RT2855A/B
Function Block Diagram
VCC VREG5
BOOT
EN
POR &
Reg
VBIAS
OC
UV & OV
V
REG5
6µA
V
REF
Min.
Off-Time
VREG5
VIN
SW
PGND
Control
Driver
SW
Ripple
Gen.
FB
+
-
-
Comparator
ZC
GND
Comparator
-
+
PGOOD
SS
V
IN
FB
VS
On-Time
FB
0.9 x V
REF
Detailed Description
The RT2855A/B are high-performance 650kHz 4A step-
down regulators with internal power switches and
synchronous rectifiers. They feature an Advanced Constant
On-Time (ACOT
TM
) 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 ACOT
TM
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.
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
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.
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.
Because no switching decisions are made during noisy
time periods, COT architectures are preferable in low duty
cycle and noisy applications. However, traditional COT
is a registered trademark of Richtek Technology Corporation.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
www.richtek.com
4
DS2855A/B-01
September 2015
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
