CS-52845D14(1997) Ver la hoja de datos (PDF) - Cherry semiconductor

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Fabricante

CS-52845D14
(Rev.:1997)

Current Mode PWM Control Circuit with 50% Max Duty Cycle

Cherry semiconductor 

VOSC

OSC

RESET

Toggle

F/F Output

EA Output

Switch

Current

VCC

IO

VO

Figure 2: Timing Diagram

Vupper

Vlower

ton

toff

tc

td

ton=tc

toff=tc+2td

Figure 3: Duty Cycle parameters.

Circuit Description:: continued

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 = 8.3A,

then

tc Å 0.5534RTCT

( ) td = RTCT ln 2.3 - 0.0083 RT

4.0 - 0.0083 RT

For better accuracy RT should be ³10k½.

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 5k½ potentiometer are used to sample

the oscillator waveform and apply an adjustable ramp to

Sense.

5

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