NCV8851B ON Semiconductor, NCV8851B Datasheet - Page 8

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

General
internal 1.5 A gate drivers designed to drive NMOS FETs.
The internal gate drivers simplify design, improve
performance and efficiency and minimize board area. The
controller uses an 800 mV, 2.0% reference, allowing for a
wide range of precise output voltage programmability.
frequency range of 170 kHz to 500 kHz, allowing more
design flexibility in compromising efficiency versus
components’ size and cost. This frequency is conveniently
set with an external resistor to ground. An external clock
signal can also be used to synchronize the NCV8851B to a
higher operating frequency during operation.
power−stage components, excessive inrush of current
during start−up is prevented by an internal soft−start, and
which almost always require the extra components of a
Type−III compensation network for adequate transient
response,
compensation. This greatly simplifies the compensator
design and optimization process, while offering much faster
transient response than a Type−I compensation network.
Additionally, the two−loop system separates the effects of
output components between the two loops, further
simplifying the compensation process.
of the error loops to offset the effects of the inherent
open−loop response. This compensation requires a resistor
and two capacitors in the feedback loop for each of the error
The NCV8851B is a synchronous buck controller with
The NCV8851B also provides a programmable fixed
To protect against possible damage of external
Unlike voltage mode control (VMC) of buck regulators,
Type−II compensation places a zero and two poles in each
ACMC
buck
Gate Drivers
V
SW
PWM
and
regulators
DETAILED OPERATING DESCRIPTION
CEA
Current
use
Inner
Loop
−
+
L
Figure 19. ACMC Loops
Type−II
http://onsemi.com
Gain=1
NCV8851B
CSA
8
R
S
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inductor current is limited via average current limiting
(ACL) and cycle−by−cycle overcurrent protection (OCP).
Thermal shutdown (TSD) is also implemented to protect the
device from overheating.
Average Current Mode Control
control (ACMC) architecture to regulate the output voltage.
ACMC uses two loops, as seen in Figure 19. Through the
current error amplifier (CEA), the inner current loop
monitors the inductor current with the unity gain current
sense amplifier (CSA). The current loop responds to input
voltage changes, affecting the line transient response. Using
the voltage error amplifier (VEA), the outer voltage loop
monitors the output voltage, responding to output load
changes, affecting the load transient response. Feedback
resistors in the voltage loop select the output voltage.
amplifiers, shown as complex impedances in Figure 19. An
input resistor from the CSA to the CEA sets the gain of the
CEA. The voltage loop also has a pair of feedback resistors
from V
Enable
activate the internal LDO. The NCV8851B is disabled when
the EN pin is pulled below the enable input logic low
threshold voltage, causing a normal shutdown to occur,
putting the part into a low quiescent current sleep mode.
When the EN pin is pulled above the enable input logic high
threshold voltage, the part is enabled, the LDO output is
brought up and then the internal soft−start begins.
The NCV8851B employs an average current mode
The enable input (EN) is a TTL−compatible input used to
OUT
Outer Voltage Loop
VEA
to set the output voltage and gain of the VEA.
−
+
C
V
REF
V
OUT
R
L
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