ON Semiconductor NCP1239FDR2G Reference Manual page 30

CLK
S
Q
Q
R
+
−
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.
Maximum Duty Cycle Limitation (NCP1239V)
Pin 9 sources a 55 mA 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 +
where Cosc and Iosc are respectively the capacitor and the
charging current of the oscillator.
Vdd
80mA/V*V
pin5
to Brown−Out
Comparator
LEB
+
0.5V
to
Current Sense
Comparator
Figure 50. NCP1239F
Cosc @ V pin9
Iosc
http://onsemi.com
NCP1239
V
pin5
5
Brown−Out
V
comp
9
Over Power
Limit
10
Current Sense
V being the product of the Pin 9 current by the Pin 9
pin9
resistance (R
pin9
results in:
Cosc @ I Dmax
(ton)max +
Pin 9 current source.
One can deduct the maximum duty cycle (Dmax) by
dividing by the period T:
Cosc @ I Dmax
Dmax +
K Dmax +
where
K is specified within the parameters' table. Please
Dmax
note that Cosc and Iosc are the internal capacitor and current
(respectively), that set the switching period (T). Hence,
Cosc
is a constant and K
Iosc @ T
switching frequency.
In the NCP1239F, the maximum duty−cycle is fixed (80%
typically).
30
Rbo1
Rbo2
Cbo
= k*R *V
comp pin5
Rcomp
V
comp
Rramp
– external resistor connected to Pin 9),
@ R pin9
, where I
Iosc
@ R pin9 + K Dmax @ R pin9
Iosc @ T
Cosc @ I Dmax
.
Iosc @ T
is independent of the
Dmax
Vin
Rsense
is the
Dmax