NCP81031
http://onsemi.com
7
Figure 3. Gate Timing Diagram
1V
1V
Switching Frequency
Connecting a resistor from ROSC/EN to external voltage
source V
pu
will configure the frequency. Normal range
would be 100 kHz to 1 MHz. With no resistor connected to
the pin, the oscillator frequency is 200 kHz. The switching
frequency will follow the relationship:
F
SW
+ 200 kHz *
V
pu
* 1.240
R
OSC
@ 10
kHz
mA
(eq. 1)
When R
osc
= infinity (no resistor connected), F
sw
=
200 kHz; when V
pu
= ground, the frequency programmed
will be higher than 200 kHz.
Soft−Start
Soft−Start will begin if VCC, VCCP are both above their
UVLO threshold and EN pin is set free. IC initially waits a
fixed delay time and then ramping−up the reference in 1024
clock cycles in closed−loop regulation. After digital soft
start, PGOOD signal will be released with three clock cycles
delay.
Protection active during soft−start:
• Overvoltage Protection always enabled;
• Undervoltage Protection is enabled after reference
voltage ramps up to 80% of the final value. In
soft−start, a UVP fault will directly restart a complete
soft−start.
Synchronization Function
Synchronize through the SYNCH pin. Synchronization
function allows different converters to share the same input
filter reducing the resulting Irms and reducing the need for
total caps to sustain the load. Synchronized systems also
exhibits higher noise immunity and better regulation.
The device synchronizes the high−side MOSFET turn−on
with the falling edge of the SYNCH pin input signal. In order
for internal switching to track the external signal, the
external signal has to fall within 0−40% frequency window
above the internal frequency set by the OSC pin. SYNCH
pin can be connected to other regulators’ PWM, phase node,
gate signals according to the desired phase−shift with proper
voltage scaling.
Protection Scheme
• PGOOD
• Overvoltage Protection (OV)
• Undervoltage Protection (UV)
• PreOVP Protection, monitor CSN/VO when IC is
disabled.
• V
in
detection: If UV is triggered during SS, it will
restart SS after a fixed delay.
