SC4810B Datasheet, PDF (12/24 Pages) Semtech Corporation – High Performance Current Mode PWM Controller with Complementary Output, Programmable Delay

POWER MANAGEMENT
Application Information (Cont.)
3V
VDMAX
VRCT
Clock Signal of SC4810
0V
t
DMAX
0V
t
Fig. 5 Illustration for DMAX
SC4810B/E
Voltage waveform on RAMP pin
VRAMP
3V
0V
OUT1
t
Maximum OUT1 of SC4810
0V
t
Fig. 6 Illustration of the programmable limits for power
transformer voltage-second product
In this application, V_DMAX was designed to be 2.8V. So the
D_MAX = 90%.
V_IN
Limit for Power Transformer
Voltage Second Product
R
The SC4810 also features programmable limits for power
transformer voltage-second product. As illustrated in Fig.
6 and Fig. 7 RAMP pin is charged up via a resistor R from
the input line voltage. The capacitor C will be discharged
via an internal FET shorted to ground and the output
OUT1 will be pulled low whenever the voltage on RAMP
pin hits 3V. By adjusting the values of the resistor R and
the value of the capacitor C, the maximum voltage-sec-
ond product imposed on the power transformer is pre-
set. The maximum voltage-second product limitation helps
prevent saturation of the power transformer.
RAMP
C
Fig. 7 Illustration for Maximum voltage-
second product on the power transformer
The selection of the R and C should consider the maxi-
mum voltage rating of the main switching FET. In this
application, the voltage rating of SI4488 is 150V. Since
Vin*D/(1-D) = 150V, D = 0.8 for low line 36V. So to get
80% at low line, R = 165kOhms and C = 220pF were
selected using volt-second product equation:
3 • Ramp • Cramp.
 2006 Semtech Corp.
12
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