KA7525B

11

The equation below describes the relationship between multiplier output and inputs.

Vmo = K

× Vm1 × (Vm2-Vref)

K

: Multiplier gain

Vm1: Voltage at Pin 3

Vm2: Error amp output voltage

Vmo: Multiplier output voltage

Current Sense Comparator

The current sense comparator RS latch configuration used ensures that only a single pulse appears at the drive output during a

given cycle. MOSFET drain current is converted to voltage using an external sense resistor in series with the external power

MOSFET. When sense voltage exceeds the threshold set by the multiplier output, the current sense comparator terminates the

gate drive to the MOSFET and resets the PWM latch. The latch insures that the output remains in a low state after the MOS-

FET drain current falls back to zero.

The peak inductor current under normal operating conditions is controlled by the multiplier output, Vmo.

Abnormal operating conditions occur during preconverter start-up at extremely high line or if output voltage sensing is lost.

Under these conditions, the multiplier output and current sense threshold will be internally clamped to 1.24V. Therefore, the

maximum peak switch current is limited to:

Ipk(max) = 1.24V / Rsense

Internal R,C filter has been included to attenuate any high frequency noise that may be present on the current waveform. These

circuit block eliminate the need for an external RC filter otherwise required for proper operation of the circuit.

+

-

+

-

+

+

Vref

2

3

1.24V

Current Sense

Comparator

Error Amp.

Vmo

Vm1

Vm2

INV

MULT

EA OUT

+

-

1

Figure 3.1 Multiplier Block

4

CS

LEB Delay(KA7525)

4

cs

1.24V

Multiplier

Output

Rsense

eliminated

Low Pass Filter can be

Figure 4.1 Current Sense Circuit

+

-

RC filter