®
RT5788A/B
6V, 4A, 1.5MHz ACOT
TM
Synchronous Step-Down Converter
in TSOT-23-8 package
General Description
The RT5788A/B is a simple, easy-to-use, 4A synchronous
step-down DC-DC converter with an input supply voltage
range of 2.5V to 6V. The device build-in an accurate 0.6V
(±1.5%) reference voltage and can operate in 100% duty
cycle as a very low dropout voltage regulator. The
RT5788A/B integrates low R
DS(ON)
power MOSFETs to
achieve high efficiency and is available in TSOT-23-8 (FC)
package.
The RT5788A/B adopts Advanced Constant On-Time
(ACOT
TM
) control architecture to provide an ultrafast
transient response with few external components and to
operate in nearly constant switching frequency over the
line, load, and output voltage range. The RT5788A/B is
designed to operate at 1.5MHz fixed frequency. While
the RT5788A automatically enters power saving mode
(PSM) operation at light load to maintain high efficiency.
The RT5788B operates in Forced PWM over the loading
range, that helps meet tight voltage regulation accuracy
requirements.
The RT5788A/B senses both FETs current for a robust
over-current protection. It prevents the device from the
catastrophic damage in output short circuit, over current
or inductor saturation. The individual enable control input
and power good indicator provides flexible system power
sequence control a built-in soft-start function prevents
inrush current during start-up. The device also includes
input under-voltage lockout, output under-voltage
protection, and over-temperature protection (thermal
shutdown) to provide safe and smooth operation in all
operating conditions.
Features
4A Converter With Built-In 22mΩ/
Ω/20mΩ
Low
Ω/
Ω
R
DS(ON)
Power FETs
Input Supply Voltage Range : 2.5V to 6V
Output Voltage Range : 0.6V to 6V
Operates Up to 100% Duty Cycle
Advanced Constant On-Time (ACOT
TM
) Control
Ultrafast Transient Response
½
No Needs for External Compensations
½
Optimized for Low-ESR Ceramic Output Capacitors
0.6V
±
1.5% High-Accuracy Feedback Reference
½
Voltage
35μA Operation Quiescent Current (RT5788A)
μ
Robust Over-Current Protection for Both FETs
Optional for Operation Modes :
½
Power Saving Mode (PSM) (RT5788A)
½
Forced PWM Mode (RT5788B)
Fixed Switching Frequency : 1.5MHz
Monotonic Start-Up for Pre-Biased Output
Individual Enable Control Input
Power Good Indicator
Built-In Internally Fixed Soft-Start (Typ. 1.5ms)
100% Duty Cycle Mode
Internal Output Discharge
Input Under-Voltage Lockout (UVLO)
Output Under-Voltage Protection (UVP) with Hiccup
Mode
Over-Temperature Protection
Available in TSOT-23-8 (FC) Package
Simplified Application Circuit
V
IN
R
PGOOD
PGOOD
Enable
C
IN
RT5788A/B
PGOOD
EN
VOUT
AGND
PGND
FB
R2
R1
C
OUT
VIN
LX
L
V
OUT
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS5788A/B-01 October 2017
www.richtek.com
1
RT5788A/B
Ordering Information
RT5788A/B
Package Type
J8F : TSOT-23-8 (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
PWM Operation Mode
A : Automatic PSM
B : Forced PWM
Note :
Richtek products are :
½
Pin Configuration
(TOP VIEW)
AGND
6
3
8
7
2
PGOOD
VIN
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.
TSOT-23-8 (FC)
Marking Information
RT5788AGJ8F
20= : Product Code
½
Applications
20=DNN
DNN : Date Code
WLAN ASIC Power / Storage (SSD and HDD)
Mobile Phones and Handheld Devices
STB, Cable Modem, and xDSL Platforms
General Purpose for POL LV Buck Converter
RT5788BGJ8F
1Z= : Product Code
1Z=DNN
DNN : Date Code
Functional Pin Description
Pin No.
1
Pin Name
PGOOD
Pin Function
Open-drain power-good indication output. The power-good function is activated after
soft-start is finished.
Power input. The input voltage range is from 2.5V to 6V. Connect a suitable input
bypass capacitor (typically of greater than 10F) between this pin and PGND. The
bypass capacitor should be placed as close to the IC as possible.
Switch node between the internal switch and the synchronous rectifier. Connect the
output LC filter from this pin to the output load.
Power ground. This pin, connected to analog ground, must be soldered to a large
PCB copper area for maximum power dissipation.
Output voltage sense input. This pin is used to monitor and adjust output voltage to
enhance load transient regulation.
Analog ground. It provides a ground return path for the control circuitry and internal
reference.
Feedback voltage input. Connect this pin to the midpoint of the external feedback
resistive divider to set the output voltage of the converter to the desired regulation
level. The device regulates the FB voltage at a feedback reference voltage, typically
0.6V.
Enable control input. A logic-high enables the converter; a logic-low forces the device
into shutdown mode.
is a registered trademark of Richtek Technology Corporation.
2
VIN
3
4
5
6
LX
PGND
VOUT
AGND
7
FB
8
EN
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
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2
DS5788A/B-01 October 2017
PGND
LX
VOUT
5
4
EN
FB
RT5788A/B
Functional Block Diagram
EN
VOUT
UVLO
OTP
Shutdown
Control
Comparator
+
-
AGND
TON
LX
VIN
FB
Error Amplifier
+
-
+
Logic
Control
Current
Limit
Detector
AZC
Driver
LX
V
REF
PGOOD
-
+
Ramp
Generator
LX
LX
PGND
FB
Operation
The RT5788A/B is a low-voltage, high-efficiency,
synchronous step-down DC-DC converter that can deliver
up to 4A output current from a 2.5V to 6V input supply.
The RT5788A/B adopts ACOT
TM
control mode, which can
reduce the output capacitance and provide ultrafast
transient responses, and allow minimal component sizes
without any additional external compensation network. The
device includes a built-in ramp voltage generator, which
takes up the virtual inductor current as an input. With the
internal ramp signal, the device can be compensated to
achieve good stability even with low-ESR ceramic
capacitors, since the need for the output capacitor's ESR
to generate an ESR ramp voltage can be eliminated.
Low V
IN
ACOT
TM
One-Shot Operation
For a low V
IN
ACOT converter, a built-in error amplifier is
used to keep track of the feedback voltage, as shown in
the Functional Block Diagram. In steady state, the error
amplifier compares the feedback voltage V
FB
and an
internal reference voltage. If the virtual inductor current
ramp voltage is lower than the output of the error amplifier,
a new pre-determined fixed on-time will be triggered by
the on-time one-shot generator, provided that minimum-
off-time one-shot is cleared and the measured inductor
current through the synchronous rectifier (low-side switch)
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is below the current limit I
LIM_L
. During the on-time, the
high-side switch is turned on 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. If the output
voltage has not reached its nominal level, another on-time,
however, can only be generated after a short blanking time,
or called minimum off-time, which is triggered by the
minimum-off-time one-shot generator. It is to prevent
another immediate on-time being triggered during the noisy
switching time and allow the feedback voltage and current
sense signals to settle. The minimum off-time t
OFF_MIN
is
kept short so that the inductor current can be raised up
quickly by rapidly repeated on-times when needed. Such
feature makes reaction speed of the ACOT-based
converters to load transients extremely fast.
Enable Control
The RT5788A/B provides an EN pin, as an external chip
enable control, to enable or disable the device. If V
EN
is
held below a logic-low threshold voltage V
ENL
of the enable
input (EN), the converter will enter into shutdown mode,
that is, the converter is disabled and switching is inhibited
even if the VIN voltage is above VIN under-voltage lockout
threshold (V
UVLO
). During shutdown mode, the supply
is a registered trademark of Richtek Technology Corporation.
DS5788A/B-01 October 2017
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3
RT5788A/B
current can be reduced to I
SHDN
(1μA or below). If the EN
voltage rises above the logic-high threshold voltage (V
ENH
)
while the V
IN
voltage is higher than UVLO threshold (V
UVLO
),
the device will be turned on, that is, switching being enabled
and soft-start sequence being initiated.
Input Under-Voltage Lockout
In addition to the EN pin, the RT5788A/B also provides
enable control through the VIN pin. It features an under-
voltage lockout (UVLO) function that monitors the internal
linear regulator (VCC). If V
EN
rises above V
ENH
first,
switching will still be inhibited until the VIN voltage rises
above V
UVLO
. It is to ensure that the internal regulator is
ready so that operation with not-fully-enhanced internal
MOSFET switches can be prevented. After the device is
powered up, if the input voltage V
IN
goes below the UVLO
falling threshold voltage (V
UVLO
− ΔV
UVLO
), this switching
will be inhibited; if V
IN
rises above the UVLO-rising
threshold (V
UVLO
), the device will resume switching.
Over-Current Protection
The RT5788A/B is protected from over current conditions
by cycle-by-cycle current limiting on both the high-side
MOSFET and the low-side MOSFET. The robust over
current protection mechanism prevents the converter to
be damaged from a catastrophic condition, i.e. the inductor
is shorted or saturated.
High-Side MOSFET Over-Current Protection
The device senses high-side current after a deglitch time
when the high-side MOSFET is turned-on. Each cycle
when the sensed current is higher than the high-side switch
peak current limit threshold, I
LIM_H
, then the device enters
high side over current protection. At the same time, the
high-side MOSFET is turned off and the low-side MOSFET
is turned on.
Low-Side MOSFET Over-Current Protection
The RT5788A/B detects low side current after a minimum-
off time while the low-side MOSFET is turned on. If the
detected low-side current is higher than the low-side switch
valley current limit threshold, I
LIM_L
, the on-time one-shot
will be inhibited until the inductor current ramps down to
the current limit level (I
LIM
). That is, another on-time can
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
only be triggered when the inductor current goes below
the low-side current limit.
If the output load current exceeds the available inductor
current (clamped by the above-mentioned low-side current
limit), the output capacitor needs to supply the extra
current such that the output voltage will begin to drop. If it
drops below the output under-voltage protection trip
threshold, the IC will stop switching to avoid excessive
heat.
Output Under-Voltage Protection
The RT5788A/B includes output under-voltage protection
(UVP) against over-load or short-circuited condition by
constantly monitoring the feedback voltage V
FB
. If V
FB
drops below the under-voltage protection trip threshold
(typically 50% of the internal reference voltage), the UV
comparator will go high to turn off both the internal high-
side and low-side MOSFET switches.
Hiccup Mode
If the output under-voltage condition continues for a period
of time, the RT5788A/B will enter output under-voltage
protection with hiccup mode. During hiccup mode, the
device remains shut down. After a period of time, a soft-
start sequence for auto-recovery will be initiated. Upon
completion of the soft-start sequence, if the fault condition
is removed, the converter will resume normal operation;
otherwise, such cycle for auto-recovery will be repeated
until the fault condition is cleared. Hiccup mode allows
the circuit to operate safely with low input current and
power dissipation, and then resume normal operation as
soon as the over-load or short-circuit condition is removed.
Soft-Start (SS)
The soft-start function is used to prevent large inrush
currents while the converter is being powered up. The
RT5788A/B provides an internal soft-start feature for inrush
control. During the start-up sequence, the internal capacitor
is charged by an internal current source I
SS
to generate a
soft-start ramp voltage as a reference voltage to an error
amplifier. The device will initiate switching and the output
voltage will smoothly ramp up to its targeted regulation
voltage only after this ramp voltage is greater than the
feedback voltage V
FB
to ensure the converters have a
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DS5788A/B-01 October 2017
RT5788A/B
smooth start-up. The typical soft-start time is 1.5ms.
Power Good Indication
The RT5788A/B provides a power-good (PGOOD) open-
drain output pin. It is to be connected to an external voltage
source through a pull-up resistor. The power-good function
is activated after soft-start is finished and is controlled by
a comparator connected to the feedback signal V
FB
. If
V
FB
rises above a power-good threshold (V
TH_PGLH
)
(typically 95% of the target value), the PGOOD pin will be
in high impedance and V
PGOOD
will be held high after a
certain delay elapsed. When V
FB
drops by a power-good
hysteresis (ΔV
TH_PGLH
) (typically 5% of the target value)
or exceeds V
TH_PGHL
(typically 110% of the target value),
the PGOOD pin will be pulled low. For V
FB
higher than
V
TH_PGHL
, V
PGOOD
can be pulled high again if V
FB
drops
back by a power-good hysteresis (ΔV
TH_PGHL
) (typically
5% of the target value). Once being started-up, if any
internal protection is triggered, PGOOD will be pulled low
to GND.
Over-Temperature Protection (Thermal Shutdown)
The RT5788A/B includes an over-temperature protection
(OTP) circuitry to prevent overheating due to excessive
power dissipation. The OTP will shut down switching
operation when junction temperature exceeds a thermal
shutdown threshold T
SD
. Once the junction temperature
cools down by a thermal shutdown hysteresis (ΔT
SD
), the
IC will resume normal operation with a complete soft-start.
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS5788A/B-01 October 2017
www.richtek.com
5
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