NCP1246 ONSEMI [ON Semiconductor], NCP1246 Datasheet - Page 27

• Manufacturer Part Number: NCP1246
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Clamped Driver
28 V, but most of the MOSFETs that will be connected to the
DRV pin cannot accept more than 20 V on their gate. The
driver pin is therefore clamped safely below 16 V. This
driver has a typical capability of 500.0 mA for source current
and 800 mA for sink current.
CurrentïMode Control With Slope Compensation and
SoftïStart
the FB voltage sets the peak current flowing in the
inductance and the MOSFET. This is done through a PWM
comparator: the current is sensed across a resistor and the
The supply voltage for the NCP1246 can be as high as
NCP1246 is a currentïmode controller, which means that
Figure 59. Skip Mode Timing Diagram
http://onsemi.com
NCP1246
27
resulting voltage is applied to the CS pin. It is applied to one
input of the PWM comparator through a 250 ns LEB block.
On the other input the FB voltage divided by 5 sets the
threshold: when the voltage ramp reaches this threshold, the
output driver is turned off. The maximum value for the
current sense is 0.7 V, and it is set by a dedicated comparator.
off and starts – or restarts – when V
softïstart is applied: the current sense setpoint is increased
by 15 discrete steps from 0 (the minimum level can be
higher than 0 because of the LEB and propagation delay)
until it reaches V
the FB loop imposes a setpoint lower than the one imposed
by the softïstart (the two comparators outputs are OR’ed).
Each time the controller is starting, i.e. the controller was
ILIM
(after a duration of t
CC
reaches V
SSTART
), or until
CC(on)
, a