6

UC1842A/3A/4A/5A

UC2842A/3A/4A/5A

APPLICATIONS DATA (cont.)

Open-Loop Laboratory Test Fixture

Oscillator Section

Oscillator Frequency vs Timing Resistance

Maximum Duty Cycle vs Timing Resistor

High peak currents associated with capacitive loads necessi-

tate careful grounding techniques. Timing and bypass capaci-

tors should be connected close to pin 5 in a single point

ground. The transistor and 5k potentiometer are used to sam-

ple the oscillator waveform and apply an adjustable ramp to

pin 3.

Slope Compensation

A fraction of the oscillator ramp can be resistively

summed with the current sense signal to provide slope

compensation for converters requiring duty cycles over

50%.

Note that capacitor, C, forms a filter with R2 to suppress

the leading edge switch spikes.