LTC3726IGN datasheet(8/16 Pages) LINER

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LTC3726

3726fb

OPERATIO

Main Control Loop

The LTC3726 is designed to work in a constant frequency,

current mode, one or two transistor forward converter.

During normal operation, the primary-side MOSFET(s)

is (are) “clocked” on with the forward MOSFET on the sec-

ondary side. This applies the reflected input voltage across

the inductor on the secondary side. When the current in

the inductor has ramped up to the peak value as com-

manded by the voltage on the ITH pin, the current sense

comparator is tripped, turning off the primary-side and

forward MOSFETs. To avoid turning on the synchronous

MOSFET prematurely and causing shoot-through, the

voltage on the SW pin is monitored. This voltage will

usually fall below 0V soon after the primary-side MOSFETs

have turned completely off. When this condition is de-

tected, the synchronous MOSFET is quickly turned on,

causing the inductor current to ramp back downwards.

The error amplifier senses the output voltage, and adjusts

the ITH voltage to obtain the peak current needed to

maintain the desired main-loop output voltage. The

LTC3726 always operates in a continuous current, syn-

chronous switching mode. This ensures a rapid transient

response as well as a stable bias supply voltage at light

loads. A maximum duty cycle (either 50% or 75%) is

internally set via clock dividers to prevent saturation of the

main transformer. In the event of an overvoltage on the

output, the synchronous MOSFET is quickly turned on to

help protect critical loads from damage.

Gate Drive Encoding

Since the LTC3726 controller resides on the secondary

side of an isolation barrier, communication to the primary-

side power MOSFETs is generally done through a trans-

former. Moreover, it is often necessary to generate a low

voltage bias supply for the primary-side gate drive cir-

cuitry. In order to reduce the number of isolated windings

present in the system, the LTC3726 uses a proprietary

scheme to encode the PWM gate drive information and

multiplex it together with bias power for the primary-side

drive and control, using a single pulse transformer. Note

that, unlike optoisolators and other modulation tech-

niques, this multiplexing scheme does not introduce a

significant time delay into the system.

For most forward converter applications, the PT+ and PT–

outputs will contain a pulse-encoded PWM signal. These

outputs are driven in a complementary fashion with an

essentially constant 50% duty cycle. This results in a

stable volt-second balance as well as an efficient transfer

of bias power across the pulse transformer. As shown in

Figure 1, the beginning of the positive half-cycle coincides

with the turn-on of the primary-side MOSFET(s). Likewise,

the beginning of the negative half-cycle coincides with

the maximum duty cycle (forced turn-off of primary

switch(es)). At the appropriate time during the positive

half-cycle, the end of the “on” time (PWM going LOW) is

signaled by briefly applying a zero volt differential across

the pulse transformer. Figure 1 illustrates the operation of

this multiplexing scheme.

The LTC3705 primary-side controller and gate driver will

decode this PWM information as well as extract the power

needed for primary-side gate drive.

Figure 1: Gate Drive Encoding Scheme (VMODE = GND)

–7V

150ns

1 CLK PER

DUTY CYCLE = 15%

VPT1+ – VPT1–

150ns

3726 F01

1 CLK PER

DUTY CYCLE = 0%

Self-Starting Architecture

When the LTC3726 is used in conjunction with the LTC3705/

LTC3725 primary-side controller and gate driver, a com-

plete self-starting isolated supply is formed. When input

voltage is first applied in such an application, the LTC3705/

LTC3725 will begin switching in an “open-loop” fashion,

causing the main output to slowly ramp upwards. This is

the primary-side soft-start mode. On the secondary side,

the LTC3726 derives its operating bias voltage from a

peak-charged capacitor. This peak-charged voltage will

rise more rapidly than the main output of the converter, so

that the LTC3726 will become operational well before the

output voltage has reached its final value.

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