NCP1239
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30
Figure 50. NCP1239F
An (averaged) portion of the input voltage is applied to the brown−out pin. A current source proportional to this voltage, flows through an
external resistor Rcomp to form an offset proportional to the (average) input voltage. Rcomp should be selected so that the offset compen-
sates the overcurrent sensed by the current sensing resistor Rsense.
+
−
CLK
to
Current Sense
Comparator
Rcomp
+
5
9
10
Current Sense
to Brown−Out
Comparator
Brown−Out
Over Power
Limit
Rramp
Rsense
V
comp
0.5V
Vdd
V
pin5
Vin
Rbo1
Rbo2
Cbo
V
comp
= k*R
comp
*V
pin5
80
A/V*V
pin5
S
R
Q
Q
LEB
PWM Latch
Maximum Duty Cycle Limitation (NCP1239V)
Pin 9 sources a 55
A current. By placing a resistor
between this pin and ground, one builds a voltage that forces
the maximum on−time. Practically the Pin 9 voltage is
compared to the positive ramp of the internal oscillator and
the power switch is allowed to be on, only when the ramp is
below V
pin9
.
Then the maximum on−time is given by:
(ton)max
+
Cosc
@
Vpin9
Iosc
where Cosc and Iosc are respectively the capacitor and the
charging current of the oscillator.
V
pin9
being the product of the Pin 9 current by the Pin 9
resistance (R
pin9
– external resistor connected to Pin 9),
results in:
(ton)max
+
Cosc
@
IDmax
Iosc
@
Rpin9
, where I
Dmax
is the
Pin 9 current source.
One can deduct the maximum duty cycle (Dmax) by
dividing by the period T:
Dmax
+
Cosc
@
IDmax
Iosc
@
T
@
Rpin9
+
KDmax
@
Rpin9
where
KDmax
+
Cosc
@
IDmax
Iosc
@
T
.
K
Dmax
is specified within the parameters’ table. Please
note that Cosc and Iosc are the internal capacitor and current
(respectively), that set the switching period (T). Hence,
ƪ
ǒ
Cosc
Iosc
@
T
Ǔ
ƫ
is a constant and K
Dmax
is independent of the
switching frequency.
In the NCP1239F, the maximum duty−cycle is fixed (80%
typically).
