ncp1239f ON Semiconductor, ncp1239f Datasheet - Page 30

• Manufacturer Part Number: ncp1239f
• Description: Lowstandby High Performance Pwm Controller
• Manufacturer: ON Semiconductor

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