®
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
RT2855A/B
Applications
Ordering Information
RT2855A/B
Package Type
QW : WQFN-16L 3x3 (W-Type)
Lead Plating System
G : Green (Halogen Free and Pb Free)
H : Hiccup Mode OVP & UVP
L : Latched OVP & UVP
A : PSM
B : PWM
Note :
Richtek products are :
½
Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
Marking Information
RT2855AHGQW
4G= : Product Code
4G=YM
DNN
YMDNN : Date Code
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
½
RT2855ALGQW
4F= : Product Code
4F=YM
DNN
YMDNN : Date Code
Pin Configurations
(TOP VIEW)
VS
VCC
VIN
VIN
16 15 14 13
RT2855BHGQW
4E= : Product Code
4E=YM
DNN
YMDNN : Date Code
FB
VREG5
SS
GND
1
2
3
4
5
6
12
GND
17
7
8
11
10
9
BOOT
SW
SW
SW
RT2855BLGQW
4D= : Product Code
4D=YM
DNN
YMDNN : Date Code
WQFN-16L 3x3
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
PGOOD
EN
PGND
PGND
DS2855A/B-01
September 2015
RT2855A/B
Functional Pin Description
Pin No.
1
2
3
4
5
6
7, 8,
17 (Exposed pad)
Pin Name
FB
VREG5
SS
GND
PGOOD
EN
Pin Function
Feedback Input Voltage. Connect FB to the midpoint of the external feedback
resistive divider to sense the output voltage. Place the resistive divider within
5mm from the FB pin. The IC regulates V
FB
at 0.765V (typical).
Internal Regulator Output. Connect a 1F capacitor to GND to stabilize
output voltage.
Soft-Start Control. Connect an external capacitor between this pin and GND
to set the soft-start time.
Ground.
Open Drain Power-good Output. PGOOD connects to PGND whenever VFB
is less than 90% of its regulation threshold (typical).
Enable Control Input. A logic-high enables the converter; a logic-low forces
the IC into shutdown mode reducing the supply current to less than 10A.
Power Ground. PGND connects to the source of the internal N-channel
MOSFET synchronous rectifier and to other power ground nodes of the IC.
The exposed pad and the 2 PGND pins should be well soldered to the input
and output capacitors and to a large PCB area for good power dissipation.
Switching Node. SW is the source of the internal N-channel MOSFET switch
and the drain of the internal N-channel MOSFET synchronous rectifier.
Connect SW to the inductor with a wide short PCB trace and minimize its
area to reduce EMI.
Bootstrap Supply for High Side Gate Driver. Connect a 0.1F capacitor
between BOOT and SW to power the internal gate driver.
Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a
suitably large (10F x 2) ceramic capacitors at this pin.
Internal Linear Regulator Supply Input. VCC supplies power for the internal
linear regulator that powers the IC. Connect VIN to the input voltage and
bypass to ground with a 0.1F ceramic capacitor.
Output Voltage Sense Input Pin.
PGND
9, 10, 11
SW
12
13, 14
15
16
BOOT
VIN
VCC
VS
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2855A/B-01
September 2015
www.richtek.com
3
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
RT2855A/B
control schemes suffer from some disadvantages that
preclude their use in many cases. Many applications require
a known switching frequency range to avoid interference
with other sensitive circuitry. True constant on-time control,
where the on-time is actually fixed, exhibits variable
switching frequency. In a step-down converter, the duty
factor is proportional to the output voltage and inversely
proportional to the input voltage. Therefore, if the on-time
is fixed, the off-time (and therefore the frequency) must
change in response to changes in input or output voltage.
Modern pseudo-fixed frequency COT architectures greatly
improve COT by making the one-shot on-time proportional
to V
OUT
and inversely proportional to V
IN
. In this way, an
on-time is chosen as approximately what it would be for
an ideal fixed-frequency PWM in similar input/output
voltage conditions. The result is a big improvement but
the switching frequency still varies considerably over line
and load due to losses in the switches and inductor and
other parasitic effects.
Another problem with many COT architectures is their
dependence on adequate ESR in the output capacitor,
making it difficult to use highly-desirable, small, low-cost,
but low-ESR ceramic capacitors. Most COT architectures
use AC current information from the output capacitor,
generated by the inductor current passing through the
ESR, to function in a way like a current mode control
system. With ceramic capacitors the inductor current
information is too small to keep the control loop stable,
like a current mode system with no current information.
ACOT
TM
Control Architecture
Making the on-time proportional to V
OUT
and inversely
proportional to V
IN
is not sufficient to achieve good
constant-frequency behavior for several reasons. First,
voltage drops across the MOSFET switches and inductor
cause the effective input voltage to be less than the
measured input voltage and the effective output voltage to
be greater than the measured output voltage. As the load
changes, the switch voltage drops change causing a
switching frequency variation with load current. Also, at
light loads if the inductor current goes negative, the switch
dead-time between the synchronous rectifier turn-off and
the high-side switch turn-on allows the switching node to
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
rise to the input voltage. This increases the effective on-
time and causes the switching frequency to drop
noticeably.
One way to reduce these effects is to measure the actual
switching frequency and compare it to the desired range.
This has the added benefit eliminating the need to sense
the actual output voltage, potentially saving one pin
connection. ACOT
TM
uses this method, measuring the
actual switching frequency (at SW) and modifying the on-
time with a feedback loop to keep the average switching
frequency in the desired range.
To achieve good stability with low-ESR ceramic capacitors,
ACOT
TM
uses a virtual inductor current ramp generated
inside the IC. This internal ramp signal replaces the ESR
ramp normally provided by the output capacitor's ESR.
The ramp signal and other internal compensations are
optimized for low-ESR ceramic output capacitors.
ACOT
TM
One-shot Operation
The RT2855A/B control algorithm is simple to understand.
The feedback voltage, with the virtual inductor current ramp
added, is compared to the reference voltage. When the
combined signal is less than the reference the on-time
one-shot is triggered, as long as the minimum off-time
one-shot is clear and the measured inductor current
(through the synchronous rectifier) is below the current
limit. The on-time one-shot turns on the high-side switch
and the inductor current ramps up linearly. After the on-
time, the high-side switch is turned off and the synchronous
rectifier is turned on and the inductor current ramps down
linearly. At the same time, the minimum off-time one-shot
is triggered to prevent another immediate on-time during
the noisy switching time and allow the feedback voltage
and current sense signals to settle. The minimum off-time
is kept short (260ns typical) so that rapidly-repeated on-
times can raise the inductor current quickly when needed.
Discontinuous Operating Mode (RT2855A Only)
After soft start, the RT2855B operates in fixed frequency
mode to minimize interference and noise problems. The
RT2855A uses variable-frequency discontinuous switching
at light loads to improve efficiency. During discontinuous
switching, the on-time is immediately increased to add
is a registered trademark of Richtek Technology Corporation.
DS2855A/B-01
September 2015
www.richtek.com
5
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