®
RT2855A/B
4A, 18V, 650kHz, ACOT
TM
Synchronous Step-Down Converter
General Description
The RT2855A/B are high-performance 650kHz 4A step-
down regulators with internal power switches and
synchronous rectifiers. They feature quick transient
response using their Advanced Constant On-Time
(ACOT
TM
) control architecture that provides stable
operation with small ceramic output capacitors and 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 improves upon other fast-
response constant on-time architectures, achieving nearly
constant switching frequency over line, load, and output
voltage ranges. 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. The RT2855A/B are stable
with and optimized for ceramic output capacitors
With internal 70mΩ switches and 30mΩ synchronous
rectifiers, the RT2855A/B display excellent efficiency and
good behavior across a range of applications, especially
for low output voltages and low duty cycles. Cycle-by-
cycle current limit, input under-voltage lock-out, externally-
adjustable soft-start, output under- and over-voltage
protection, and thermal shutdown provide safe and smooth
operation in all operating conditions.
The RT2855A and RT2855B are each available in
WQFN-16L 3x3 package, with exposed thermal pads. The
RT2855B switches continuously even at light loads to
avoid low-frequency interference while the RT2855A
features a power-saving discontinuous operating mode at
light loads.
Features
Fast Transient Response
Steady 650kHz Switching Frequency at all Load
Current (RT2855B)
Discontinuous operating mode at Light Load
(RT2855A)
4A Output Current
Advanced Constant On-Time (ACOT
TM
) Control
Optimized for Ceramic Output Capacitors
4.5V to 18V Input Voltage Range
Internal 70mΩ Switch and 30mΩ Synchronous
Ω
Ω
Rectifier
0.765V to 7V Adjustable Output Voltage
Externally-adjustable, Pre-biased Compatible Soft-
Start
Cycle-by-Cycle Current Limit
Optional Output Discharge Function
Output Over- and Under-voltage Shut-down
�½
Latched (RT2855ALGQW/RT2855BLGQW Only)
�½
Hiccup Mode (RT2855AHGQW/RT2855BHGQW
Only)
Input Under Voltage Lockout
Thermal Shutdown
Simplified Application Circuit
V
IN
Power Good
VREG5
Input Signal
RT2855A/B
VIN
SW
VCC
PGOOD
BOOT
VS
V
OUT
FB
EN
SS
VREG5
GND PGND
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2855A/B-01
September 2015
www.richtek.com
1
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
