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KA7525B View Datasheet(PDF) - Fairchild Semiconductor

Part Name
Description
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KA7525B
Fairchild
Fairchild Semiconductor Fairchild
KA7525B Datasheet PDF : 18 Pages
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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
KA7525B
Current Sense
Comparator
CS
+
-
4
+
1.24V
Vref
3
MULT
Vm1
Vmo
Error Amp.
- Vm2 +
+
+
-
1
INV
2
EA OUT
Figure 3.1 Multiplier Block
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.
LEB Delay(KA7525R)C filter
cs
+
4
-
Rsense
1.24V
Multiplier
Output
Low Pass Filter can be
eliminated
Figure 4.1 Current Sense Circuit
11
 

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