Datasheet

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LTC3765

3765fb

For more information www.linear.com/LTC3765

TYPICAL APPLICATION

FEATURES DESCRIPTION

Active Clamp Forward

Controller and Gate Driver

The LT C

3765 is a start-up controller and gate driver for

use in a self-starting secondary-side control forward con-

verter. When

combined with the LTC3766 secondary-side

ynchronous forward controller, a complete isolated power

supply is created using a minimum of discrete parts. A

proprietary scheme is used to multiplex gate drive signals

and bias power across the isolation barrier through a

small pulse transformer. The LTC3765 contains an on-

chip bridge rectifier that extracts gate drive bias power

from this pulse transformer, eliminating the need for a

separate bias supply. A precision undervoltage lockout

circuit and linear regulator controller ensure a quick and

well-controlled start-up.

The LTC3765 includes an active clamp output for driving an

external PMOS, as well as an adjustable delay to optimize

efficiency. The active clamp architecture reduces voltage

stress on the main power switch, and provides the highest

possible efficiency. Overcurrent protection and the Direct

Flux Limit guarantee no transformer saturation without

compromising transient response.

36V-72V to 5V/15A Active Clamp Isolated Forward Converter

APPLICATIONS

Supports Self-Starting Secondary-Side Control

Direct Flux Limit™ Guarantees No Saturation

Active Clamp Drive with Delay Adjustment

On-Chip Bridge Rectifier Eliminates Need for

Separate Gate Drive Bias Supply

Wide Input Supply Range: 8V and Up (Limited Only

by External Components)

Linear Regulator Controller for Fast Start-Up

High Speed Gate Drivers

Precision UVLO with Adjustable Hysteresis

Overcurrent Protection

Overtemperature Protection

Adjustable Start-Up Frequency and Soft-Start

Tiny 16-Lead MSOP Package

Isolated Battery Chargers

Isolated 48V Telecommunication Systems

Servers and Embedded Computing

Automotive and Heavy Equipment

L, LT , LT C , LTM, PolyPhase, Linear Technology and the Linear logo are registered trademarks

and Direct Flux Limit is a trademark of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents, including 7200014 and

6144194. Other patents pending.

–

ITH

–

SW SG

GND PGND SGD FGD MODE

RUN V

NDRV

LTC3766

RUN

Si3437DV

(SOT-23)

15mΩ

0.5W

3mΩ

–

FS/UVSSFLT

NDRV PG

CORE

DELAY SGND

14k

18.2k

100Ω 100Ω

L1: PULSE PA1393.152

T1: PULSE PA0810

T2: PULSE PA0297

15k26.1k33nF 22k 17.8k

3765 TA01

604Ω

4.42k

0.1µF

2:1

6:2

10.5k15k

365k

1/8W

100k

Si3440DV

–

36V TO 72V

FDMS86201

BSC0901NS

••

SiR414DP

33nF

4.7µF

LTC3765

PGND

–

1Ω

168Ω

SMAG

FS/SYNC V

–

1.0µF

1nF

2.2nF

250VAC

33nF

200V

47pF

470pF

2.2µF

100V

×3

220µF

6.3V

×2

1.4µH

OUT

15A

OUT

–

100nF

200V

•

Summary of content (24 pages)

PAGE 1
LTC3765 Active Clamp Forward Controller and Gate Driver FEATURES n n n n n n n n n n n n DESCRIPTION Supports Self-Starting Secondary-Side Control Direct Flux Limit™ Guarantees No Saturation Active Clamp Drive with Delay Adjustment On-Chip Bridge Rectifier Eliminates Need for Separate Gate Drive Bias Supply Wide Input Supply Range: 8V and Up (Limited Only by External Components) Linear Regulator Controller for Fast Start-Up High Speed Gate Drivers Precision UVLO with Adjustable Hysteresis Overcurrent Pr
PAGE 2
LTC3765 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION VCC, NDRV Voltages.................................... –0.3V to 15V (NDRV-VCC) Voltage...................................... –0.3V to 6V IN+, IN–, SSFLT Voltages............................. –0.3V to 15V ISMAG Voltage................................................. –5V to 18V RUN Voltage................................................–0.3V to 12V DELAY, RCORE, FS/UV, IS+, IS – Voltages........ –0.
PAGE 3
LTC3765 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VCC = 12V unless otherwise noted.
PAGE 4
LTC3765 TYPICAL PERFORMANCE CHARACTERISTICS VCC Supply Current vs Supply Voltage VCC UVLO Thresholds vs Temperature UVLO THRESHOLD (V) SUPPLY CURRENT (mA) 1.5 1.0 0.5 7.6 100 7.5 90 CHARGE PUMP CURRENT (µA) 2.0 RISING 7.4 7.3 7.2 7.1 FALLING 7.0 6.
PAGE 5
LTC3765 TYPICAL PERFORMANCE CHARACTERISTICS PG Rising Delay Time vs DELAY Pin Resistor 600 1000 200 RDELAY = 50k 500 190 100 300 DELAY (ns) 400 DELAY (ns) DELAY TIME (ns) AG Falling Delay Time vs Temperature PG Rising Delay vs Temperature RDELAY = 22k 200 1 10 100 DELAY RESISTOR (kΩ) 160 0 –55 1000 –25 65 95 5 35 TEMPERATURE (°C) 125 3765 G10 2.5 VCC = 12V 25 VCC = 12V 2.6 2.5 1.5 1.0 0.5 2.4 2.
PAGE 6
LTC3765 PIN FUNCTIONS PGND (Pin 1): Power Ground. Ground return for the high current gate drivers. VCC (Pin 3): Main Supply Pin. A ceramic bypass capacitor should be tied between this pin and ground. RUN (Pin 12): Run Control and Undervoltage Lockout (UVLO). Connect to a resistor divider to monitor the input voltage VIN, which is required for proper operation of the Direct Flux Limit. Converter operation is enabled for VRUN > 1.25V.
PAGE 7
LTC3765 BLOCK DIAGRAM CHARGE PUMP VIN RECTIFIER MAIN PWM REGULATOR SHUTDOWN SGND SGND IN– VCC 7.4V/7.0V VCC + VCC UNDERVOLTAGE LOCKOUT – 1.25V/1.
PAGE 8
LTC3765 TIMING DIAGRAM PULSE ENCODED PWM VIN+ – VIN– PWM ON TIME LTC3765 AG LTC3765 PG ~ VIN 1 – DUTY CYCLE VIN SWP NODE 0V LTC3766 SG LTC3766 FG ~ VOUT 1 – DUTY CYCLE SWB NODE 0V VIN • SW NODE NS NP 0V 3765 TD01 SET BY LTC3766 FGD PIN SET BY LTC3766 SGD PIN SET BY LTC3765 DELAY PIN FIXED 180ns DELAY SW VIN+ • SWP LTC3765 AG VIN– AG PG IN+ IN– VOUT+ • SWB PG FG • • SW FG PT+ LTC3766 SG SG PT – 3765 F01 VOUT– Figure 1.
PAGE 9
LTC3765 OPERATION The LTC3765 is a forward converter start-up controller and gate driver for use in a single-switch forward converter with active clamp reset. When connected through a pulse transformer to the LTC3766 secondary-side synchronous forward controller, it forms a highly efficient forward converter with secondary-side regulation, galvanic isolation between input and output, and synchronous rectification.
PAGE 10
LTC3765 OPERATION The delay time between the primary switch turn-off and the active clamp turn-on is substantially less critical. Relative to the power loss due to turning on the primary switch, the power loss from switching the active clamp is significantly less. This difference results from both the lower current that the active clamp switches and the natural resonance of the system which facilitates zero voltage switching.
PAGE 11
LTC3765 OPERATION core by a resistor from the RCORE pin to ground. The magnetizing current is then limited during the on-time by this accurate internal approximation. Unlike previous methods, the Direct Flux Limit directly measures and monitors flux accumulation and guarantees that the transformer will not saturate, even when starting into a pre-biased output, without compromising transient response.
PAGE 12
LTC3765 APPLICATIONS INFORMATION RUN Pin Resistor Selection Normal operation is enabled when the voltage on the RUN pin rises above its 1.25V threshold. As shown in Figure 3, the RUN pin is typically used with an external resistive divider as an accurate undervoltage lockout (UVLO) on the VIN supply. A 5µA current is pulled by the RUN pin when it is below its threshold that, when combined with the value chosen for R1, increases the UVLO hysteresis beyond the internal minimum of 4%.
PAGE 13
LTC3765 APPLICATIONS INFORMATION These two equations result in a wide range of values for RNDRV. For many applications, a 100k resistor will satisfy these requirements. The rate of charge of VCC from 0V to 8.5V is controlled by the LTC3765 to be approximately 35µs regardless of the size of the capacitor connected to the VCC pin. The charging current for this capacitor can be approximated as: IC1 = 8.
PAGE 14
LTC3765 APPLICATIONS INFORMATION Choose the overcurrent trip current ITRIP to be less than the maximum pulsed drain current rating of the primary NMOS but greater than the sum of the peak inductor current at full load and the current required to charge the output capacitor at start-up, reflected through the transformer.
PAGE 15
LTC3765 APPLICATIONS INFORMATION Self-Starting Start-Up When starting up, the LTC3765 begins switching in an open-loop fashion to supply power to the secondary side LTC3766. When the LTC3766 has adequate bias voltage and has met other conditions for start-up, it begins sending both duty cycle information and power through the pulse transformer connected to the IN+/IN– pins. The LTC3765’s start-up switching frequency is set by a resistor from FSUV to ground.
PAGE 16
LTC3765 APPLICATIONS INFORMATION In many applications, the LTC3766 supply is initially biased by this open-loop duty cycle ramp. The maximum duty cycle that the LTC3765 can provide therefore dictates whether the LTC3766 has adequate bias for start-up. The maximum duty cycle is typically 70%; however, the setting of the DELAY pin (tDPG) decreases the maximum on-time. Therefore, the maximum duty cycle during startup is 70%-(tDPG • fSW • 100%).
PAGE 17
LTC3765 APPLICATIONS INFORMATION The above equation assumes that there is no load current, which is the worst-case condition for output voltage rise. If tOUT is less than tBIAS, then the soft-start capacitor value should be increased. Note that these equations are approximations and the actual times will vary somewhat with circuit parameters.
PAGE 18
LTC3765 APPLICATIONS INFORMATION ripple on the VCC capacitor (CVCC) due to the magnetizing current can be approximated by the following equation: ∆VCC = VOUT (NP / NS )  VOUT (NP / NS )   1–  6.8 • C VCC •LMAG • fSW 2  VIN(MAX)  In general, a 4.7µF capacitor is a good choice for most application circuits when the active clamp current is returned to VCC.
PAGE 19
LTC3765 APPLICATIONS INFORMATION When the NMOS is on, the magnetizing current and reflected inductor current are both flowing through the NMOS. The inductor current is generally much larger than the magnetizing current, which makes the magnetizing current difficult to measure directly.
PAGE 20
LTC3765 APPLICATIONS INFORMATION capacitor is also the voltage seen at the drains of the PG and AG MOSFETs. As shown in Figure 8, the VCL voltage has a minimum when the converter is operating at 50%. For a given range on VIN, therefore, the maximum clamp voltage (VCL(MAX)) will occur either at the minimum or maximum VIN, depending on which input voltage causes the converter to operate furthest from 50% duty cycle.
PAGE 21
LTC3765 APPLICATIONS INFORMATION Maximum Duty Cycle During the delay time between AG rising and PG rising, power is not transferred from the input supply to the output supply. In most forward converter systems, the maximum on-time is artificially limited by the delay, which then drives a trade-off between the optimal delay time and the maximum achievable duty cycle.
PAGE 22
LTC3765 PACKAGE DESCRIPTION MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.10 (.201) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 8 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 0.305 ±0.038 (.0120 ±.0015) TYP 16 0.50 (.0197) BSC 4.039 ±0.102 (.159 ±.004) (NOTE 3) RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 0.
PAGE 23
LTC3765 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 1/12 Removed ThinSOT and No RSENSE from disclaimer. 1 Changed Fault Output High condition to VCC = 6.7V (VCC UVLO Min Falling). 3 Updated PG Rising Delay Typical Performance graphs. 5 Changed 6V to 5.75V to match Electrical Characteristics table. 11 Added text to explain open-loop start-up maximum duty cycle 16 Added text to clarify what value of magnetizing inductance to use in the Direct Flux Limit equations.
PAGE 24
LTC3765 TYPICAL APPLICATION 36V-60V to 14V at 25A Isolated 350W Bus Converter L1 1.8µH +VIN 36V TO 60V 2.2µF 100V ×3 2.2µF 100V • 5T 4mΩ 1W ES1PD 0.68Ω 1/8W 100Ω 100Ω 66.5k SW FG IS+ IS+ 1nF LTC3766 NPO 33nF D2 NDRV AG VCC 0.1µF IN+ DELAY • 220pF IN– 27.4k 33nF SS IS– MODE PT+ • FB 27.4k 2.2nF 250VAC 15.0k 60.4k 150k 10nF 604Ω 13.7k SSFLT 68µF 16V: SANYO 16TQC68M D1-D2: ZHCS506 D5: BAS21 L1: VISHAY IHLP4040DZER1R8M11 L2: COILCRAFT SER2814L-472KL VCC 1µF T2 2.5:2 4.