RT2855A/B
Function Block Diagram
VCC VREG5
BOOT
POR &
Reg
EN
Min.
VREG5
Off-Time
VIN
VBIAS
OC
V
REF
Control
Driver
ZC
SW
UV & OV
PGND
SW
V
REG5
Ripple
Gen.
GND
6µA
+
-
-
Comparator
-
PGOOD
SS
FB
FB
V
IN
+
Comparator
0.9 x V
REF
FB
VS
On-Time
Detailed Description
duty cycle applications.After the on-time one-shot period,
there is a minimum off-time period before any further
regulation decisions can be considered. This arrangement
avoids the need to make any decisions during the noisy
time periods just after switching events, when the
switching node (SW) rises or falls. Because there is no
fixed clock, the high-side switch can turn on almost
immediately after load transients and further switching
pulses can ramp the inductor current higher to meet load
requirements with minimal delays.
The RT2855A/B are high-performance 650kHz 4A step-
down regulators with internal power switches and
synchronous rectifiers. They feature anAdvanced Constant
On-Time (ACOTTM) control architecture that provides
stable operation with ceramic output capacitors without
complicated external compensation, among other benefits.
The input voltage range is from 4.5V to 18V and the output
is adjustable from 0.765V to 7V.
The proprietary ACOTTM control scheme improves upon
other constant on-time architectures, achieving nearly
constant switching frequency over line, load, and output
voltage ranges. The RT2855A/B are optimized for ceramic
output capacitors. Since there is no internal clock,
response to transients is nearly instantaneous and inductor
current can ramp quickly to maintain output regulation
without large bulk output capacitance.
Traditional current mode or voltage mode control schemes
typically must monitor the feedback voltage, current
signals (also for current limit), and internal ramps and
compensation signals, to determine when to turn off the
high-side switch and turn on the synchronous rectifier.
Weighing these small signals in a switching environment
is difficult to do just after switching large currents, making
those architectures problematic at low duty cycles and in
less than ideal board layouts.
Constant On-Time (COT) Control
The heart of any COT architecture is the on-time one-
shot. Each on-time is a pre-determined “fixed” period
that is triggered by a feedback comparator. This robust
arrangement has high noise immunity and is ideal for low
Because no switching decisions are made during noisy
time periods, COT architectures are preferable in low duty
cycle and noisy applications. However, traditional COT
Copyright 2015 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS2855A/B-01 September 2015
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
