UCC28130/1/2/3/4/5Q1

LOW POWER BICMOS CURRENT MODE PWM

SGLS245D − MAY 2004 − REVISED APRIL 2008

7

POST OFFICE BOX 655303

detailed descriptions (continued)

RC

RC is the oscillator timing pin. For fixed frequency operation, set timing capacitor charging current by connecting

a resistor from REF to RC. Set frequency by connecting timing capacitor from RC to GND. For best perfomance,

keep the timing capacitor lead to GND as short and direct as possible. If possible, use separate ground traces

for the timing capacitor and all other functions.

The frequency of oscillation can be estimated with the following equations:

UCC2813−0 1 2 4: F

+ 1.5

R

C

UCC2813−3 UCC2813−5 : F

+ 1.0

R

C

where frequency is in Hz, resistance is in ohms, and capacitance is in farads. The recommended range of the

timing resistors is between 10 k

Ω and 200 kΩ and the timing capacitor is 100 pF to 1000 pF. Never use a timing

resistor less than 10 k.

An electrolytic capacitor may also be used in addition to the ceramic capacitor.

voltage reference (REF)

REF is the voltage reference for the error amplifier and also for many other functions on the IC. REF is also used

as the logic power supply for high-speed switching logic on the IC.

This means that REF can be used as a logic output indicating power system status. It is important for reference

stability that REF is bypassed to GND with a ceramic capacitor as close to the pin as possible. An electrolytic

capacitor may also be used in addition to the ceramic capacitor. A minimum of 0.1-

µF ceramic capacitor is

required. Additional REF bypassing is required for external loads greater than 2.5 mA on the reference.

To prevent noise problems with high-speed switching transients, bypass REF to ground with a ceramic capacitor

close to the IC package.

I

F.

(1)

(2)