®
RT6255A/B
5A, 18V, 500kHz, ACOT
TM
Step-Down Converter
General Description
The RT6255A/B is a high-efficiency, synchronous step-
down DC-DC converter that can deliver up to 5A output
current from a 4.5V to 18V input supply. The RT6255A/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 line, load, and output voltage
ranges.
The RT6255A/B features cycle-by-cycle current limit
protection against short-circuit outputs, and soft-start
function to prevent inrush current during start-up. It 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
Input Supply Voltage Range : 4.5V to 18V
Output Current : 5A
Advanced Constant On-Time (ACOT
TM
) Control for
Ultrafast Transient Response
Steady Switching Frequency : 500kHz
½
Forced PWM Mode (RT6255B)
½
Pulse-Skipping Mode (PSM) at Light Load
(RT6255A)
Optimized for Low-ESR Ceramic Output Capacitors
Programmable Output Voltage : 0.6V to 5V
Internal 45mΩ Switch and 23mΩ Synchronous
Ω
Ω
Rectifier
Cycle-by-cycle Current Limit Protection
Internal Soft-Start (Typ 1.5ms) or Externally
Adjustable, Pre-biased Compatible Soft-Start (Only
for Option with SS Pin)
Power Good Indicator (90%) (Only for Option with
PGOOD Pin)
Output Under-Voltage Protection (UVP)
Over-Temperature Protection (OTP)
Input Under-Voltage Lockout (UVLO)
Available in TSOT-23-6 (FC) and TSOT-23-8 (FC)
Packages
Ordering Information
RT6255A/B
( )
Package Type
J6F : TSOT-23-6 (FC)
J8F : TSOT-23-8 (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Pin8 Definition
Empty means PGOOD pin
S : SS pin
UVP Option
H : Hiccup
PSM/PWM
A : PSM/PWM
B : Force-PWM
Note :
Richtek products are :
½
Applications
Set Top Box
Portable TV
Access Point Router
DSL Modem
LCD TV
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
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6255A/B-02 March 2017
www.richtek.com
1
RT6255A/B
Operation
The RT6255A/B is a high-efficiency, monolithic
synchronous step-down
DC-DC converter that can deliver
up to 5A output current from a 4.5V to 18V input supply.
Using the ACOT control mode can reduce the output
capacitance and perform fast transient response. It can
minimize the component size without additional external
compensation network.
Current Limit
The RT6255A/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, adding temperature compensation
for greater accuracy. If the current exceeds the current
limit, the on-time one-shot is inhibited until it drops below
the current limit level. 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 (see next section)
the IC will stop switching to avoid excessive heat.
Hiccup Mode
The RT6255A/B 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 the overload or short circuit
is removed.
Input Under-Voltage Lockout
To protect the chip from operating at insufficient supply
voltage, the UVLO is needed. When the input voltage of
V
IN
is lower than the UVLO falling threshold voltage, the
device will be lockout.
Shut-Down, Start-Up and Enable (EN)
The enable input (EN) has a logic-low level. When V
EN
is
below this level the IC enters shutdown mode. When V
EN
exceeds its logic-high level the IC is fully operational.
External Bootstrap Capacitor
Connect a 0.1μF low ESR ceramic capacitor between
BOOT and LX. This bootstrap capacitor provides the gate
driver supply voltage for the high side N- MOSFET switch.
Over-Temperature Protection
The RT6255A/B includes an Over-Temperature Protection
(OTP) circuitry to prevent overheating due to excessive
power dissipation. The OTP will shut down switching
operation when the junction temperature exceeds 150°C.
Once the junction temperature cools down by
approximately 15°C, the IC will resume normal operation.
For continuous operation, provide adequate cooling so
that the junction temperature does not exceed 150°C.
UVP Protection
The RT6255A/B detects under-voltage conditions by
monitoring the feedback voltage on FB pin. When the
feedback voltage is lower than 60% of the target voltage,
the UVP comparator will go high to turn off both internal
high-side and low-side MOSFETs.
Soft-Start (SS)
(For RT6255AHSGJ8F/RT6255BHSGJ8F)
The soft-start function is used to prevent large inrush
currents while the converter is being powered up. The
RT6255A/B provides a soft-start feature for inrush control.
For some options, it provides an SS pin so that the soft-
start time can be programmed by selecting the value of
the external capacitor C
SS
connected from the SS pin to
GND. During the start-up sequence, the external capacitor
is charged by an internal current source I
SS
(typically,
4μA) to generate a soft-start ramp voltage as a reference
voltage to the PWM comparator. 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 smooth start-up. For soft-start control,
the SS pin should never be left unconnected.
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6255A/B-02 March 2017
www.richtek.com
5
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