MC33368 View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MC33368

HIGH VOLTAGE GREENLINE POWER FACTOR CONTROLLER

Motorola => Freescale 

MC33368

Output Switching Frequency Clamp

In normal operation, the MC33368 operates the boost

inductor in the critical mode. That is, the inductor current

ramps to a peak value, ramps down to zero, then immediately

begins ramping positive again. The peak current is

programmed by the multiplier output within the IC. As the

input voltage haversine declines to near zero, the output

switch on–time becomes constant, rather than going to zero

because of the small integrated dc voltage at Pin 5 caused by

C2, R3 and R5. Because of this, the average line current

does not exactly follow the line voltage near the zero

crossings. The Output Switching Frequency Clamp remedies

this situation to improve power factor and minimize EMI

generated in this operating region. The values of R10 and

C7, as shown in Figure 15, program a minimum off–time in

the frequency clamp which overrides the zero current detect

signal, forcing a minimum off–time. This allows discontinuous

conduction operation of the boost inductor in the zero

crossing region, and the average line current more nearly

follows the voltage. The Output Switching Frequency Clamp

function can be disabled by connecting the FC input, Pin 13,

to the VCC supply Pin 12.

For best results, the minimum off–time, determined by

the values of R10 and C7, should be chosen so that

ts(min) = t(on) + t(off)fc. Output drive is inhibited when the

voltage at the frequency clamp input is less than 2.0 V. When

the output drive is high, C7 is discharged through an internal

100 µA current source. When the output drive switches low,

C7 is charged through R10. The drive output is inhibited until

+ * ƪ * ǒ Ǔƫ the voltage across C7 reaches 2.0 V, establishing a minimim

off–time where:

t(off)fc

R10 C7 loge 1

2

VCC

Output

The IC contains a CMOS output driver that was

specifically designed for direct drive of power MOSFETs. The

Gate Output is capable of up to ±1500 mA peak current with

a typical rise and fall time of 50 ns with a 1.0 nF load.

Additional internal circuitry has been added to keep the Gate

Output in a sinking mode whenever the Undervoltage

Lockout is active. This characteristic eliminates the need for

an external gate pull–down resistor. The totem–pole output

has been optimized to minimize cross–conduction current

during high speed operation.

MOTOROLA ANALOG IC DEVICE DATA

9

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