4

UCC1808-1/-2

UCC2808-1/-2

UCC3808-1/-2

f

RC

141

.

where frequency is in Hertz, resistance in Ohms, and ca-

pacitance in Farads. The recommended range of timing

resistors is between 10k

Ω and 200kΩ and range of tim-

ing capacitors is between 100pF and 1000pF. Timing re-

sistors less than 10k

Ω should be avoided.

For best performance, keep the timing capacitor lead to

GND as short as possible, the timing resistor lead from

VDD as short as possible, and the leads between timing

components and RC as short as possible. Separate

ground and VDD traces to the external timing network

are encouraged.

VDD: The power input connection for this device. Al-

though 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 fre-

quency and the MOSFET gate charge (Qg), average

OUT current can be calculated from

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.

PIN DESCRIPTIONS (cont.)

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 ½ the oscillator frequency, with guaranteed duty cycle of <

50% for both outputs.

Figure 1. Block diagram for oscillator.

UDG-97009

A 200kHz push-pull application circuit with a full wave rec-

75W maximum and is electrically isolated from the input.

Since the UCC3808 is a peak current mode controller the

2N2222A emitter following amplifier (buffers the CT wave-

form) 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 con-

troller supply is a series RC for startup, 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 UC3965 Precision

Reference with Low Offset Error Amplifier. Small signal

compensation with tight voltage regulation is achieved

using this part on the secondary side. Many choices ex-

ist for the output inductor depending on cost, volume,

and mechanically strength. Several design options are

iron powder, molypermalloy (MPP), or a ferrite core with

an air gap as shown here. The main power transformer

is a low profile design, EFD size 25, using Magnetics

Inc. P material which is a good choice at this frequency

and temperature. The input voltage may range from 36V

dc to 72V dc. Refer to application note U-170 for addi-

tion design information.

APPLICATION INFORMATION