®
RT6238A/B
8A, 18V, 500kHz, ACOT
TM
Synchronous Step-Down Converter
General Description
The RT6238A/B is a high-performance 500kHz, 8A 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 RT6238A/B is stable with and optimized
for ceramic output capacitors. With internal 35mΩ switches
and 14mΩ synchronous rectifiers, the RT6238A/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
lockout, externally-adjustable soft-start, output under- and
over-voltage protection, and thermal shutdown provide safe
and smooth operation in all operating conditions. The
RT6238A/B is available in the UQFN-14L 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
8A Output Current
Ω
Ω
35mΩ Internal High-Side N-MOSFET and 14mΩ
Internal Low-Side N-MOSFET
Steady 500kHz 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
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
EN Signal
Power Good
RT6238A/B
VIN
SW
BOOT
FB
V
OUT
EN
PGOOD
PVCC
SS
GND
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6238A/B-08 July 2017
www.richtek.com
1
RT6238A/B
Ordering Information
RT6238A/B
Package Type
QUF : UQFN-14L 2x3 (U-Type) (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
UVP Option
H : Hiccup Mode UVP
L : Latched OVP & UVP
A : PSM
B : PWM
Note :
Richtek products are :
½
Marking Information
RT6238ALGQUF
0K : Product Code
W : Date Code
0KW
RT6238BLGQUF
0H : Product Code
0HW
W : 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.
RT6238AHGQUF
0L : Product Code
0LW
W : Date Code
½
Pin Configurations
(TOP VIEW)
GND
EN
SS
RT6238BHGQUF
0J : Product Code
0JW
W : Date Code
14
13
12
11
10
9
8
AGND
FB
PVCC
PGOOD
1
2
3
4
5
6
7
GND
GND
GND
VIN
BOOT
SW
UQFN-14L 2x3 (FC)
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
VIN
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS6238A/B-08 July 2017
RT6238A/B
Functional Pin Description
Pin No.
1
2
3
4
5
6
7, 8
9, 10, 11, 12
Pin Name
AGND
FB
PVCC
PGOOD
BOOT
SW
VIN
GND
Analog GND.
Feedback Voltage Input. It is used to regulate the output of the converter to a set
value via an external resistive voltage divider. The feedback reference voltage is
0.7V typically.
Internal Regulator Output. Connect a 1F capacitor to GND to stabilize output
voltage.
Power Good Indicator Open-Drain Output.
Bootstrap Supply for High-Side Gate Driver. This capacitor is needed to drive the
power switch's gate above the supply voltage. It is connected between the SW and
BOOT pins to form a floating supply across the power switch driver. A 0.1F
capacitor is recommended for use.
Switch Node. Connect this pin to an external L-C filter.
Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a
suitably large (10F x 2) ceramic capacitor.
Ground.
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. The EN pin
can be connected to VIN with a 100k pull-up resistor for automatic start-up. The
pull-up resistance should not small than 60k to prevent EN pin voltage over than
absolute maximum rating.
Soft-Start Time Setting. An external capacitor should be connected between this
pin and GND.
Pin Function
13
EN
14
SS
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
0.4
EN
5k
3V
-
+
GND SW
SW
Ripple
Gen.
+
+
-
0.9 V
REF
FB
V
IN
Comparator
1.2/1.01
-
+
On-Time
SW
Comparator
+
-
PGOOD
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6238A/B-08 July 2017
www.richtek.com
3
RT6238A/B
Detailed Description
The RT6238A/B is a high-performance 500kHz 8A 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 provides 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
RT6238A/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
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
is a registered trademark of Richtek Technology Corporation.
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
www.richtek.com
4
DS6238A/B-08 July 2017
RT6238A/B
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
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 RT6238A/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.
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
Discontinuous Operating Mode (RT6238A Only)
After soft-start, the RT6238A operates in fixed frequency
mode to minimize interference and noise problems. The
RT6238A uses variable-frequency discontinuous switching
at light loads to improve efficiency. During discontinuous
switching, the on-time is immediately increased to add
“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
500kHz switching and encouraging the circuit to remain
in continuous conduction, preventing repetitive mode
transitions between continuous switching and
discontinuous switching.
Current Limit
The RT6238A/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 Under-Voltage Protection
Hiccup Mode
The RT6238AH/RT6238BH provide Hiccup Mode Under-
Voltage Protection (UVP). When the FB voltage drops
below 60% of the feedback reference voltage, the output
voltage drops below the UVP trip threshold for longer than
270μs (typical) then IC's UVP is triggered. UVP function
is a registered trademark of Richtek Technology Corporation.
DS6238A/B-08 July 2017
www.richtek.com
5
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