LTC3705IGN datasheet(8/20 Pages) LINER

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LTC3705

3705fb

Mode Setting

The LTC3705 is a controller and gate driver designed for

use in a 2-switch forward converter. When used in con-

junction with the LTC3706 PolyPhase secondary-side

synchronous forward controller it forms a complete

2-switch forward converter with secondary-side regula-

tion, galvanic isolation between input and output, and

synchronous rectification. In this mode, upon start-up,

the FB/IN+ and FS/IN– pins are effectively shorted by one

winding of the pulse transformer. The LTC3705 detects

this short circuit to determine that it is in secondary-side

control mode. Operation in this mode is confirmed when

the LTC3706 begins switching the pulse transformer.

Alternately, the LTC3705 can be used as a standalone

primary-side controller. In this case, the FB/IN+ and FS/IN–

pins operate independently. The FB/IN+ pin is connected to

the collector of an optoisolator to provide feedback and the

FS/IN– pin is connected to the frequency set resistor.

Gate Drive Encoding

In secondary-side control with the LTC3706, after a start-

up sequence, the LTC3706 transmits multiplexed PWM

information through a pulse transformer to the FB/IN+ and

FS/IN– inputs of the LTC3705. In the LTC3705, the PWM

receiver extracts the duty cycle and uses it to control the

top and bottom gate drivers.

Figure 1 shows that the LTC3706 drives the pulse trans-

former in a complementary fashion, with a duty cycle of

approximately 50%. At the appropriate time during the

positive half cycle, the LTC3706 applies a short (150ns)

zero-voltage pulse across the pulse transformer, indicat-

ing the end of the “on” time. Although this scheme allows

the transmission of 0% to 50% duty cycle, it is necessary

to establish a minimum controllable “on” time of approxi-

mately 100ns. This ensures that 0% duty cycle can be

reliably distinguished from 50% duty cycle.

On-Chip Rectifier

Simultaneously with duty-cycle decoding, and through

the same pulse transformer, the near-square-wave gener-

ated by the LTC3706 provides primary-side VCC gate drive

bias power by way of the LTC3705’s on-chip full-wave

bridge rectifier. No auxiliary bias supply is necessary and

forward converter design and circuitry are considerably

simplified.

External Series Pass Linear Regulator

The LTC3705 features an external series pass linear regu-

lator that eliminates the long start-up time associated with

the conventional trickle charger. The drain of an external

NMOS is connected to the input voltage and the source is

connected to VCC. The gate of the NMOS is connected to

NDRV. To power the gate, an external pull-up resistor is

connected from the input voltage to NDRV. The NMOS

must be a standard 3V threshold type (i.e. not logic level).

An on-chip circuit manages the start up and operation of

the linear regulator. It takes approximately 45

µs for the

linear regulator to charge VCC to its target value of 8V

(unless limited by a slower rise of VIN). The LTC3705

begins operating the gate drives when VCC reaches 7.4V.

Often, the thermal rating of the NMOS prevents it from

operating continuously, and the LTC3705 “times out” the

linear regulator to prevent overheating. This is accom-

plished using the capacitor connected to the SSFLT pin as

described subsequently.

Trickle Charger Shunt Regulator

Alternately, a trickle charger can be implemented by

eliminating the external NMOS and connecting NDRV to

VCC and using the pull-up resistor to charge VCC. To allow

extra headroom for starting, the LTC3705 detects this

mode and increases the threshold for starting the gate

drives to 13.4V. An internal shunt regulator limits the

voltage on the trickle charger to 15V.

OPERATIO

Figure 1. Gate Drive Multiplexing Scheme

DUTY CYCLE = 15%

DUTY CYCLE = 0%

150ns

1 CLK PER

+7V

–7V

VPT1

+ – V

PT1

–

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