Converter's protection schemes, overcurrent, overvoltage

AN2530

8/13

voltage can be used to set the opamp's inverting input (pin 6) to 0.4 V through the R6 & R7

divider network.

The inverting input of internal opamp is set to 0.4 V, so 0.4 V/2 A = 0.2

Ω.

A set of 1

Ω/0.25 W resistances was chosen to be readily available and by paralleling them

we get 0.25

Ω/1 W, which "generates" 0.25 Ω*2 A = 0.5 V at maximum current.

Then, we have to choose the values for R17 & R18 (a resistor divider network) to get the

0.4 V at Pin 7 (OPin+), R17 = 39 k

Ω and R18 = 10 kΩ in our case.

Concerning feedback, regulation is achieved by means of varying the driver's frequency. A

heavier load determines a lower operational frequency and the contrary is true for a lighter

R11 defines the maximum operational frequency and R10, R12 and optocoupler's internal

resistance (that varies according to the current supplied to the load) set the actual operating

frequency.