GENERAL INFORMATION

Overview

Programming the Operating Frequency

105

.

0

C

R

1

f

RC

RC

SW

´

´

=

A

750

R

V

RC

IN

m

£

(1)

Synchronizing the Oscillator

TPS40200

The TPS40200 is a non-synchronous controller with a built in 200-mA driver designed to drive high speed

P-channel FETS up to 500 kHz. Small size combined with complete functionality makes the part both versatile

and easy to use.

The controller uses a low-value current-sensing resistor in series with the input voltage and the power FET’s

source connection to detect switching current. When the voltage drop across this resistor exceeds 100 mV, the

part enters a hiccup fault mode at about 2% of the operating frequency.

The device uses voltage feedback to an error amplifier that is biased by a precision 700-mV reference.

Feed-forward compensation from the input keeps the PWM gain constant over the full input voltage range,

eliminating the need to change frequency compensation for different input voltages.

The part also incorporates a soft-start feature where the output follows a slowly rising soft-start voltage,

preventing output-voltage overshoot.

pin to VDD and a capacitor attached from the RC pin to ground. This connection and the two oscillator

comparators inside the device, are shown in Figure 26. The oscillator frequency can be calculated from the

following equation:

Where :

• f

• R

• C

(shown in Figure 26) is discharging the timing capacitor. This condition may be expressed by Equation 1.

Figure 26 shows the functional diagram of the oscillator. When synchronizing the oscillator to an external clock,

the RC pin must be pulled below 150 mV for 20 ns or more. The external clock frequency must be higher than

the free running frequency of the converter as well. When synchronizing the controller, if the RC pin is held low

for an excessive amount of time, erratic operation may occur. The maximum amount of time that the RC pin

should be held low is 50% of a nominal output pulse, or 10% of the period of the synchronization frequency

whichever is less.

Under circumstances where the input voltage is high and the duty cycle is less than 50%, a Schottky diode

connected from the RC pin to an external clock may be used to synchronize the oscillator. The cathode of the

diode is connected to the RC pin. The trip point of the oscillator is set by an internal voltage divider to be 1/10 of

the input voltage. The clock signal must have an amplitude higher than this trip point. When the clock goes low, it

allows the reset current to restart the RC ramp, synchronizing the oscillator to the external clock. This provides a

simple, single-component method for clock synchronization.

10

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