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NCP1253 Datasheet(PDF) 14 Page - ON Semiconductor

Part No. NCP1253
Description  Current-Mode PWM Current-Mode PWM Power Supplies
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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NCP1253 Datasheet(HTML) 14 Page - ON Semiconductor

 
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NCP1253
http://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:
Sramp +
Vramp,peakDmax
TSW
+
2.5
0.8
15m
+ 133 kV sor133 mV ms
(eq. 6)
In our flyback design, let’s assume that our primary
inductance Lp is 770 mH, and the SMPS delivers 19 V with
a Np:Ns ratio of 1:0.25. The off−time primary current slope
Sp is thus given by:
SP +
Vout ) Vf
Ns
NP
LP
+
(19 ) 0.8)
4
770m
+ 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:
Ssense + SPRsense + 103k 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/
ms, the divider ratio (divratio) between Rcomp and
the internal 20 k
W resistor is:
divratio +
17m
133m
+ 0.127
(eq. 9)
The series compensation resistor value is thus:
Rcomp + Rrampdivratio + 20k
0.127 [ 2.5 kW (eq. 10)
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.


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