TL1454A, TL1454AY

DUALCHANNEL PULSEWIDTHMODULATION (PWM)

CONTROL CIRCUIT

SLVS423 A− MAY 2002 − REVISED SEPTEMBER 2002

5

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

error amplifier

R1 and R2 should be tight-tolerance (

±1% or better) devices with low and/or matched temperature coefficients

to minimize output voltage errors. A device with a

±5% tolerance is suitable for R3.

_

+

IN −

IN +

R2

COMP

To PWM

VO

Compensation

Network

REF

R1

R3

Converter

Output

Figure 2. Sense Divider/Error Amplifier Configuration for Converters with Negative Outputs

Figure 2 shows the divider network and error-amplifier configuration for negative output voltages. In general,

the comments for positive output voltages also apply for negative outputs. The output voltage is given by:

V

O +*

R

1

V

ref

R

2

The design procedure for choosing the resistor value is to select a convenient value for R2 (instead of R3 in

the procedure for positive outputs) and calculate R1 and R3 using:

R

1 +*

R

2

V

O

V

ref

R

3 +

R

1

R

2

R

1 )

R

2

Values in the 10-k

Ω to 20-kΩ range work well for R2. R3 can be omitted and the noninverting amplifier connected

to ground in applications where the output voltage tolerance is not critical.

oscillator

The oscillator frequency can be set between 50 kHz and 2 MHz with a resistor connected between RT and GND

and a capacitor between CT and GND (see Figure 3). Figure 6 is used to determine RT and CT for the desired

operating frequency. Both components should be tight-tolerance, temperature-stable devices to minimize

frequency deviation. A 1% metal-film resistor is recommended for RT, and a 10%, or better, NPO ceramic

capacitor is recommended for CT.

RT

CT

RT

CT

TL1454A

21

Figure 3. Oscillator Timing