11

LTC1433/LTC1434

Power-On Reset Function (POR)

The power-on reset function monitors the output voltage

and turns on an open-drain device when it is out of

regulation. An external pull-up resistor is required on the

POR pin.

When power is first applied or when coming out of

shutdown, the POR output is pulled to ground. When the

output voltage rises above a level which is 5% below the

regulated output value, an internal counter starts. After

device turns off.

The POR output will go low whenever the output voltage

drops below 7.5% of its regulated value for longer than

approximately 30

µs, signaling an out-of-regulation condi-

tion. In shutdown the POR output is pulled low even if the

regulator’s output is held up by an external source.

Run/Soft Start Function

The RUN/SS pin is a dual purpose pin which provides the

soft start function and a means to shut down the LTC1433/

LTC1434. Soft start reduces input surge currents by

providing a gradual ramp-up of the internal current limit.

Power supply sequencing can also be accomplished using

this pin.

An internal 3

µA current source charges up an external

the LTC1433/LTC1434 begins operating. As the voltage

on RUN/SS continues to ramp from 1.3V to 2.4V the

internal current limit is also ramped at a proportional

linear rate. The current limit begins at approximately

2.4V). The output voltage thus ramps up slowly, charging

the output capacitor while input surge currents are re-

duced. If RUN/SS has been pulled all the way to ground

there is a delay of approximately 0.5s/

µF before starting,

followed by a like time to reach full current.

By pulling the RUN/SS pin below 1.3V, the LTC1433/

LTC1434 are put in low current shutdown. This pin can be

driven directly from logic as shown in Figure 8. Diode D1

up slowly providing the soft start function. This diode can

be deleted if soft start is not needed. The RUN/SS pin has

an internal 6V Zener clamping the voltage on this pin (see

Functional Diagram).

1433/34 F08

D1

RUN/SS

RUN/SS

Figure 8. RUN/SS Pin Interfacing

Phase-Locked Loop and Frequency Synchronization

The LTC1434 has an internal voltage-controlled oscilla-

tor and phase detector comprising a phase-locked loop.

This allows the MOSFET turn-on to be locked to the rising

edge of an external source. The frequency range of the

voltage-controlled oscillator is

±30% around the center

expression (assuming the phase-locked loop is locked,

C

Frequency

kHz









()





















206 10

11

4

.

–

Instead of using the above expression, Figure 2 graphi-

cally shows the relationship between the oscillator fre-

conditions at the PLL LPF pin.

The phase detector used is an edge sensitive digital type

which provides zero degrees phase shift between the

external and internal oscillators. This type of phase detector

will not lock up on input frequencies close to the harmonics

equal to the capture range,

The output of the phase detector is a pair of complemen-

tary current sources charging or discharging the external

filter network on the PLL LPF pin. The relationship

between the voltage on the PLL LPF pin and operating

frequency is shown in Figure 9. A simplified block diagram

is shown in Figure 10.