TL5002

PULSEWIDTHMODULATION CONTROL CIRCUIT

SLVS304A − SEPTEMBER 2000 − REVISED AUGUST 2002

2

POST OFFICE BOX 655303

detailed description

voltage reference

error amplifier

The error amplifier compares a sample of the dc-to-dc converter output voltage to an external reference voltage

and generates an error signal for the PWM comparator. The dc-to-dc converter output voltage is set by selecting

the error-amplifier gain (see Figure 1), using the following expression:

To PWM

Comparator

4

VI(FB)

3

+

−

R2

R1

COMP

INV

Compensation

Network

TL5002

GND

8

5

NI

Vref

Figure 1. Error-Amplifier Gain Setting

The error-amplifier output is brought out as COMP for use in compensating the dc-to-dc converter control loop

for stability. Because the amplifier can only source 45

µA, the total dc load resistance should be 100 kΩ or more.

oscillator/PWM

and GND. Acceptable resistor values range from 15 k

Ω to 250 kΩ. The oscillator frequency can be determined

by using the graph shown in Figure 5.

The oscillator output is a triangular wave with a minimum value of approximately 0.7 V and a maximum value

of approximately 1.3 V. The PWM comparator compares the error-amplifier output voltage and the DTC input

voltage to the triangular wave and turns the output transistor off whenever the triangular wave is greater than

the lesser of the two inputs.

dead-time control (DTC)

DTC provides a means of limiting the output-switch duty cycle to a value less than 100 %, which is critical for

equal to the current at the oscillator timing terminal, RT. Connecting a resistor between DTC and GND generates

source impedance of RT is relatively high (1250

accomplished using the following equation and the voltage limits for the frequency in question as found in