NCV8851B ON Semiconductor, NCV8851B Datasheet - Page 11

• Manufacturer Part Number: NCV8851B
• Description: Automotive Grade Synchronous Buck Controller
• Manufacturer: ON Semiconductor

required to realize the full benefit of the onboard drivers.
and V
current paths for the GH and GL connections must be
optimized, minimizing parasitic resistance and inductance.
Current Limiting and Overcurrent Protection
and cycle−by−cycle overcurrent protection (OCP) to protect
the power switches, inductor, current sense resistor and
other external components. The current through the inductor
is continuously sensed using the CSP and CSN pins. A sense
resistor is placed between these pins to translate the output
current to a proportional voltage. This voltage is compared
to a fixed internal voltage threshold.
the PWM pulse is terminated for this cycle, limiting the
current through the inductor. In steady−state operation,
decreasing the load resistance while in ACL will cause the
duty cycle and V
skipping pulses or jitter.
differential voltage exceeds the OCP threshold, which is
Careful selection and layout of external components is
The capacitors between V
The NCV8851B contains average current limiting (ACL)
When the differential voltage exceeds the ACL threshold,
There is also a fast OCP path which is tripped when the
SW
must be placed as close as possible to the IC. The
OUT
to decrease proportionally without
Figure 24. Synchronization from 170 kHz to an external 600 kHz signal
IN
and GND and between BST
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NCV8851B
11
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above the ACL threshold. This causes the PWM pulse to be
terminated very quickly and disables the part from switching
back on until the current through the inductor has dropped
below the OCP threshold. Once the inductor current is below
the OCP threshold, the part will begin switching again and
the current will be limited by ACL, until the inductor current
drops below the ACL threshold.
NCV8851B will limit large transient currents yet resume
normal operation on the following cycle. Additionally, the
current will not run away, nor will the part latch off in case
of a short, which is typical of other current limiting schemes
employing high−side current sensing.
SYNC Feature
to a higher frequency. The rising edge of the SYNC pulse
turns on the power switch to start a new switching cycle, as
shown in Figure 24. There is a 0.5 ms delay between the
rising edge of the SYNC pulse and rising edge of the V
pin voltage. The SYNC threshold is TTL logic compatible,
and duty cycle of the SYNC pulses can vary from 10% to
90%. The SYNC frequency must be higher than the internal
oscillator frequency set by R
An advantage of this current limiting scheme is that the
An external clock signal can synchronize the NCV8851B
OSC
.
SW
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