7

UCC15701/2

UCC25701/2

UCC35701/2

Transitioning From UCC3570 To UCC35701

The UCC35701/2 is an advanced version of the popular,

low power UCC3570 PWM.

Significant improvements

were made to the IC’s oscillator and PWM control sec-

tions to enhance overall system performance. All of the

key attributes and functional blocks of the UCC3570

were maintained in the UCC35701/2. A typical applica-

tion using UCC3570 and UCC35701/2 is shown in Fig. 6

for comparison.

The advantages of the UCC35701/2 over the UCC3570

are as follows.

Improved oscillator and PWM control section.

A precise maximum volt-second clamp circuit. The

UCC3570 has a dual time base between oscillator and

feedforward circuitry. The integated time base in

UCC35701/2 improves the duty cycle clamp accuracy,

providing better than

± 5% accurate volt- second

clamp over full temperature range.

Separately programmable oscillator timing resistor

(RT) and capacitor (CT) circuits provide a higher

degree of versatility.

An independent SYNC input pin for simple external

synchronization.

A smaller value filter capacitor (0.1

mF) can be used

with the enhanced reference voltage.

UCC35701/2 is pin to pin compatible to UCC3570 but is

not a direct drop-in replacement for UCC3570 sockets.

The changes required to the power supply printed circuit

board of for existing UCC3570 designs are minimal. For

conversion, only one extra resistor to set the volt-second

clamp needs to be added to the existing PC board lay-

outs. In addition, some component values will need to

be changed due to the functionality change in of four of

the IC pins.

The Pinout Changes from UCC3570 are as follows.

Pin 7 was changed from SLOPE to RT (for timing

resistor)

Pin 8 was changed from ISET to VSCLAMP (requiring

one additional resistor from pin 9 to VREF)

Pin 10 was changed from RAMP to CT (single timing

capacitor)

Pin 11 was changed from FREQ to SYNC (input only)

Additional Information

Please refer to the following two Unitrode application

topics on UCC3570 for additional information.

[1] Application Note U-150, Applying the UCC3570 Volt-

age-Mode PWM Controller to Both Off-line and DC/DC

Converter Designs by Robert A. Mammano

[2] Design Note DN-62, Switching Power Supply Topol-

ogy,

Voltage

Mode

vs.

Current

Mode

by

Robert

Mammano

APPLICATION INFORMATION (cont.)

VSCLAMP

CT

FEEDBK

SOFT START

HIGH DC

LOW DC

ZERO DC

SOFTST

V-S CLAMP

SOFT STOP

Figure 1. Timing diagram for PWM action with forward, soft start and volt-second clamp.

TYPICAL WAVEFORMS

UDG-98207