ncv8881 ON Semiconductor, ncv8881 Datasheet - Page 23

• Manufacturer Part Number: ncv8881
• Description: Ncv8881 1.5a Automotive Buck Regulator With Watchdog
• Manufacturer: ON Semiconductor

regulation, a soft−start circuit slowly raises the switching
regulator error amplifier reference to V
overloading the input supply.
VOLTAGE REFERENCE
reference provides the SMPS Error Amplifier and the 5P0
and 8P5 linear regulators with a stable, precision reference
voltage.
SMPS ERROR AMPLIFIER
Voltage Mode control method employed by the NCV8881
requires Type III compensation for optimum regulator
response to load and line transients.
cycle of the power switch by controlling the moment at
which the power switch shuts off (power switch turn−ons
occur at a fixed rate).
SMPS OSCILLATOR
NCV8881 switching frequency will be the factory−set
default frequency f
ROSC SMPS FREQUENCY CONTROL
ground will raise the switching frequency above the
factory−set default according to the following equation.
Connection of a resistor between the ROSC pin and 5P0 will
lower the switching frequency below the default. The
OUTPUT
ENABLE
An internal, temperature compensated Bandgap voltage
The error amplifier is an operational amplifier. The
The output voltage of the error amplifier controls the duty
With no connections to the ROSC or SYNC pins, the
Connection of a resistor between the ROSC pin and
SMPS
8P5
5P0
VIN
0
Regulation Threshold
Short−Circuit Threshold
F
SW
+ 6840
1
OSC
of the internal oscillator.
R
2
ROSC
SOFT−START TIMER
−0.97
3
Current is Limited
FBR
Figure 46. SMPS Overload During Startup
) 170
in order to avoid
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programmed switching frequency should be no higher than
the highest synchronization frequency if synchronization is
used.
SMPS SYNCHRONIZATION
switch turn−on edges to coincide with rising edges of the
applied clock signal. When synchronization will be
significantly higher than the default frequency, an ROSC
resistor which sets the internal oscillator frequency at (but
no higher than) the synchronization frequency can be used
to maintain the switching frequency approximately the same
as the synchronization frequency in the absence of the
SYNC signal.
resistor controls the internal ramp slope, and can be used to
adjust the gain of the pulse width modulator.
operation to the factory−set default or ROSC programmed
frequency after the De−synchronization delay.
OUTPUT VOLTAGE REGULATION MONITOR
low, and the POR, BOOT and Watchdog Delays are
initialized. When FB voltage exceeds V
Delay begins to time out. If, when the FB voltage is below
V
input is low, the NCV8881 will completely shut off (see
Figures 46 through 48).
6
FBMONL
Applying a clock signal to the SYNC pin will cause power
Besides controlling the switching frequency, the ROSC
A steady low or high SYNC input will restore SMPS
When the FB voltage is below V
7
, the Soft−Start Timer has expired and the EN
8
9
FBMONL
10
FBMONH
, RESB is pulled
11
the POR