®
RT6251A/B
3A, 18V, Synchronous Step-Down Converter
General Description
The RT6251A/B is a high efficiency, monolithic
synchronous step-down DC/DC converter that can deliver
up to 3A output current from a 4.5V to 18V input supply.
The RT6251A/B current-mode architecture with external
compensation allows the transient response to be
optimized over a wide range of loads and output capacitors.
Cycle-by-cycle current limit provides protection against
shorted outputs and soft-start eliminates input current
surge during start-up. Fault condition protections include
output under-voltage protection, output over-voltage
protection, and over-temperature protection. The low
current shutdown mode provides output disconnection,
enabling easy power management in battery-powered
systems.
Features
Ω
Ω
Low R
DS(ON)
Power MOSFET Switches 46mΩ/36mΩ
Input Voltage Range : 4.5V to 18V
Adjustable Switching Frequency : 200kHz to 1.2MHz
Current-Mode Control
Synchronous to External Clock : 200kHz to 1.2MHz
Accurate Voltage Reference 0.8V
±
1%
Monotonic Start-Up into Pre-biased Outputs
Adjustable Soft-Start
Power Good Indicator
Under-Voltage and Over-Voltage Protection
Input Under-Voltage Lockout
Ordering Information
RT6251A/B
Package Type
QW : WQFN-14AL 3.5x3.5 (W-Type)
Lead Plating System
G : Green (Halogen Free and Pb Free)
H : Hiccup UVP
N : Non-UVP
A : PSM
B : PWM
Note :
Richtek products are :
½
Applications
High Performance Point of Load Regulation
Notebook Computers
High Density and Distributed Power Systems
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
Simplified Application Circuit
V
IN
C
IN
RT6251A/B
BOOT
VIN
C
BOOT
PVIN
Enable
PGOOD
R
OSC
C
SS
SS/TR
GND
EN
PGOOD
RT/SYNC
FB
COMP
R
COMP1
C
COMP2
C
COMP1
R2
LX
L
R1
C
OUT
V
OUT
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6251A/B-00 November 2015
www.richtek.com
1
RT6251A/B
Marking Information
RT6251AHGQW
0L= : Product Code
Pin Configurations
(TOP VIEW)
RT/SYNC
PGOOD
14
13
12
0L=YM
DNN
YMDNN : Date Code
1
RT6251ANGQW
0K= : Product Code
FB
RT6251BHGQW
0J= : Product Code
WQFN-14AL 3.5x3.5
0J=YM
DNN
YMDNN : Date Code
RT6251BNGQW
0H= : Product Code
0H=YM
DNN
YMDNN : Date Code
Functional Pin Description
Pin No.
1
Pin Name
RT/SYNC
Pin Function
Oscillator Resistor and External Frequency Synchronization Input. Connecting a
resistor from this pin to GND sets the switching frequency or connecting an
external clock to this pin changes the switching frequency.
System Ground. Provide the ground return path for the control circuitry and
low-side power MOSFET. The exposed pad must be soldered to a large PCB
and connected to GND for minimum power dissipation.
Power Input. Supplies the power switches of the device.
Supply Voltage Input. Supplies the control circuitry and internal reference of the
device.
Feedback Voltage Input. This pin is used to set the desired output voltage via an
external resistive divider. The feedback reference voltage is 0.8V typically.
Compensation Node. The current comparator threshold increases with this
control voltage. Connect external compensation elements to this pin to stabilize
the control loop.
Soft-Start and Tracking Control Input. Connect a capacitor from SS to GND to
set the soft-start period. The soft-start period can be used to track and sequence
when the external voltage on this pin overrides the internal reference.
Enable Control Input. Floating this pin or connecting this pin to logic high can
enable the device and connecting this pin to GND can disable the device.
is a registered trademark of Richtek Technology Corporation.
2, 3,
GND
15 (Exposed Pad)
4, 5
6
7
8
PVIN
VIN
FB
COMP
9
10
SS/TR
EN
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
DS6251A/B-00 November 2015
COMP
0K=YM
DNN
YMDNN : Date Code
GND
GND
PVIN
PVIN
VIN
2
3
4
5
6
7
15
8
GND
11
10
9
BOOT
LX
LX
EN
SS/TR
RT6251A/B
Pin No.
11, 12
13
14
Pin Name
LX
BOOT
PGOOD
Pin Function
Switch Node. LX is the switching node that supplies power to the output and
connect the output LC filter from LX to the output load.
Bootstrap Supply for High-Side Gate Driver. Connect a 100nF or greater
capacitor from LX to BOOT to power the high-side switch.
Power Good Indicator Output. This pin is an open-drain logic output that is pulled
to ground when the output voltage is lower or higher than its specified threshold
under the conditions of OVP, OTP, dropout, EN shutdown, or during slow start.
Function Block Diagram
PGOOD EN
UV
Comparator
91% V
REF
PGOOD
Logic
109% V
REF
OV
Comparator
V
EN
= 1.21V
Slope
Compensation
I
SS
SS/TR
FB
V
REF
+
+
-
VIN
PVIN
I
P
I
hys
Thermal
Detector
UVLO
Regulator
Shutdown
Logic
Shutdown
BOOT
High-Side
Current Sense
Current
Comparator
Power Stage
Control
Logic,
Driver, and
Boot UVLO
LX
Voltage
Reference
Oscillator with
RT/SYNC
Function
COMP
RT/SYNC
Low-Side
Current Sense
GND
Operation
UV Comparator
If the feedback voltage (V
FB
) is lower than threshold voltage
(91% of V
REF
), the UV Comparator's output goes high
and the logic control circuit is allowed to turn on the
MOSFET to pull PGOOD pin to low.
OV Comparator
If the feedback voltage (V
FB
) is higher than threshold voltage
(109% of V
REF
), the OV Comparator's output goes high
and the logic control circuit is allowed to turn on the
MOSFET to pull PGOOD pin to low.
Voltage Reference
The converter produces a precise
±1%
voltage reference
over-temperature by scaling the output of a temperature
stable bandgap circuit.
Error Amplifier
The device uses a transconductance error amplifier. The
error amplifier compares the FB pin voltage with the SS/
TR pin voltage and the internal reference voltage which is
0.8V. The transconductance of the error amplifier is 1300
μA/V
during normal operation. The compensation network
should be connected between the COMP pin and ground.
Oscillator with RT/SYNC function
The switching frequency is adjustable by an external
resistor connected between the RT/SYNC pin and GND.
The available frequency range is from 200kHz to 1.2MHz.
An internal synchronized circuit has been implemented
to switch from RT mode to SYNC mode. To implement
the synchronization function, connect a square wave clock
signal to the RT/SYNC pin with a duty cycle between
10% to 90%. The switching cycle is synchronized to the
falling edge of the external clock at RT/SYNC pin.
is a registered trademark of Richtek Technology Corporation.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
DS6251A/B-00 November 2015
www.richtek.com
3
RT6251A/B
Absolute Maximum Ratings
(Note 1)
−0.3V
to 20V
−0.3V
to (V
IN
+ 0.3V)
−5V
to (V
IN
+ 6.3V)
−0.3V
to (V
IN
+ 6.3V)
−0.3V
to 6V
2.083W
Supply Input Voltage, VIN, PVIN --------------------------------------------------------------------------------
Switch Node Voltage, LX ------------------------------------------------------------------------------------------
LX (t
≤
10ns) ---------------------------------------------------------------------------------------------------------
BOOT Pin Voltage --------------------------------------------------------------------------------------------------
Other Pins ------------------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
WQFN-14AL 3.5x3.5 ----------------------------------------------------------------------------------------------
Package Thermal Resistance (Note 2)
WQFN-14AL 3.5x3.5,
θ
JA
----------------------------------------------------------------------------------------- 48°C/W
WQFN-14AL 3.5x3.5,
θ
JC
---------------------------------------------------------------------------------------- 3.8°C/W
Junction Temperature ---------------------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------ 260°C
Storage Temperature Range -------------------------------------------------------------------------------------
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------------- 2kV
CDM (Charged Device Model) ----------------------------------------------------------------------------------- 2kV
(Note 4)
Recommended Operating Conditions
Power Input Voltage, PVIN --------------------------------------------------------------------------------------- 1.6V to 18V
Supply Input Voltage, VIN ---------------------------------------------------------------------------------------- 4.5V to 18V
Junction Temperature Range -------------------------------------------------------------------------------------
−40°C
to 125°C
Electrical Characteristics
(V
IN
= 4.5V to 18V, V
PVIN
= 1.6V to 18V, T
A
= 25°C, unless otherwise specified)
Parameter
Supply Voltage
PVIN Power Input
Operating Voltage
VIN Supply Input
Operating Voltage
Under-Voltage Lockout
Threshold
Under-Voltage Lockout
Threshold Hysteresis
VIN Shutdown Current
Symbol
Test Conditions
Min
Typ
Max
Unit
V
PVIN
V
IN
V
UVLO
V
UVLO
V
SHDN
V
EN
= 0V
RT6251A, V
FB
= 0.83V, Not
Switching
RT6251B, V
FB
= 0.83V, Not
Switching
V
EN
Rising
V
EN
Falling
V
IN
Rising
1.6
4.5
--
--
--
--
--
--
--
4
150
3
800
600
18
18
4.5
--
9
1150
1000
A
mV
V
VIN Quiescent Current
I
Q
Enable Voltage
EN Threshold Voltage
V
IH
V
IL
--
1.1
1.21
1.17
1.27
--
V
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6251A/B-00 November 2015
RT6251A/B
Parameter
Pull-Up Current
Hysteresis Current
Reference Voltage
Reference Voltage
V
REF
0A
I
LOAD
3A
R
OSC
= 37k
Switching Frequency
f
OSC
R
OSC
= 110k
R
OSC
= 270k
Switching Frequency Range
Minimum Sync Pulse Width
SYNC Threshold Voltage
SYNC Falling Edge to LX
Rising Edge Delay
Internal MOSFET
High-Side On-Resistance
Low-Side On-Resistance
LX and BOOT
Minimum On-Time
Minimum Off-Time
BOOTLX UVLO
Soft-Start and Tracking
Internal Charge Current
SS to Feedback Offset
Current Limit
High-Side Switch Current
Limit
Low-Side Switch Sourcing
Current Limit
Low-Side Switch Sinking
Current Limit
Error Amplifier
Error Amplifier
Trans-conductance
Error Amplifier DC Gain
Error Amplifier Sink/Source
Current
COMP to I
switch
gm
gm
2A
< I
COMP
< 2A,
V
COMP
= 1V
V
FB
= 0.8V
V
COMP
= 1V, 100mV input
overdrive
--
1000
--
--
1300
3100
110
12
--
--
--
--
A/V
V/V
A
A/V
4.2
3.8
--
6.2
5.8
1.2
--
--
--
A
V
SS
= 0.4V
--
--
2
20
--
60
A
mV
V
BL-UVLO
Measured at 90% to 90% of V
LX
,
I
LX
= 2A, 25°C
V
BOOT
V
LX
3V
--
--
--
--
0
--
135
--
3
V
ns
R
DS(ON)_H
R
DS(ON)_L
V
BOOT
V
LX
= 5.5V
V
IN
= 12V
--
--
46
36
60
50
m
High-Level
Low-Level
Measure at 500kHz with R
OSC
resistor in series
Include Sync mode and RT
mode set point
0.792
1080
400
160
200
--
--
0.8
--
0.8
1200
480
200
--
20
--
--
66
0.808
1320
560
240
1200
--
2
--
--
ns
V
ns
kHz
V
Timing Resistor and External Clock
Symbol
Test Conditions
V
EN
= 1.1V
V
EN
= 1.3V
Min
--
--
Typ
1
3
Max
--
--
Unit
A
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6251A/B-00 November 2015
www.richtek.com
5
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