LTC3900

7

3900i

APPLICATIO S I FOR ATIO

Figure 2. Synchronization Waveforms

GATE

SG

SYNC

FG

CG

3900 F02

Overview

In a typical forward converter topology, a power trans-

former is used to provide the functions of input/output

isolation and voltage step-down to achieve the required

low output voltage. Schottky diodes are often used on

the secondary-side to provide rectification. Schottky di-

odes, though easy to use, result in a loss of efficiency due

to relatively high voltage drops. To improve efficiency,

synchronous output rectifiers utilizing N-channel MOSFETs

can be used instead of Schottky diodes. The LTC3900

provides all of the necessary functions required to drive

the synchronous rectifier MOSFETs.

Figure 1 shows a simplified forward converter application.

T1 is the power transformer; Q1 is the primary-side power

transistor driven by the primary controller, LT1950 GATE

output. The pulse transformer T2 provides synchroniza-

tion and is driven by either the inverted GATE output or a

synchronization signal, SG from the primary controller.

Q3 and Q4 are secondary-side synchronous switches

steady DC output voltage for the load. Also shown in

optocoupler driver LT1797 and an optocoupler, back to

Each full cycle of the forward converter operation consists

of two periods. In the first period, Q1 turns on and the

primary-side delivers power to the load through T1. SG

goes low and T2 generates a negative pulse at the LTC3900

SYNC input. The LTC3900 forces FG to turn on and CG to

turn off, Q3 conducts. Current flows to the load through

high and T2 generates a positive pulse at the LTC3900

SYNC input. The LTC3900 forces FG to turn off and CG to

turn on, Q4 conducts. Current continues to flow to the load

nization waveforms.

External MOSFET Protection

A programmable timer and a differential input current

sense comparator are included in the LTC3900 for protec-

tion of the external MOSFET during power down and Burst

Mode

When the primary controller is powering down, the pri-

mary controller shuts down first and the LTC3900 contin-

is no SYNC pulse to the LTC3900. The LTC3900 keeps one

of the drivers turned on depending on the polarity of the

last SYNC pulse. If the last SYNC pulse is positive, CG will

remain high and the catch MOSFET, Q4 will stay on. The

inductor current will start falling down to zero and con-

tinue going in the negative direction due to the voltage that

is still present across the output capacitor (the current

the inductor current is negative, the inductor current will

produce high voltage across Q4, resulting in a MOSFET

avalanche. Depending on the amount of energy stored in

the inductor, this avalanche energy may damage Q4.

The timer circuit and current sense comparator in LTC3900

are used to prevent reverse current buildup in the output

inductor.

Timer

Figure 3 shows the LTC3900 timer internal and external

circuits. The timer operates by using an external R-C

charging network to program the time-out period. On

every negative transition at the SYNC input, the chip

generates a 200ns pulse to reset the timer cap. If the SYNC

signal is missing or incorrect, allowing the timer cap

voltage to go high, it shuts off both drivers once the

voltage reaches the time-out threshold. Figure 4 shows

the timer waveforms.

Burst Mode is a registered trademark of Linear Technology Corporation.