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CS52843ED14

Current Mode PWM Control Circuit

Cherry semiconductor 

Circuit Description

When the power supply sees a sudden large output cur-

rent increase, the control voltage will increase allowing the

duty cycle to momentarily increase. Since the duty cycle

tends to exceed the maximum allowed to prevent trans-

former saturation in some power supplies, the internal

oscillator waveform provides the maximum duty cycle

clamp as programmed by the selection of oscillator timing

components.

VOSC

OSC

RESET

Toggle

F/F Output

EA Output

Switch

Current

VCC

Setting the Oscillator

The times Tc and Td can be determined as follows:

( ) tc = RTCT ln

VREF - VLOWER

VREF - VUPPER

( ) td = RTCT ln

VREF - IdRT - VLOWER

VREF - IdRT - VUPPER

Substituting in typical values for the parameters in the

above formulas:

VREF = 5.0V, VUPPER = 2.7V, VLOWER = 1.0V, Id = 8.3mA,

then

tc ≈ 0.5534RTCT

IO

VO

Figure 2: Timing Diagram

VREF

RT

OSC

CT

Gnd

( ) td = RTCT ln

2.3 - 0.0083 RT

4.0 - 0.0083 RT

For better accuracy RT should be ≥10kΩ.

Grounding

High peak currents associated with capacitive loads neces-

sitate careful grounding techniques. Timing and bypass

capacitors should be connected close to Gnd in a single

point ground.

The transistor and 5kΩ potentiometer are used to sample

the oscillator waveform and apply an adjustable ramp to

Sense.

Vupper

Vlower

tc

td

Sawtooth Mode

LARGE RT (≈10kΩ)

VOSC

Triangular Mode

SMALL RT (≈700kΩ)

Internal Clock

VREF

Internal Clock

Figure 3: Oscillator Timing Network and Parameters

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