD CURRENT (A)

EFFICIENCY (%)

3825 TA01b

8 10

36V

48V

72V

Regulation

LOAD CURRENT (A)

3.13

OUTPUT (V)

3.18

3.28

3.33

3.38

3.43

3825 TA01c

3.23

8 10

36V

48V

72V

100k12k

3.01k

28.7k

402k

20Ω

47Ω

47k

20Ω

•

100pF

470µF

×4

OUT

3.3V

12A

36V TO 72V

0.02Ω

47µF

15k

2.1k 150k

0.22µF

100k

10k

15Ω

330Ω

47pF

2.2µF

0.1µF

10nF

2.2nF

0.1µF

3825 TA01a

1µF

×2

SYNC

PGDLY

UVLO

SENSE

–

SENSE

CMP

ENDLY OSC

LT3825

GND SFST

•

CMP

L, LT, LT C , LTM, Burst Mode, SwitcherCAD, Linear Technology and the Linear logo are registered

trademarks and No R

SENSE

, ThinSOT are trademarks of Linear Technology Corporation. All other

trademarks are the property of their respective owners. Protected by U.S. Patents, including

6948466, 5841643.

Summary of content (32 pages)

PAGE 1
LT3825 Isolated No-Opto Synchronous Flyback Controller with Wide Input Supply Range DESCRIPTION FEATURES n n n n n n n n n The LT®3825 is an isolated switching regulator controller designed for medium power flyback topologies. A typical application is 10W to 60W with input voltage limited only by external power path components. A third transformer winding provides output voltage feedback.
PAGE 2
LT3825 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION VCC to GND Low Impedance Source......................... –0.3V to 18V Current Fed (VCC Has Internal 19.5V Clamp)............30mA Into VCC UVLO, SYNC Pin Voltage............................ –0.3V to VCC SENSE–, SENSE+ Pin Voltage.......................–0.5V, +0.5V FB Pin Current.........................................................±2mA VC Pin Current........................................................
PAGE 3
LT3825 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VCC = 14V; PG, SG Open; VC = 1.5V, VSENSE– = 0V; RCMP = 1k, RtON = 90k, RPGDLY = 27.4k, RENDLY = 90k, unless otherwise specified (Note 3). PARAMETER CONDITIONS Reference Voltage Line Regulation 12V ≤ VCC ≤ 18V Feedback Amplifier Voltage Gain VC = 1.2V to 1.7V MIN l TYP MAX UNITS 0.005 0.
PAGE 4
LT3825 TYPICAL PERFORMANCE CHARACTERISTICS VCC(0N) and VCC(OFF) vs Temperature VCC Start-Up Current vs Temperature VCC Current vs Temperature 10 300 16 VCC(ON) 15 9 250 14 12 11 150 8 –50 –25 0 50 75 25 TEMPERATURE (°C) 100 4 0 –50 –25 125 50 25 75 0 TEMPERATURE (°C) 3825 G01 125 110 SENSE = VSENSE+ – 215 WITH VSENSE = 0V 102 100 98 96 94 106 210 104 205 200 195 90 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 –25 0 50 75 25 TEMPERATURE (°C) 90 –50 125 300 1.237 VFB (V) 1.
PAGE 5
LT3825 TYPICAL PERFORMANCE CHARACTERISTICS 70 70 125°C 25°C 50 65 IVC (µA) –10 1050 1000 55 50 –30 950 45 –50 –70 1100 SINK CURRENT VFB = 1.4V 60 10 Feedback Amplifier gm vs Temperature SOURCE CURRENT VFB = 1.1V –40°C 30 IVC (µA) Feedback Amplifier Source and Sink Current vs Temperature gm (µmho) Feedback Amplifier Output Current vs VFB 0.9 1 1.1 1.2 VFB (V) 1.3 40 –50 1.5 1.4 50 25 75 0 TEMPERATURE (°C) –25 100 Feedback Amplifier Voltage Gain vs Temperature 3.6 1550 3.
PAGE 6
LT3825 TYPICAL PERFORMANCE CHARACTERISTICS Minimum PG On-Time vs Temperature RtON(MIN) = 158k 330 320 250 200 tPGDLY (ns) 310 300 290 RENDLY = 90k 300 RPGDLY = 27.4k 320 tON(MIN) (ns) Enable Delay Time vs Temperature PG Delay Time vs Temperature 300 280 tED (ns) 340 150 RPGDLY = 16.
PAGE 7
LT3825 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
LT3825 BLOCK DIAGRAM CLAMPS VCC 15.3V 19.5V 0.7 + 1.3 – 1.
PAGE 9
LT3825 FLYBACK FEEDBACK AMPLIFIER T1 VFLBK FLYBACK LT3825 FEEDBACK AMP R1 FB 1V R2 + 8 • – VFB 1.237V VC + 9 CVC VIN PRIMARY • SECONDARY • COUT ISOLATED OUTPUT MP – COLLAPSE DETECT MS R ENABLE S Q 3825 FFA TIMING DIAGRAM PRIMARY SIDE MOSFET DRAIN VOLTAGE VFLBK VIN 0.
PAGE 10
LT3825 OPERATION The LT3825 is a current mode switcher controller IC designed specifically for use in an isolated flyback topology employing synchronous rectification. The LT3825 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 opto-isolator in isolated designs greatly improving dynamic response and reliability.
PAGE 11
LT3825 OPERATION that developed under these conditions, forced continuous operation normally occurs. See Applications Information for further details. Enable Delay (ENDLY) The flyback pulse appears when the primary-side switch shuts off. However, it takes a finite time until the transformer primary-side voltage waveform represents the output voltage. This is partly due to rise time on the primary-side MOSFET drain node but, more importantly, is due to transformer leakage inductance.
PAGE 12
LT3825 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 where : • • MP + Q3 A1 – RCMPF + 50k SENSE 12 K1 = VOUT VIN • Eff So the effective change in VOUT target is: ∆VOUT = K1• ∆IOUT • 14 RCMP 13 CCMP RSENSE 3825 F01 Figure 1. Load Compensation Diagram RSENSE • R1• NSF RCMP thus : R ∆VOUT = K1• SENSE • R1• NSF ∆IOUT RCMP where: node.
PAGE 13
LT3825 APPLICATIONS INFORMATION Transformer Design Transformer design/specification is the most critical part of a successful application of the LT3825. The following sections provide basic information about designing the transformer and potential trade-offs. If you need help, the LTC Applications group is available to assist in the choice and/or design of the transformer. Turns Ratios The design of the transformer starts with determining duty cycle (DC).
PAGE 14
LT3825 APPLICATIONS INFORMATION As a rough guide, leakage inductance of several percent (of mutual inductance) or less may require a snubber, but exhibit little to no regulation error due to leakage spike behavior. Inductances from several percent up to perhaps ten percent cause increasing regulation error. Avoid double digit percentage leakage inductances as there is a potential for abrupt loss of control at high load current.
PAGE 15
LT3825 APPLICATIONS INFORMATION Ripple current and percentage ripple is largest at minimum duty cycle; in other words, at the highest input voltage. LP is calculated from: LP 2 2 VIN(MAX) • DCMIN ) VIN(MAX) • DCMIN ) • Eff ( ( = = fOSC • XMAX • POUT fOSC • XMAX • PIN where: fOSC is the oscillator frequency DCMIN is the DC at maximum input voltage XMAX is ripple current ratio at maximum input voltage For a 48V (VIN = 36V to 72V) to 5V/8A converter with 90% efficiency, POUT = 40W and PIN = 44.44W.
PAGE 16
LT3825 APPLICATIONS INFORMATION Setting Feedback Resistive Divider The expression for VOUT developed in the Operation section is rearranged to yield the following expression for the feedback resistors: ( )  V    OUT +ISEC • ESR + RDS(ON)    R1= R2 –1   VFB • NSF   Continuing the example, if ESR + RDS(ON) = 8mΩ, R2 = 3.32k, then:  5 + 8 • 0.008  R1= 3.4k  – 1 = 37.6k  1.232 • 1/ 3  choose 37.4k.
PAGE 17
LT3825 APPLICATIONS INFORMATION 4. Compute: RSENSE • R1• NSF RS(OUT) 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 a 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
LT3825 APPLICATIONS INFORMATION The tON(MIN) resistor is set with the following equation: RtON(MIN) (kΩ) = tON(MIN)(ns) – 104 1.063 Keep RtON(MIN) greater than 70k. A good starting value is 160k. 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.
PAGE 19
LT3825 APPLICATIONS INFORMATION The bias current on this pin depends on the pin voltage and UVLO state. The change provides the user with adjustable UVLO hysteresis. When the pin rises above the UVLO threshold a small current is sourced out of the pin, increasing the voltage on the pin. As the pin voltage drops below this threshold, the current is stopped, further dropping the voltage on UVLO. In this manner, hysteresis is produced.
PAGE 20
LT3825 APPLICATIONS INFORMATION Switching action commences and the converter begins to deliver power to the output. Initially the output voltage is low and the flyback voltage is also low, so CTR supplies most of the LT3825 current (only a fraction comes from RTR.) VCC voltage continues to drop until after some time, typically tens of milliseconds, the output voltage approaches its desired value. The flyback winding then provides the LT3825 supply current and the VCC voltage stabilizes.
PAGE 21
LT3825 APPLICATIONS INFORMATION during the current comparator sense action (current mode switching). AN19 provides a method for empirically tweaking frequency compensation. Basically it involves introducing a load current step and monitoring the response. Slope Compensation This part incorporates current slope compensation. Slope compensation is required to ensure current loop stability when the DC is greater than 50%.
PAGE 22
LT3825 APPLICATIONS INFORMATION Output Voltage Error Sources The LT3825’s feedback sensing introduces additional sources of errors. The following is a summary list. The internal bandgap voltage reference sets the reference voltage for the feedback amplifier. The specifications detail its variation. The external feedback resistive divider ratio proportional directly affects regulated voltage. Use 1% components.
PAGE 23
LT3825 APPLICATIONS INFORMATION CMILLER is calculated from the gate charge curve included on most MOSFET data sheets (Figure 6). The flat portion of the curve is the result of the Miller (gate-to-drain) capacitance as the drain voltage drops. The Miller capacitance is computed as: CMILLER Q – QA = B VDS b QA QB GATE CHARGE (QG) 3825 F06 Figure 6.
PAGE 24
LT3825 APPLICATIONS INFORMATION Capacitor Selection In a flyback converter, the input and output current flows in pulses, placing severe demands on the input and output filter capacitors. The input and output filter capacitors are selected based on RMS current ratings and ripple voltage. Select an input capacitor with a ripple current rating greater than: IRMS = PIN VIN(MIN) 1 – DCMAX DCMAX IRMS = IOUT DCMAX 1– DCMAX Continuing the example: IRMS = 8A 52.6% = 8.43A 1– 52.
PAGE 25
LT3825 APPLICATIONS INFORMATION The other 1% is due to the bulk C component, so use: COUT ≥ IOUT 1% • VOUT • fOSC In many applications the output capacitor is created from multiple capacitors to achieve desired voltage ripple, reliability and cost goals. For example, a low ESR ceramic capacitor can minimize the ESR step, while an electrolytic capacitor satisfies the required bulk C.
PAGE 26
LT3825 APPLICATIONS INFORMATION PC Board Layout Considerations In order to minimize switching noise and improve output load regulation, connect the GND pin of the LT3825 directly to the ground terminal of the VCC decoupling capacitor, the bottom terminal of the current sense resistor, the ground terminal of the input capacitor, and the ground plane (multiple vias). Place the VCC capacitor immediately adjacent to the VCC and GND pins on the IC package.
PAGE 27
LT3825 TYPICAL APPLICATIONS 48V to 5V at 8A Isolated Supply 4 T1 7 8 9 • 5 VIN+ 36V TO 72V 2.2µF 100V 1 47k 1/4W + BAS21 402k 1% 29.4k 1% P6SMB100A 47µF 20V R7 20Ω 3 SG PG SENSE+ SG LT3825 3.01k 1% 15k 1% PGDLY tON SYNC RCMP ENDLY 12k 2.1k 1% 100k VC OSC GND SFST 100k 47pF Si7336ADP ×2 47Ω 1µF 2.2nF 0.1µF CCMP 10nF 680pF 0.22µF 100k 0.
PAGE 28
LT3825 TYPICAL APPLICATIONS 48V to 3.3V at 6A Isolated Supply T1 PULSE PB2134 4 7 • 8 9 5 VIN+ 36V TO 72V 0.82µF 100V 47k 1/4W + BAS21 402k 1% 1 26.1k 1% • 47µF 20V R7 20Ω • 3 0.1µF + 10 11 12 B0540W SG 0.04Ω 1W 330Ω FB LT3825 UVLO 15k 1% 3.01k 1% PGDLY tON SYNC RCMP 15k 866Ω 1% 100k SENSE– VC ENDLY 62k 47pF OSC GND SFST 1µF FMMT718 SENSE+ SG 47Ω FMMT618 Si7892DP SG PG 0.1µF 2.2nF 250V Si4490DY VCC 150µF 6TPB150ML ×3 VOUT+ 3.3V 6A CCMP 3.3nF 680pF 0.
PAGE 29
LT3825 TYPICAL APPLICATIONS 48V to 3.3V at 12A Isolated Supply L1 0.1µH + VIN 36V TO 72V 28.7k 1% BAS21 T1 • 3.01k 1% 402k 1% 2.2µF 100V 47µF FB 47k 1/4W 20Ω PG 12k 100k 750Ω 2.2nF 250V 47Ω SG VC CCMP 0.01µF 0.1µF 1µF 330Ω SENSE– OSC GND SFST 150k 47pF Si7336ADP Q2 SENSE+ 0.02Ω 1/2W LT3825 PGDLY tON SYNC RCMP ENDLY B0540W • C5 470µF Q1 UVLO 15k 1% C1 TO C4 47µF ×3 Si4490DY SG SG T1 20Ω + VCC • 100pF 250V VOUT+ 3.3V 12A 15Ω 2.2nF 0.
PAGE 30
LT3825 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. 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 (.
PAGE 31
LT3825 REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 12/09 Change to Absolute Maximum Ratings Change to Electrical Characteristics C D E 01/10 11/12 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 Change to Typical Application 30 Change to Related Parts 32 Change TA = 25°C to TJ = 25°C 2, 3 Addition to Note 3 3 Changed Typical Application sch
PAGE 32
LT3825 RELATED PARTS PART NUMBER LT3748 LT3758 DESCRIPTION 100V Isolated Flyback Controller Boost, Flyback, SEPIC and Inverting Controller LT3958 LT3837 80V Flyback/Boost Converters Isolated No Opto Synchronous Flyback Controller LT3798 Off-Line, Isolated, No Opto-Coupler, Flyback Controller with Active PFC LT3799 Off-Line, Isolated, Flyback LED Controller with Active PFC LTC®3803/LTC3803-3/ 200kHz Flyback DC/DC Controller LTC3803-5 LTC3873/LTC3873-5 No RSENSE™, Constant-Frequency, Flyback, Boost, SEPI