LTC3726IGN datasheet(10/16 Pages) LINER

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LTC3726

3726fb

pin (see Slave Mode Operation). The Phase angle cannot

be adjusted when the FB/PHASE pin is being used for

voltage loop regulation.

Soft-Start

The soft-start circuitry has four functions: 1) to provide a

shutdown, 2) to provide a smooth ramp on the output

voltage during start-up, 3) to limit the output current in a

short-circuit situation by entering a hiccup mode, and

4) to communicate fault and shutdown information

between multiple LTC3726s in a PolyPhase application.

When the RUN/SS pin is pulled to GND, the chip is placed

into shutdown mode. If this pin is released, the RUN/SS

pin is initially charged with a 50

µAcurrentsource.Afterthe

RUN/SS pin gets above 0.5V, the chip is enabled. At the

instant that the LTC3726 is first enabled, the RUN/SS

voltage is rapidly preset to a voltage that will correspond

to the main output voltage of the DC/DC converter. (See

the Self-Starting Architecture section.) After this preset

interval has completed, the normal soft-start interval

begins and the charging current is reduced to 5

µA. The

external soft-start voltage is used to internally ramp up the

0.6V reference (positive) input to the error amplifier.

When fully charged, the RUN/SS voltage remains at 3V.

In the event that the sensed switch or inductor current

exceeds the overcurrent trip threshold, an internal fault

latch is tripped. When such a fault is detected, the LTC3726

immediately goes to zero duty cycle and initiates a soft-

start retry. Prior to discharging the soft-start capacitor,

however, the LTC3726 first puts a voltage pulse on the

RUN/SS pin, which trips the fault latch in any other

LTC3726 that shares the RUN/SS. This ensures an orderly

shutdown of all phases in a PolyPhase application. After

the soft-start capacitor is fully discharged, the LTC3726

attempts a restart. If the fault is persistent, the system

enters a “hiccup” mode.

Note that in self-starting secondary-side control applica-

tions (with the LTC3705 or LTC3725), the presence of the

LT3726 bias voltage is dependent upon the regular switch-

ing of the primary-side MOSFETs. Therefore, depending

on the details of the application circuit, the LTC3726 may

lose its bias voltage after a fault has been detected and

before completing a soft-start retry. In this case, the

“hiccup-mode” operation is actually governed by the

LTC3705/LTC3725 soft-start circuitry (see the LTC3705/

LTC3725 data sheets).

Drive Mode and Maximum Duty Cycle

Although the LTC3726 is primarily intended to be used

with the LTC3705/LTC3725 in forward converter applica-

tions, the MODE pin provides the flexibility to use the

LTC3726 in a wide variety of additional applications. This

pin can be used to defeat the gate drive encoding scheme,

as well as change the maximum duty cycle from its default

value of 50%. The use of the MODE pin is summarized in

Table 3.

When the gate drive encoding scheme is defeated, a

standard PWM-style signal will be present at the PT+ pin

and a reference clock (in phase with the PWM signal) will

be present at the PT– pin. These outputs can be used in

“standalone” applications (without the LTC3705/LTC3725)

to drive the gates of MOSFETs in a conventional manner.

Table 3

PT+/PT– Mode

INTENDED

MODE PIN

(MAX DUTY CYCLE)

APPLICATION

GND

Encoded PWM

2-Switch Forward

(DMAX = 50%)

with LTC3705

VCC

Encoded PWM

1-Switch Forward

(DMAX = 75%)

with LTC3725

200k

Ω to GND

Standard PWM

2-Switch Forward

(DMAX = 50%)

Standalone

100k

Ω to GND

Standard PWM

1-Switch Forward

(DMAX = 75%)

Standalone

Overvoltage Protection

This circuit monitors the voltage on the FB input. If the

voltage on the FB pin exceeds 117% of 0.6V (0.7V), an

overvoltage (OVP) is detected. For overvoltage protection,

the secondary-side synchronous MOSFET is turned on

while all other MOSFETs are turned off. This protection

mode is not latched, so that the overvoltage detection is

cleared if the FB voltage falls below 115% of 0.6V (0.69V).

OPERATIO

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