NCP1253

www.onsemi.com

14

Rsense

Rcomp

20k

0V

2.5 V

CS

+

−

L.E.B

from FB

setpoint

latch

reset

ON

Figure 29. Inserting a Resistor in Series with the Current Sense Information Brings Ramp Compensation and

Stabilizes the Converter in CCM Operation

In the NCP1253 controller, the oscillator ramp features a

2.5 V swing. If the clock operates at a 65 kHz frequency,

then the available oscillator slope corresponds to:

V

T

SW

+

2.5

0.8

15

m

+ 133 kV sor133 mV ms

(eq. 6)

In our flyback design, let’s assume that our primary

mH, and the SMPS delivers 19 V with

S

N

P

L

P

+

(19

) 0.8)

4

770

m

+ 103 kA s

(eq. 7)

Given a sense resistor of 330 m

W, the above current ramp

turns into a voltage ramp of the following amplitude:

0.33

+ 34 kV sor34mV ms

(eq. 8)

If we select 50% of the downslope as the required amount

of ramp compensation, then we shall inject a ramp whose

slope is 17 mV/

ms. Our internal compensation being of

133 mV/

the internal 20 k

W resistor is:

divratio

+

17m

133m

+ 0.127

(eq. 9)

The series compensation resistor value is thus:

0.127

A resistor of the above value will then be inserted from the

sense resistor to the current sense pin. We recommend

adding a small 100 pF capacitor, from the current sense pin

to the controller ground for improved noise immunity.

Please make sure both components are located very close to

the controller.