NCP1581 Datasheet, PDF (9/14 Pages) ON Semiconductor – High Frequency Synchronous Step Down PWM Controller for Tracking Applications

High Side
Logic
Signal
Low Side
Logic
Signal
NCP1581
t_DEAD
t_DEAD
HDRV
V_ADT
LDRV
V_ADT
R _DSmax
High Side
MOSFET
R _DS
RDS _{(ON )min}
R _DSmax
Low Side
MOSFET
R _DS
RDS _{(ON )min}
t _{d (on )}
tr
tf
t _{d (off )}
tr
tf
t _{d (on )}
t _{d (off)}
Figure 11. MOSFETs Timing Diagram
Soft Start
The soft start time is set by a capacitor connected between
the SS pin and ground. This function is used for controlling
the output voltage slope and limiting start-up currents. The
start-up sequence initiates when the Power On Ready (POR)
internal signal rises to logic level high. That means the
supply voltage and V_P/EN voltage are over the set thresholds.
The soft start capacitor is charged by a 22 mA current source.
If POR is low, the SS pin is internally pulled to GND, which
means that the NCP1581 is in a shutdown state. The SS pin
voltage (0 V to 2 V) controls the internal current source
(64 mA to 0 mA) with a negative linear characteristic. This
current source injects current into the resistor (25 kW)
connected between the FB pin and the negative input of the
error amplifier and into the external feedback resistor
network. Voltage drop on these resistors is over 1.6 V, which
is enough to force the error amplifier into a negative
saturation state and to block switching.
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