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AN827 APPLICATION NOTE

Pin 3 OVP (Overvoltage protection input)

The overvoltage protection trip level is determined by the voltage divider across the output bulk capacitor, C6.

The resistor values R11, R21 and R22 are calculated as follows:

Where

The overvoltage limit selection is dependent upon the voltage rating of the output bulk capacitor (450VDC) and

the power mosfet (500BVdss). Care must be taken that the level is not set too low, thus causing false tripping

of the OVP.

Pin 4 IAC (AC current input)

This pin must be connected through resistors R1 and R2 to the rectified line to drive the multiplier with a current

IIAC proportional to the instantaneous line voltage as shown below:

scribed in section Pin 8 (MULTOUT).

Pin 5 CA-OUT (Current amplifier output)

The current amplifier output delivers its signal to the PWM comparator. An external network defines the suitable

loop gain to process the multiplier output and the inductor current signals. To avoid oscillation problems, the

maximum inductor downslope (Vout/L) must be lower than the oscillator ramp-slope (Vsrp*fsw). The current am-

plifier high frequency gain can be described as follows:

Where:

Thus, use R14=R16=2.7k, and R15=36K.

To define the value of the compensation capacitor, C9, it is useful to consider the open loop current gain, defined

by the ratio of the voltage across the sense resistor and the current amplifier output voltage. The crossover fre-

quency is given by the following equation:

R21

R22

+

R11

-----------------------------

V

out

∆V

out

+

V

ref

---------------------------------

1

400

47

+

5.1

-----------------------

1

909k

909k

+

21k

---------------------------------

=

–

=

–

=

I

()

V

i npk

R1

R2

+

----------------------

288

⋅

806k

806k

+

---------------------------------

77

µA

==

=

I

()

2264

⋅

806k

806k

+

---------------------------------

231

µA

==

G

ca

R15

R14

-----------

1

V

sr p

f

SW

L

⋅⋅

V

out

R

sens e

⋅

----------------------------------

5.0 80k 0.5m

⋅⋅

400 0.033

⋅

-----------------------------------------

=

≤

+

=

f

c

f

sw

2

π

⋅

-----------

80k

2

π

⋅

-----------

12.7kHz

===