UCC2808A1Q1, UCC2808A2Q1

LOW POWER CURRENT MODE PUSHPULL PWM

SGLS183A − AUGUST 2003 − REVISED APRIL 2008

6

POST OFFICE BOX 655303

pin assignments (continued)

SQ

R

4

OSCILLATOR

OUTPUT

0.2 V

VDD

2

RC

(APPROXIMATE

FREQUENCY)

FREQUENCY =

1.41

RC

UDG-00095

Figure 1. Block Diagram for Oscillator

NOTE A: The oscillator generates a sawtooth waveform on RC. During the RC rise time, the output stages alternate on time, but both stages are

off during the RC fall time. The output stages switch a 1/2 the oscillator frequency, with ensured duty cycle of < 50% for both outputs.

VDD: The power input connection for this device. Although quiescent VDD current is very low, total supply

current will be higher, depending on OUTA and OUTB current, and the programmed oscillator frequency. Total

VDD current is the sum of quiescent VDD current and the average OUT current. Knowing the operating

frequency and the MOSFET gate charge (Qg), average OUT current can be calculated from:

I

OUT +

Qg

F, where F is frequency

To prevent noise problems, bypass VDD to GND with a ceramic capacitor as close to the chip as possible along

with an electrolytic capacitor. A 1-

µF decoupling capacitor is recommended.

APPLICATION INFORMATION

5 V at 50 W maximum and is electrically isolated from the input. Since the UCC2808A is a peak-current-mode

controller the 2N2907 emitter following amplifier (buffers the CT waveform) provides slope compensation which

is necessary for duty ratios greater than 50%. Capacitor decoupling is very important with a single ground IC

controller, and a 1

µF is suggested as close to the IC as possible. The controller supply is a series RC for start-up,

paralleled with a bias winding on the output inductor used in steady state operation.

Isolation is provided by an optocoupler with regulation done on the secondary side using the TL431 adjustable

precision shunt regulator. Small signal compensation with tight voltage regulation is achieved using this part

on the secondary side. Many choices exist for the output inductor depending on cost, volume, and mechanicall

strength. Several design options are iron powder, molypermalloy (MPP), or a ferrite core with an air gap as

shown here. The main power transformer has a Magnetics Inc. ER28 size core made of P material for efficient

operation at this frequency and temperature. The input voltage may range from 36 V dc to 72 V dc.