LTC3706IGN datasheet(9/20 Pages) LINER

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LTC3706

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OPERATION

Main Control Loop

The LTC3706 is designed to work in a constant frequency,

current mode 2-transistor forward converter. During

normal operation, the primary-side MOSFETs (both top

and bottom) are “clocked” on together with the forward

MOSFET on the secondary 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 commanded 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 detected, 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 LTC3706 always operates in a continuous

current, synchronous 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 LTC3706 controller resides on the secondary side

of an isolation barrier, communication to the primary-side

power MOSFETs is generally done through a transformer.

Moreover, it is often necessary to generate a low voltage

bias supply for the primary-side gate drive circuitry. In

order to reduce the number of isolated windings present

in the system, the LTC3706 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 techniques, 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 MOSFETs. Likewise,

the beginning of the negative half-cycle coincides with the

maximum duty cycle (forced turn-off of primary switches).

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.

Self-Starting Architecture

When the LTC3706 is used in conjunction with the

LTC3705 primary-side controller and gate driver, a

complete self-starting isolated supply is formed. When

input voltage is ﬁrst applied in such an application, the

LTC3705 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 LTC3706 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 LTC3706 will become operational well before

the output voltage has reached its ﬁnal value.

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