®
RT6251A/B
3A, 18V, Synchronous Step-Down Converter
General Description
Features
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.
Ω/36mΩ
Ω
Low RDS(ON) Power MOSFET Switches 46mΩ
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)
Applications
Lead Plating System
G : Green (Halogen Free and Pb Free)
H : Hiccup UVP
N : Non-UVP
A : PSM
B : PWM
High Performance Point of Load Regulation
Notebook Computers
High Density and Distributed Power Systems
Note :
Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
VIN
CIN
RT6251A/B
BOOT
VIN
CBOOT
PVIN
Enable
VOUT
R1
EN
PGOOD
PGOOD
ROSC
RT/SYNC
FB
COMP
SS/TR
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
COUT
RCOMP1
CCOMP2
CSS
DS6251A/B-00 November 2015
L
LX
CCOMP1
R2
GND
is a registered trademark of Richtek Technology Corporation.
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1
RT6251A/B
Marking Information
Pin Configurations
YMDNN : Date Code
RT6251ANGQW
0K= : Product Code
14
2
13
3
12
GND
4
11
5
10
15
6
7
FB
0K=YM
DNN
YMDNN : Date Code
GND
GND
PVIN
PVIN
VIN
1
9
BOOT
LX
LX
EN
SS/TR
8
COMP
0L=YM
DNN
PGOOD
0L= : Product Code
RT/SYNC
(TOP VIEW)
RT6251AHGQW
WQFN-14AL 3.5x3.5
RT6251BHGQW
0J= : Product Code
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
2, 3,
GND
15 (Exposed Pad)
4, 5
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.
PVIN
Power Input. Supplies the power switches of the device.
6
VIN
Supply Voltage Input. Supplies the control circuitry and internal reference of the
device.
7
FB
8
COMP
9
SS/TR
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.
10
EN
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.
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.
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is a registered trademark of Richtek Technology Corporation.
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RT6251A/B
Pin No.
Pin Name
Pin Function
LX
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.
13
BOOT
Bootstrap Supply for High-Side Gate Driver. Connect a 100nF or greater
capacitor from LX to BOOT to power the high-side switch.
14
PGOOD
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.
11, 12
Function Block Diagram
PGOOD EN
VIN
UV
Comparator
Thermal
Detector
Ihys
IP
91% VREF
PGOOD
Logic
109% VREF
Shutdown
Logic
VEN = 1.21V
OV
Comparator
High-Side
Current Sense
ISS
VREF
UVLO
Regulator
Slope
Compensation
SS/TR
FB
PVIN
+
+
-
Current
Comparator
Voltage
Reference
Oscillator with
RT/SYNC
Function
COMP
Shutdown
Power Stage
Control
Logic,
Driver, and
Boot UVLO
Low-Side
Current Sense
BOOT
LX
GND
RT/SYNC
Operation
UV Comparator
Error Amplifier
If the feedback voltage (VFB) is lower than threshold voltage
(91% of VREF), 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.
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.
OV Comparator
If the feedback voltage (VFB) is higher than threshold voltage
(109% of VREF), 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.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS6251A/B-00 November 2015
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.
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RT6251A/B
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, VIN, PVIN -------------------------------------------------------------------------------Switch Node Voltage, LX -----------------------------------------------------------------------------------------LX (t ≤ 10ns) --------------------------------------------------------------------------------------------------------BOOT Pin Voltage -------------------------------------------------------------------------------------------------Other Pins -----------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
WQFN-14AL 3.5x3.5 ---------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
WQFN-14AL 3.5x3.5, θJA ----------------------------------------------------------------------------------------WQFN-14AL 3.5x3.5, θJC ---------------------------------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Storage Temperature Range ------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------------CDM (Charged Device Model) -----------------------------------------------------------------------------------
Recommended Operating Conditions
−0.3V to 20V
−0.3V to (VIN + 0.3V)
−5V to (VIN + 6.3V)
−0.3V to (VIN + 6.3V)
−0.3V to 6V
2.083W
48°C/W
3.8°C/W
150°C
260°C
−65°C to 150°C
2kV
2kV
(Note 4)
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
(VIN = 4.5V to 18V, VPVIN = 1.6V to 18V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Supply Voltage
PVIN Power Input
Operating Voltage
VPVIN
1.6
--
18
VIN Supply Input
Operating Voltage
VIN
4.5
--
18
Under-Voltage Lockout
Threshold
VUVLO
--
4
4.5
Under-Voltage Lockout
Threshold Hysteresis
VUVLO
--
150
--
VIN Shutdown Current
VSHDN
VEN = 0V
--
3
9
RT6251A, VFB = 0.83V, Not
Switching
--
800
1150
RT6251B, VFB = 0.83V, Not
Switching
--
600
1000
VIH
VEN Rising
--
1.21
1.27
VIL
VEN Falling
1.1
1.17
--
VIN Quiescent Current
IQ
VIN Rising
V
mV
A
Enable Voltage
EN Threshold Voltage
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V
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DS6251A/B-00 November 2015
RT6251A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
Pull-Up Current
VEN = 1.1V
--
1
--
Hysteresis Current
VEN = 1.3V
--
3
--
0A ILOAD 3A
0.792
0.8
0.808
ROSC = 37k
1080
1200
1320
ROSC = 110k
400
480
560
ROSC = 270k
160
200
240
Include Sync mode and RT
mode set point
200
--
1200
--
20
--
High-Level
--
--
2
Low-Level
0.8
--
--
Measure at 500kHz with ROSC
resistor in series
--
66
--
Unit
A
Reference Voltage
Reference Voltage
VREF
V
Timing Resistor and External Clock
Switching Frequency
fOSC
Switching Frequency Range
Minimum Sync Pulse Width
SYNC Threshold Voltage
SYNC Falling Edge to LX
Rising Edge Delay
kHz
ns
V
ns
Internal MOSFET
High-Side On-Resistance
RDS(ON)_H
VBOOT VLX = 5.5V
--
46
60
Low-Side On-Resistance
RDS(ON)_L
VIN = 12V
--
36
50
Minimum On-Time
Measured at 90% to 90% of VLX,
ILX = 2A, 25°C
--
--
135
Minimum Off-Time
VBOOT VLX 3V
--
0
--
--
--
3
V
--
2
--
A
--
20
60
mV
High-Side Switch Current
Limit
4.2
6.2
--
Low-Side Switch Sourcing
Current Limit
3.8
5.8
--
--
1.2
--
--
1300
--
A/V
1000
3100
--
V/V
--
110
--
A
--
12
--
A/V
m
LX and BOOT
BOOTLX UVLO
VBL-UVLO
ns
Soft-Start and Tracking
Internal Charge Current
SS to Feedback Offset
VSS = 0.4V
Current Limit
Low-Side Switch Sinking
Current Limit
A
Error Amplifier
Error Amplifier
Trans-conductance
gm
2A < ICOMP < 2A,
VCOMP = 1V
Error Amplifier DC Gain
VFB = 0.8V
Error Amplifier Sink/Source
Current
VCOMP = 1V, 100mV input
overdrive
COMP to Iswitch gm
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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RT6251A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Power Good
Power Good Rising
Threshold
VFB Rising (Good)
--
94
--
VFB Rising (Fault)
--
109
--
Power Good Falling
Threshold
VFB Falling (Fault)
--
91
--
VFB Falling (Good)
--
106
-
Power Good Sink Current
Capability
PGOOD signal fault, IPGOOD sinks
2mA
--
--
0.3
V
Power Good Leakage
Current
PGOOD signal good, VPGOOD = 5.5V
--
30
100
nA
Minimum VIN for Indicating
PGOOD
VPGOOD 0.5V, IPGOOD sinks 100A
--
0.6
1
%VREF
%VREF
V
Minimum SS/TR Voltage for
Indicating PGOOD
--
--
2.6
160
175
--
--
10
--
Over-Temperature Protection
Thermal Shutdown
TSD
Thermal Shutdown
Hysteresis
TSD
°C
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
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RT6251A/B
Typical Application Circuit
RT6251A/B
VIN
4.5V to 18V
6
BOOT
VIN
13
CIN
4, 5
10
Enable
PVIN
LX
ROSC
100k
CSS
10nF
1
11, 12
L
VOUT
R1
EN
FB
14 PGOOD
PGOOD
CBOOT
0.1µF
COMP
7
R2
CCOMP1
CCOMP2
RT/SYNC
9 SS/TR
COUT
RCOMP1
8
GND
2, 3,
15 (Exposed Pad)
Table 1. Suggested Component Values
VOUT (V)
R1 (k)
R2 (k)
RCOMP1
(k)
CCOMP1
(nF)
CCOMP2
(pF)
COUT (F)
L (H)
5
126
24
5.23
15
180
44
6.8
3.3
75
24
3.48
15
180
44
4.7
2.5
51
24
2.67
15
180
44
4.7
1.8
30
24
1.91
15
180
44
3.6
1.5
21
24
1.6
15
180
44
3.6
1.2
12
24
1.27
15
180
44
2.2
1
6
24
1
15
180
44
2.2
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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RT6251A/B
Typical Operating Characteristics
Efficiency vs. Output Current
Efficiency vs. Output Current
100
100
90
90
VIN = 9V
VIN = 12V
VIN = 18V
Efficiency (%)
70
80
Efficiency (%)
80
60
50
40
30
VOUT = 5V
VOUT = 3.3V
VOUT = 1.2V
70
60
50
40
30
20
20
10
10
RT6251A, VOUT = 3.3V
RT6251A, VIN = 12V
0
0
0
0.5
1
1.5
2
2.5
0
3
0.5
1
Output Current (A)
Efficiency vs. Output Current
90
90
VIN = 9V
VIN = 12V
VIN = 18V
Efficiency (%)
Efficiency (%)
80
60
50
40
30
20
3
VOUT = 5V
VOUT = 3.3V
VOUT = 1.2V
70
60
50
40
30
20
10
10
RT6251B, VOUT = 3.3V
0
RT6251B, VIN = 12V
0
0
0.5
1
1.5
2
2.5
3
0
0.5
1
Output Current (A)
3.45
3.35
Output Voltage (V)
3.37
RT6251A
3.35
3.30
2
2.5
3
Output Voltage vs. Input Voltage
3.50
3.40
1.5
Output Current (A)
Output Voltage vs. Output Current
Output Voltage (V)
2.5
Efficiency vs. Output Current
100
70
2
Output Current (A)
100
80
1.5
RT6251B
3.25
3.20
3.33
3.31
IOUT = 1A
IOUT = 2A
IOUT = 3A
3.29
3.27
3.25
3.15
VIN = 12V, VOUT = 3.3V
VOUT = 3.3V
3.10
3.23
0
0.5
1
1.5
2
2.5
Output Current (A)
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3
4
6
8
10
12
14
16
18
Input Voltage (V)
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RT6251A/B
Reference Voltage vs. Temperature
Switching Frequency vs. Temperature
650
Switching Frequency (kHz)1
0.95
Reference Voltage (V)
0.90
0.85
0.80
0.75
0.70
600
550
500
450
400
ROSC = 100k
0.65
350
-50
-25
0
25
50
75
100
125
-50
-25
0
Temperature (°C)
4.5
4.2
3.9
3.6
3.3
VEN = 0V
3.0
8
10
12
125
14
16
10
8
6
4
2
VEN = 0V
0
18
-50
-25
0
25
50
75
100
125
Temperature (°C)
Quiescent Current vs. Input Voltage
Quiescent Current vs. Temperature
1400
1400
1200
1200
Quiescent Current (μA)
Quiescent Current (μA)
100
12
Input Voltage (V)
RT6251A
1000
75
Shutdown Quiescent Current vs. Temperature
Shutdown Quiescent Current (μA)1
Shutdown Quiescent Current (μA)1
4.8
6
50
Temperature (°C)
Shutdown Quiescent Current vs. Input Voltage
4
25
800
RT6251B
600
400
RT6251A
1000
800
RT6251B
600
400
VFB = 0.83V
200
VFB = 0.83V
200
4
6
8
10
12
14
16
Input Voltage (V)
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DS6251A/B-00 November 2015
18
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT6251A/B
Current Limit vs. Temperature
9
8
8
Current Limit (A)
Current Limit (A)
Current Limit vs. Input Voltage
9
7
6
5
4
7
6
5
4
3
3
4
6
8
10
12
14
16
18
-50
-25
0
Input Voltag (V)
4.4
1.30
Rising
4.2
Falling
4.1
50
75
100
125
Enable Threshold vs. Temperature
1.35
Enable Voltage (V)
Input Voltage (V)
Input UVLO vs. Temperature
4.5
4.3
25
Temperature (°C)
4.0
1.25
Rising
1.20
Falling
1.15
1.10
VIN = 4.5V
VEN = 3.3V
3.9
1.05
-50
-25
0
25
50
75
100
125
-50
0
25
50
75
100
Temperature (°C)
Load Transient Response
Load Transient Response
RT6251A
125
RT6251B
VOUT
(1V/Div)
VOUT
(1V/Div)
IOUT
(1A/Div)
IOUT
(1A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0 to 3A
Time (100μs/Div)
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-25
Temperature (°C)
VIN = 12V, VOUT = 3.3V, IOUT = 0 to 3A
Time (100μs/Div)
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RT6251A/B
Voltage Ripple
Voltage Ripple
VIN
(200mV/Div)
VIN
(500mV/Div)
VOUT
(10mV/Div)
VOUT
(10mV/Div)
VLX
(20V/Div)
VLX
(20V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN = 12V, VOUT = 3.3V, IOUT = 1A
Time (1μs/Div)
Time (1μs/Div)
Power On from Input Voltage
Power Off from Input Voltage
VIN
(20V/Div)
VIN
(20V/Div)
VPGOOD
(5V/Div)
VPGOOD
(5V/Div)
VOUT
(2V/Div)
VOUT
(2V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (2.5ms/Div)
Time (10ms/Div)
Power On from Enable Voltage
Power Off from Enable Voltage
VEN
(5V/Div)
VEN
(5V/Div)
VPGOOD
(5V/Div)
VPGOOD
(5V/Div)
VOUT
(2V/Div)
VOUT
(2V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (2.5ms/Div)
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VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (2.5ms/Div)
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RT6251A/B
Application Information
This IC is a single phase Buck PWM converter with two
integrated N-MOSFETs. It provides good performance
during load and line transients by implementing a single
feedback loop, current-mode control, and external
compensation. The integrated synchronous power
switches can increase efficiency and it is suitable for lower
duty cycle applications. The switching frequency can be
externally set from 200kHz to 1.2MHz which allows for
high efficiency and optimal size selection of output filter
components. In additional, there is a synchronization
mode control in this device which can be synchronized to
the external clock frequency, and easily switched from
internal switching mode to synchronization mode.
The device contains a power good protection and an
external soft-start function that is able to monitor the
system output voltage for normal regulation and provides
a programmable power up sequence for avoiding inrush
currents efficiently. Furthermore, the device incorporates
a lot of protections such as OVP, OCP, OTP and etc.
Main Control Loop
The device implements an adjustable fixed frequency with
peak current-mode control which offers an excellent
performance over various line and loading. During normal
operation, the internal high-side power switch is turned
on by the internal oscillator initiating. Current in the
inductor increases until the high-side switch current reaches
the current reference converted by the output voltage VCOMP
of the error amplifier. The error amplifier adjusts its output
voltage by comparing the feedback signal from a resistive
voltage divider on the FB pin with an internal 0.8V
reference. When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier increases its current reference until
the average inductor current matches the new load current.
When the high-side power MOSFET turns off, the lowside synchronous power switch (N-MOSFET) turns on
until the beginning of the next clock cycle.
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VIN and PVIN Pins
The VIN and PVIN pins can be used together or separately
for a variety of applications. In this device, the VIN pin is
an input for supplying internal reference and control
circuitry and the PVIN pin is an input for providing main
power to device system and internal high-side power
MOSFET. When the VIN and PVIN pins are tied together,
both pins can operate from 4.5V to 18V. When the VIN
and PVIN pins are used separately, VIN pin must be ranged
from 4.5V to 18V, and the PVIN pin can be applied down
to as low as 1.6V to 18V.
The device incorporates an internal Under-Voltage Lockout
(UVLO) circuitry on the VIN pin. If the VIN pin voltage
exceeds the UVLO rising threshold voltage 4V, the
converter resets and prepares the PWM for operation. If
the VIN pin voltage falls below the falling threshold voltage
3.85V during normal operation, the device is disabled.
Such wide internal UVLO hysteresis of 150mV can
efficiently prevent noise caused reset. There is also an
external UVLO circuitry which can be achieved by
configuring a resistive voltage divider on EN pin for both
input VIN and PVIN pins and it is able to provide either
input pins an adjustable UVLO function to ensure a proper
power up behavior. More discussions are located in the
section of Enable Operation.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense
the output voltage as shown in Figure 1.
VOUT
R1
FB
RT6251A/B
R2
GND
Figure 1. Setting the Output Voltage
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RT6251A/B
For high efficiency, the divider resistance must adopt larger
values, but too large values may induce noises and voltage
errors by the coupled FB pin input current. It is
recommended to use the values between 10kΩ and
100kΩ. The output voltage is set by an external resistive
voltage divider according to the following Equation (1) :
VOUT = VREF 1 R1
(1)
R2
where VREF is the feedback reference voltage (0.8V typ.).
Soft-Start
The device contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timing
is programmed by the external capacitor between SS/TR
pin and GND. The device provides an internal 2μA charge
current for the external capacitor. If a 10nF capacitor is
used to set the soft-start, the period can be 4ms. The
calculations for external charge capacitor CSS and soft-
separated inductor current signal is used to monitor overcurrent condition, so the maximum output current stays
relatively constant regardless of duty cycle. More
discussions about over-current protection are described
in a later section.
Enable Operation
The EN pin is an device enable input. Pulling the EN pin
to logic low that is typically less than the set threshold
voltage 1.17V, the device shuts down and enters to low
quiescent current state about 2μA. The regulator starts
switching again once the EN pin voltage exceeds the
threshold voltage 1.21V. In additional, the EN pin is
implemented with an internal pull-up current source which
allows to enable the device when the EN pin is floating.
For general external timing control, the EN pin can be
externally pulled high by adding a capacitor and a resistor
from the VIN pin as Figure 2.
start time TSS are shown in Equation (2) :
TSS =
CSS VREF
ISS
(2)
where CSS is the external soft-start capacitor, ISS is the
soft-start charge current (2μA), VREF is the feedback
reference voltage (0.8V).
Once the input voltage falls below UVLO threshold, the
EN pin is pulled low, or the OTP is triggered, the device
stops switching and the SS/TR pin starts to discharge. It
is held such shutdown condition until the event is cleared
and the SS/TR pin has already discharged to ground
ensuring proper soft-start behavior.
During the pre-biased start-up sequence, the output of
device is not discharged by low-side power switch
because the device is designed to prevent low-side
MOSFET sinking. It is allowed to sink when the SS/TR
pin exceeds 2.1V.
Slope Compensation
Slope compensation provides stability in constant
frequency architectures by preventing sub-harmonic
oscillations at duty cycles greater than 50%. It is
accomplished internally by adding a compensating ramp
to the inductor current signal. Normally, the peak inductor
current is remained constant under the whole duty cycle
range when slope compensation is added. For the device,
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS6251A/B-00 November 2015
VIN
VIN
REN
CEN
RT6251A/B
EN
GND
Figure 2. Enable Timing Control
An external MOSFET can be added to implement digital
control from the EN pin to ground, as shown in Figure 3.
In this case, there is no need to connect a pull-up resistor
between the VIN and EN pins since the EN pin is pulled
up by the internal current source. The device can simply
achieve the digital control only through an external
MOSFET on EN pin.
VIN
External
Digital Control
VIN
RT6251A/B
EN
GND
Figure 3. Digital Enable Control
The EN pin can also be applied to adjust its Under-Voltage
Lockout (UVLO) threshold with two external resistors
divider from the both input VIN and PVIN pins used together
or separately, and the application structures can refer to
Figure 4, Figure 5, and Figure 6.
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13
RT6251A/B
Separated
Supply
REN1
PVIN
RT6251A/B
EN
REN2
GND
Figure 4. Resistor Divider for PVIN UVLO Setting
Separated
Supply
REN1
VIN
RT6251A/B
EN
REN2
GND
Figure 5. Resistor Divider for VIN UVLO Setting,
VIN ≥ 4.5V
Combined
Supply
REN1
PVIN
VIN
RT6251A/B
EN
Adjustable Operating Frequency-RT mode
Selection of the operating frequency is a tradeoff between
efficiency and component size. Higher operating frequency
allows the use of smaller inductor and capacitor values
but it may press the minimum controllable on-time to affect
devices stability. Lower operating frequency improves
efficiency by reducing internal gate charge and switching
losses but requires larger inductance and capacitance to
maintain low output ripple voltage.
The operating frequency of the device is determined by
an external resistor ROSC, that is connected between the
RT/SYNC pin and ground. The value of the resistor sets
the ramp current which is used to charge and discharge
an internal timing capacitor within the oscillator. The
practical switching frequency ranges from 200kHz to
1.2MHz. Determine the ROSC resistor value by examining
the curve in Figure 7.
REN2
1200
Figure 6. Resistor Divider for PVIN and VIN UVLO
Setting
Under above application structures, the adjustable UVLO
function of EN pin allows to achieve a secondary UVLO
on PVIN pin, a higher UVLO on VIN pin or even a common
UVLO on both VIN and PVIN pins. For example, if the EN
pin is configured as Figure 5 and the output voltage is set
to a higher value 10V. The device may shut down after
soft-start sequence is over, and the reason for the result
is that the VOUT is still lower than its set target during the
VIN rising period even though VIN has already risen to its
internal UVLO threshold 4V. To prevent this situation, an
adjustable UVLO threshold from EN pin is useful to avoid
such high output transfer condition. The exact UVLO
thresholds can be calculated by Equation (3). The setting
VOUT is 10V and VIN is from 0V to 18V. When VIN is higher
than 12V, the device is triggered to enable the converter.
Assume REN1 = 56kΩ. Then,
R
V
REN2 = EN1 IH
(3)
VIN_S VIH
where VIH is the typical threshold of enable rising (1.21V)
and VIN_S is the target turn on input voltage (12V in this
example). According to the equation, the suggested
resistor REN2 is 6.28kΩ.
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14
Switching Frequency (kHz)1
GND
1000
800
600
400
200
0
50
100
150
200
250
300
ROSC (k Ω)
Figure 7. Switching Frequency vs. ROSC Resistor
Synchronization-SYNC mode
The device is allowed to synchronize with an external
square wave clock ranging from 200kHz to 1.2MHz applied
to the RT/SYNC pin. The range of sync duty cycle must
be from 20% to 80%, and the amplitude of sync signal
must be higher than 2V and lower than 0.8V. During the
SYNC mode operation, the switching cycle of LX pin is
synchronized to the falling edge of the external sync
signal.
is a registered trademark of Richtek Technology Corporation.
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RT6251A/B
Before the external sync signal is provided to the RT/
SYNC pin, the device operates at the original switching
frequency set by resistor ROSC. When the sync signal is
provided, the SYNC mode overrides the RT mode to force
the device synchronizing to external frequency. This IC
can easily switch between RT mode and SYNC mode,
and the application structure can be configured as Figure
8.
External
Sync Signal
RT/SYNC
ROSC
RT6251A/B
GND
Figure 8. External Sync Signal Control
Power Good Output
The power good output is an open-drain output and needs
to connect a voltage source below 5.5V with a pull-up
resistor for avoiding the PGOOD floating. When the output
voltage is 9% above or 9% below its set voltage, PGOOD
is pulled low. It is held low until the output voltage returns
within the allowed tolerances ±6% once more. During softstart, PGOOD is actively held low when VIN is greater
than 1V and is only allowed to be high when soft-start
period is over that means the SS/TR pin exceeds 2.1V
typically and the output voltage reaches 94% of its set
voltage. Besides, the PGOOD pin is also pulled low when
the input UVLO or OVP are triggered, EN pin is pulled
below 1.21V or the OTP is occurred.
External Bootstrap Diode
Connect a 100nF low ESR ceramic capacitor between
the BOOT and SW pins. This capacitor provides the gate
driver voltage for the high-side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and the BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65% .The bootstrap diode can be a
low cost one such as IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
RT6251A/B. Note that the external boot voltage must be
lower than 5.5V.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS6251A/B-00 November 2015
5V
BOOT
RT6251A/B
1µF
SW
Figure 9. External Bootstrap Diode
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
V
V
(4)
IL = OUT 1 OUT
VIN
f L
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Highest efficiency operation is achieved by reducing
ripple current at low frequency, but it requires a large
inductor to attain this goal.
For the ripple current selection, the value of ΔIL = 0.24
(IMAX) is a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
ripple current stays below a specified maximum, the
inductor value should be chosen according to the following
equation :
VOUT
VOUT
L =
(5)
1 VIN(MAX)
f
I
L(MAX)
In this device, 3.7μH is recommended for initial design.
The current rating of the inductor (caused a 40°C
temperature rising from 25°C ambient) must be greater
than the maximum load current and ensure that the peak
current does not saturate the inductor during short-circuit
condition. Referring the Table 2 for the inductor selection
reference.
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Series
Dimensions (mm)
TDK
TDK
TAIYO YUDEN
WE
WE
VLF10045
SLF12565
NR8040
744325
744355
10 x 9.7 x 4.5
12.5 x 12.5 x 6.5
8x8x4
10.2 x 10.2 x 4.7
12.8 x 12.8 x 6.2
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RT6251A/B
Input and Output Capacitors Selection
The input capacitance CIN is needed to filter the trapezoidal
current at the Source of the high-side MOSFET. To prevent
large ripple current, a low ESR input capacitor sized for
the maximum RMS current should be used. The RMS
current is given by Equation (6) :
V
IRMS = IOUT(MAX) OUT
VIN
VIN
1
VOUT
(6)
The formula above has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst condition is commonly
used for design because even significant deviations do
not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. For the
input capacitor, Two 10μF and one 4.7μF low ESR ceramic
capacitors are recommended for bypassing the PVIN pin
and VIN pin respectively and an additional 0.1μF is
recommended to place as close as possible to the IC
input side for high frequency filtering. All the recommended
input and output capacitors can refer to Table 3 for more
detail.
Table 3. Suggested Capacitors for CIN and COUT
Location
Component Supplier
Part No.
Capacitance (F)
Case Size
CIN
MURATA
GRM32ER71C226M
22
1210
CIN
TDK
C3225X5R1C226M
22
1210
COUT
MURATA
GRM31CR60J476M
47
1206
COUT
TDK
C3225X5R0J476M
47
1210
COUT
MURATA
GRM32ER71C226M
22
1210
COUT
TDK
C3225X5R1C226M
22
1210
The selection of COUT is determined by the required ESR
to minimize voltage ripple. Moreover, the amount of bulk
capacitance is also a key for COUT selection to ensure
that the control loop is stable. Loop stability can be
checked by viewing the load transient response. The
output ripple ΔVOUT is determined by Equation (7) :
1
VOUT IL ESR
8fCOUT
(7)
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input VIN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at VIN large enough to damage the
part.
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16
Level Frequency Shift
While the FB pin drops, switching frequency is
proportional to the feedback voltage, this is a level
frequency reduced function which is implemented in the
device. For the same short-circuit example, when the
output voltage drops during over-current condition, the
switching frequency is reduced in direct proportion to the
output voltage, so the low-side MOSFET is turned off long
enough to reduce the inductor current to prevent a current
runaway issue. With function of level frequency reducing,
the switching frequency can reduce from 100%, 50%, then
25% as the voltage decreases from 0.8V to 0V on FB pin.
The principle of level frequency reducing is also allowed
to cover the soft-start sequence to increase the switching
frequency as feedback voltage increases from 0V to 0.8V.
Output Over-Voltage Protection
The device provides an output Over-Voltage Protection
(OVP) once the output voltage exceeds 109% of VOUT,
the OVP function turns off the high-side power MOSFET
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RT6251A/B
to stop current flowing to the output which can only be
released when the output voltage drops below 106% of
VOUT. There is a 5μs delay also built into the over-voltage
protection circuit to prevent false transition. Using this
OVP feature can easily minimize the output overshoot.
incorporating this additional protection, the device is able
to prevent an excessive sinking current from the load during
the condition of pre-biased output and the SS/TR pin is
asserted high that is 2.1V or above.
Over-Temperature Protection
High-Side MOSFET Over-Current Protection
The Over-Current Protection (OCP) of high-side MOSFET
is implemented in this device, it adopts monitoring inductor
current during the on-state to control the COMP pin voltage
for turning off the high-side MOSFET. Each cycle the
separated inductor current signal is compared through
sensing the external inductor current to the COMP pin
voltage from an error amplifier output. If the separated
inductor current peak value exceeds the set current limit
threshold, the high-side power switch is turned off.
Low-Side MOSFET Over-Current Protection
The device not only implements the high-side over-current
protection but also provides the over sourcing current
protection and over sinking current protection for low-side
MOSFET. With these three current protections, the IC
can easily control inductor current at both side power
switches and avoid current runaway for short-circuit
condition.
For the sourcing current protection, there is a specific
comparator in internal circuitry to compare the low-side
MOSFET sourcing current to the internal set current limit
at the end of every clock cycle. When the low-side
sourcing current is higher than the set sourcing limit, the
high-side power switch is not turned on and low-side power
switch is kept on until the following clock cycle for releasing
the above sourcing current to the load. It is allowed to
turn on the high-side MOSFET again when the low-side
current is lower than the set sourcing current limit at the
beginning of a new cycle.
For the sinking current protection, it is implemented by
detecting the voltage across the low-side power switch. If
the low-side reverse current exceeds the set sinking limit,
both power switches are off immediately, and it is held to
stop switching until the beginning of next cycle. By
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An Over-Temperature Protection (OTP) is contained in the
device. The protection is triggered to force the device
shutdown for protecting itself when the junction
temperature exceeds 175°C typically. Once the junction
temperature drops below the hysteresis 10°C typically,
the device is re-enable and automatically reinstates the
power up sequence.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WQFN-14AL 3.5x3.5 package, the thermal resistance, θJA,
is 48°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formula :
PD(MAX) = (125°C − 25°C) / (48°C/W) = 2.083W for
WQFN-14AL 3.5x3.5 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 10 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
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RT6251A/B
Maximum Power Dissipation (W)1
2.5
Layout Considerations
Four-Layer PCB
2.0
Follow the PCB layout guidelines for optimal performance
of the device.
1.5
Keep the traces of the main current paths as short and
wide as possible.
1.0
Put the input capacitor as close as possible to VIN and
PVIN pins.
LX node is with high frequency voltage swing and should
be kept at small area. Keep analog components away
from the LX node to prevent stray capacitive noise pickup.
Connect feedback network behind the output capacitors.
Keep the loop area small. Place the feedback
components near the device.
Connect all analog grounds to a common node and then
connect the common node to the power ground behind
the output capacitors.
An example of PCB layout guide is shown in Figure 11
for reference.
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 10. Derating Curve of Maximum Power
Dissipation
Place the input and output capacitors
as close to the IC as possible.
PGOOD
1
14
2
13
3
12
GND
4
11
5
10
15
6
8
FB
7
CVIN
9
BOOT
LX
LX
EN
SS/TR
R2
COMP
CPVIN
GND
GND
PVIN
PVIN
VIN
RT/SYNC
ROSC
CBOOT
L
COUT
VOUT
LX should be connected
to inductor by wide and
short trace, and keep
sensitive components
away from this trace.
CSS
RCOMP
Place the feedback
components as close
to the IC as possible.
R1 CCOMP
Figure 11. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
DS6251A/B-00 November 2015
RT6251A/B
Outline Dimension
1
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min.
Max.
Min.
Max.
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.200
0.300
0.008
0.012
D
3.400
3.600
0.134
0.142
D2
2.000
2.100
0.079
0.083
D3
0.200
0.008
E
3.400
3.600
0.134
0.142
E2
2.000
2.100
0.079
0.083
E3
0.325
0.013
e
0.500
0.020
e1
1.500
0.059
K
0.350
0.014
L
0.350
0.450
0.014
0.018
W-Type 14AL QFN 3.5x3.5 Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
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