®
RT6236A/B
6A, 18V, 650kHz, ACOT
TM
Synchronous Step-Down Converter
General Description
The RT6236A/B is a high-performance 650kHz, 6A step-
down regulator with internal power switches and
synchronous rectifiers. It features quick transient response
using its 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 0.7V to 8V. 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 RT6236A/B is stable with and optimized
for ceramic output capacitors. With internal 51mΩ switches
and 18mΩ synchronous rectifiers, the RT6236A/B displays
excellent efficiency and good behavior across a range of
applications, especially for low output voltages and low
duty cycles. Cycle-by-cycle current limit provides
protection against shorted outputs, 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
RT6236A/B is available in the UQFN-13JL 2x3 (FC)
package, with exposed thermal pad.
Features
Fast Transient Response
Advanced Constant On-Time (ACOT
TM
) Control
4.5V to 18V Input Voltage Range
Adjustable Output Voltage from 0.7V to 8V
6A Output Current
51mΩ Internal High-Side N-MOSFET and 18mΩ
Ω
Ω
Internal Low-Side N-MOSFET
Steady 650kHz Switching Frequency
Up to 95% Efficiency
Optimized for All Ceramic Capacitors
Externally-Adjustable, Pre-Biased Compatible Soft-
Start
Cycle-by-Cycle Current Limit
Input Under-Voltage Lockout
Output Over- and Under-Voltage Protection
Power Good Output
Externally-Adjustable, Pre-Biased Compatible Soft-
Start
Thermal Shutdown
Applications
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
Simplified Application Circuit
V
IN
C
IN
V
PVCC
Enable
C
SS
R
PG
PGOOD
EN
SS
FB
PVCC
GND
C
PVCC
V
PVCC
R2
RT6236A/B
SW
VIN
BOOT
L
V
OUT
C
BOOT
C
FF
R1
C
OUT
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6236A/B-02 August 2016
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1
RT6236A/B
Ordering Information
RT6236A/B
Package Type
QUF : UQFN-13JL 2x3 (U-Type) (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
H : Hiccup Mode UVP
L : Latched OVP & UVP
A : PSM
B : PWM
Note :
Richtek products are :
½
Pin Configurations
(TOP VIEW)
BOOT
11
10
9
8
7
5
6
GND
13
GND
EN
FB
NC
SW
12
1
2
3
4
VIN
VIN
PGOOD
NC
SS
UQFN-13JL 2x3 (FC)
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
Marking Information
RT6236AHGQUF
0Z : Product Code
RT6236BHGQUF
0X : Product Code
W : Date Code
W : Date Code
0ZW
0XW
RT6236ALGQUF
0Y : Product Code
RT6236BLGQUF
0W : Product Code
W : Date Code
0YW
0WW
W : Date Code
Functional Pin Description
Pin No.
1, 13
2
3
4, 7
5
6
8
9, 10
11
12
Pin Name
GND
EN
FB
NC
SS
PVCC
PGOOD
VIN
BOOT
SW
Ground.
Enable Control Input.
Converter Feedback Input. Connect to output voltage feedback resistor divider.
No Internal Connection.
Soft-Start Control. A external capacitor should be connected to GND.
5V Power Supply Output. A capacitor (typical 1F) should be connected to GND.
Open Drain Power Good Output.
Power Input and Connected to High-Side MOSFET Drain.
Bootstrap. This capacitor is needed to drive the power switch's gate above the
supply voltage. It is connected between SW and BOOT pins to form a floating
supply across the power switch driver. A 0.1F capacitor is recommended for use.
Switch Output.
is a registered trademark of Richtek Technology Corporation.
Pin Function
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
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2
PVCC
DS6236A/B-02 August 2016
RT6236A/B
Function Block Diagram
PVCC
V
IN
Reg
VIBIAS
BOOT
PVCC
Min.
Off
V
REF
Control
Driver
LGATE
PVCC
UGATE
SW
GND
VIN
OC
UV & OV
PVCC
6µA
SS
FB
V
IN
SW
On-Time
GND SW
SW
Ripple
Gen.
+
-
-
Comparator
Comparator
0.9 V
REF
FB
+
-
PGOOD
EN
EN
Detailed Description
The RT6236A/B is a high-performance 650kHz 6A step-
down regulators with internal power switches and
synchronous rectifiers. It features 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 ACOT
TM
control mode also provide fast transient
response, especially for low output voltages and low duty
cycles.
The input voltage range is from 4.5V to 18V and the output
is adjustable from 0.7V to 8V. 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
RT6236A/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
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
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
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DS6236A/B-02 August 2016
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3
RT6236A/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 VOUT and inversely proportional to VIN. 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 VOUT and inversely
proportional to VIN 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
©
2016 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 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 RT6236A/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 and 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 (230ns typical) so that rapidly-repeated on-
times can raise the inductor current quickly when needed.
Discontinuous Operating Mode (RT6236A Only)
After soft-start, the RT6236A operates in fixed frequency
mode to minimize interference and noise problems. The
RT6236A uses variable-frequency discontinuous switching
at light loads to improve efficiency. During discontinuous
switching, the on-time is immediately increased to add
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DS6236A/B-02 August 2016
RT6236A/B
“hysteresis”
to discourage the IC from switching back to
continuous switching unless the load increases
substantially.
The IC returns to continuous switching as soon as an on-
time is generated before the inductor current reaches zero.
The on-time is reduced back to the length needed for
650kHz switching and encouraging the circuit to remain
in continuous conduction, preventing repetitive mode
transitions between continuous switching and
discontinuous switching.
Current Limit
The RT6236A/B current limit is a cycle-by-cycle
“valley”
type, measuring the inductor current through the
synchronous rectifier during the off-time while the inductor
current ramps down. The current is determined by
measuring the voltage between source and drain of the
synchronous rectifier. If the inductor current exceeds the
current limit, the on-time one-shot is inhibited (Mask high
side signal) until the inductor current ramps down below
the current limit. Thus, only when the inductor current is
well below the current limit is another on time permitted.
This arrangement prevents the average output current from
greatly exceeding the guaranteed current limit value, as
typically occurs with other valley-type current limits. If
the output current exceeds the available inductor current
(controlled by the current limit mechanism), the output
voltage will drop. If it drops below the output under-voltage
protection level, the IC will stop switching (see next
section).
Output Over-Voltage Protection and Under-Voltage
Protection
If the output voltage V
OUT
rises above the regulation level
and lower 1.2 times regulation level, the high-side switch
naturally remains off and the synchronous rectifier turns
on. For RT6236B, if the output voltage remains high the
synchronous rectifier remains on until the inductor current
reaches the low side current limit. If the output voltage
still remains high, then IC's switches remain that the
synchronous rectifier turns on and high-side MOS keeps
off to operate at typical 500kHz switching protection, again
if inductor current reaches low side current, the
synchronous rectifier will turn off until next protection
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
clock. If the output voltage exceeds the OVP trip threshold
(1.25 times regulation level) for longer than 10μs (typical),
then IC's output Over-Voltage Protection (OVP) is
triggered. RT6236BL chip enters latch mode.
For RT6236A If the output voltage Vo rises above the
regulation level and lower 1.2 times regulation level, the
high-side switch naturally remains off and the synchronous
rectifier turns on until the inductor current reaches zero
current. If the output voltage remains high, then IC's
switches remain off. If the output voltage exceeds the OVP
trip threshold (1.25 times regulation level) for longer than
10μs (typical), the IC's OVP is triggered. RT6236AL chip
enters latch mode.
The RT6236A/B include output Under-Voltage Protection
(UVP). If the output voltage drops below the UVP trip
threshold for longer than 250μs (typical) then IC's UVP is
triggered. Chip into latch or hiccup mode. (see next
section).
Hiccup Mode
The RT6236AH/BH, use hiccup mode for UVP. When the
protection function is triggered, the IC will shut down for a
period of time and then attempt to recover automatically.
Hiccup mode allows the circuit to operate safely with low
input current and power dissipation, and then resume
normal operation as soon as UVP is removed. During
hiccup mode, the shutdown time is determined by the
capacitor at SS. A 2μA current source discharges V
SS
from its starting voltage (normally V
PVCC
). The IC remains
shut down until V
SS
reaches 0.2V, about 10ms for a 3.9nF
capacitor. At that point the IC begins to charge the SS
capacitor at 6μA, and a normal start-up occurs. If the fault
remains, UVP protection will be enabled when V
SS
reaches
2.2V (typical). The IC will then shut down and discharge
the SS capacitor from the 2.2V level, taking about 4ms for
a 3.9nF SS capacitor.
Latch-Off Mode
The RT6236AL/BL, use latch-off mode OVP and UVP.
When the protection function is triggered the IC will shut
down in Latch-Off Mode. The IC stops switching, leaving
both switches open, and is latched off. To restart operation,
toggle EN or power the IC off and then on again.
is a registered trademark of Richtek Technology Corporation.
DS6236A/B-02 August 2016
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5
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