®
RT8251
5A, 24V, 570kHz Step-Down Converter
General Description
The RT8251 is a monolithic step-down switch mode
converter with a built-in internal power MOSFET. It achieves
5A continuous output current over a wide input supply
range with excellent load and line regulation. Current mode
operation provides fast transient response and eases loop
stabilization.
The RT8251 provides protection functions such as
cycle-by-cycle current limiting and thermal shutdown. In
shutdown mode, the regulator draws 25μA of supply
current. Programmable soft-start minimizes the inrush
supply current and the output overshoot at initial startup.
The RT8251 requires a minimum number of external
components. The RT8251 is available in WQFN-16L 3x3
and SOP-8 (Exposed Pad) packages.
Features
Wide Operating Input Voltage Range : 4.75V to 24V
Adjustable Output Voltage Range : 0.8V to 15V
Output Current up to 5A
25μA Low Shutdown Current
μ
Internal Power MOSFET : 70mΩ
Ω
High Efficiency up to 95%
570kHz Fixed Switching Frequency
Stable with Low ESR Output Ceramic Capacitors
Thermal Shutdown Protection
Cycle-By-Cycle Over Current Protection
RoHS Compliant and Halogen Free
Applications
Distributed Power Systems
Battery Charger
DSL Modems
Pre-regulator for Linear Regulators
Pin Configurations
(TOP VIEW)
VIN
VIN
SW
SW
16 15 14 13
Ordering Information
12
VIN
VIN
VIN
GND
1
2
3
4
5
6
7
8
SW
RT8251
Package Type
QW : WQFN-16L 3x3 (W-Type)
SP : SOP-8 (Exposed Pad-Option 1)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
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.
GND
17
SW
10
SW
9
BOOT
11
WQFN-16L 3x3
BOOT
VIN
SW
GND
2
3
4
8
7
FB
COMP
EN
SS
SS
EN
COMP
FB
GND
9
6
5
SOP-8 (Exposed Pad)
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS8251-04 February 2013
www.richtek.com
1
RT8251
Marking Information
RT8251GQW
GE= : Product Code
YMDNN : Date Code
RT8251GSP
RT8251GSP : Product Number
GE=YM
DNN
RT8251
GSPYMDNN
YMDNN : Date Code
Typical Application Circuit
V
IN
4.75V to 24V
Chip Enable
1, 2, 3,
15, 16
C
IN
10µF x 2
VIN
BOOT
RT8251
7 EN
8 SS
9
C
BOOT
L
100nF 4.7µH
D
B540C
R
C
22k
R1
30.9k
R2
10k
V
OUT
3.3V/5A
C
OUT
22µF x 2
SW 10 to 14
FB 5
C
SS
10nF
4,
Exposed Pad (17)
GND
COMP
6
C
C
2.2nF
C
P
(Open)
Figure 1. Typical Application Circuit for WQFN-16L 3x3
V
IN
4.75V to 24V
Chip Enable
2
C
IN
10µF x 2
VIN
BOOT
RT8251
SW
1
3
C
BOOT
100nF
D
B540C
C
C
2.2nF
L
4.7µH
R1
30.9k
R2
10k
V
OUT
3.3V/5A
C
OUT
22µF x 2
7 EN
8 SS
4,
Exposed Pad(9)
GND
C
SS
10nF
FB 5
COMP
6
R
C
22k
C
P
NC
Figure 2. Typical Application Circuit for SOP-8 (Exposed Pad)
Table 1. Recommended Component Selection
V
OUT
(V)
R1 (kΩ)
R2 (kΩ)
R
C
(kΩ)
C
C
(nF)
L1 (μH)
C
OUT
(μF)
15
10
8
5
3.3
2.5
1.8
1.2
182
115
91
52.3
30.9
21.5
12.4
4.99
10
10
10
10
10
10
10
10
51
43
39
30
22
16
13
13
1
1.2
1.5
1.5
2.2
2.2
2.2
2.2
22
10
10
6.8
4.7
4.7
2.2
2.2
44
44
44
44
44
44
44
44
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS8251-04 February 2013
RT8251
Functional Pin Description
Pin No.
WQFN-16L 3x3
Pin Name
SOP-8
(Exposed Pad)
Pin Function
Power Input. VIN supplies the power to the IC, as well as the
step-down converter switches. Connect VIN with a 4.75V to 24V
power source. Connect VIN to GND with a capacitor that the
capacitance is large enough to eliminate noise on the input to the
IC.
Ground. This pin is the voltage reference for the regulated output
voltage. For this reason, care must be taken in its layout. This
node should be placed outside of the D1 to C
IN
ground path to
prevent switching current spikes from inducing voltage noise into
the part. The exposed pad must be soldered to a large PCB and
connected to GND for maximum power dissipation.
Feedback Input. An external resistor divider from the output to
GND, tapped to the FB pin, sets the output voltage.
Compensation Node. This node is the output of the
transconductance error amplifier and the input to the current
comparator. Frequency compensation is done at this node by
connecting a series R-C to ground.
Enable Input. EN is a digital input that turns the regulator on or
off. Drive EN higher than 1.4V to turn on the regulator, lower
than 0.4V to turn it off. For automatic startup, leave EN
unconnected.
Soft-Start Control Input. SS controls the soft start period.
Connect a capacitor (≧10nF) from SS to GND to set the
soft-start period. A 10nF capacitor sets the Soft-Start period to
1ms.
Bootstrap. This capacitor C
BOOT
is needed to drive the power
switch’s gate above the supply voltage. It is connected between
the SW and BS pins to form a floating supply across the power
switch driver. The voltage across C
BOOT
is about 5V and is
supplied by the internal +5V supply when the SW pin voltage is
low.
Power Switching Output. SW is the switching node that supplies
power to the output. Connect the output LC filter from SW to the
output load. Note that a capacitor is required from SW to BOOT
to power the high-side switch.
1, 2, 3, 15, 16
2
VIN
4,
4,
GND
17 (Exposed Pad) 9 (Exposed Pad)
5
5
FB
6
6
COMP
7
7
EN
8
8
SS
9
1
BOOT
10, 11, 12, 13,
14
3
SW
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS8251-04 February 2013
www.richtek.com
3
RT8251
Function Block Diagram
VIN
V
CC
1µA
EN
10k
1.1V
3V
VA V
CC
-
+
Shutdown
Comparator
0.4V
Foldback
Control
+
+
-
Current
Comparator
Logic
Internal
Regulator
Oscillator
570kHz
Current Sense
Slope Comp Amplifier
+
-
BOOT
SW
VA
V
CC
10µA
SS
-
UV
Comparator
GND
+
+
EA
-
Gm = 820µA/V
0.8V
FB
COMP
Absolute Maximum Ratings
(Note 1)
Supply Voltage, V
IN
------------------------------------------------------------------------------------------
−0.3V
to 26V
Switching Voltage, V
SW
-------------------------------------------------------------------------------------
−0.3V
to (V
IN
+
0.3V)
BOOT Voltage, V
BOOT
--------------------------------------------------------------------------------------- (V
SW
−
0.3V) to (V
SW
+
6V)
All Other Pins -------------------------------------------------------------------------------------------------
−0.3V
to 6V
Power Dissipation, P
D
@ T
A
= 25°C
WQFN-16L 3x3 ----------------------------------------------------------------------------------------------- 1.471W
SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------- 1.333W
Package Thermal Resistance (Note 2)
WQFN-16L 3x3,
θ
JA
------------------------------------------------------------------------------------------ 68°C/W
WQFN-16L 3x3,
θ
JC
----------------------------------------------------------------------------------------- 7.5°C/W
SOP-8 (Exposed pad),
θ
JA
--------------------------------------------------------------------------------- 75°C/W
SOP-8 (Exposed Pad),
θ
JC
-------------------------------------------------------------------------------- 15°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
Recommended Operating Conditions
(Note 4)
Supply Voltage, V
IN
------------------------------------------------------------------------------------------ 4.75V to 24V
Enable Voltage, V
EN
----------------------------------------------------------------------------------------- 0V to 5.5V
Junction Temperature Range -------------------------------------------------------------------------------
−
40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------
−
40°C to 85°C
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS8251-04 February 2013
RT8251
Electrical Characteristics
(V
IN
= 12V, T
A
= 25°C unless otherwise specified)
Parameter
Feedback Reference Voltage
High-Side Switch-On Resistance
Low-Side Switch-On Resistance
High-Side Switch Leakage
Current Limit
Current Sense Transconductance
Error Amplifier Tansconductance
Oscillator Frequency
Short Circuit Oscillation Frequency
Maximum Duty Cycle
Minimum On-Time
UVLO Threshold Rising
UVLO Threshold Hysteresis
EN Input Voltage
Logic Low
Logic High
Symbol
V
FB
R
DS(ON)1
R
DS(ON)2
Test Conditions
4.75V
≦
V
IN
≦
24V
Min
0.784
--
--
Typ
0.8
70
15
--
6.8
4.6
920
570
185
85
100
4.1
200
--
--
1
25
0.8
10
1
150
Max
0.816
--
--
10
--
--
--
720
--
--
--
--
--
0.4
5.5
--
--
1
--
--
--
Unit
V
mΩ
Ω
μA
A
A/V
μA/V
kHz
kHz
%
ns
V
mV
V
μA
μA
mA
μA
ms
°C
V
EN
= 0V, V
SW
= 0V
I
LIM
G
CS
gm
f
SW
V
FB
= 0V
D
MAX
t
ON
V
FB
= 0.7V
Duty = 85%; V
BOOT−SW
= 4.8V
Output Current to V
COMP
ΔI
C
= ±10μA
--
--
--
--
420
--
--
--
--
--
V
IL
V
IH
V
EN
= 0V
I
SHDN
I
Q
I
SS
T
SD
V
EN
= 0V
V
EN
= 2V, V
FB
= 1V
V
SS
= 0V
C
SS
= 10nF
--
1.4
--
--
--
--
--
--
Enable Pull Up Current
Shutdown Current
Quiescent Current
Soft-Start Current
Soft-Start Period
Thermal Shutdown
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.
θ
JA
is measured at T
A
= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
θ
JC
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.
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS8251-04 February 2013
www.richtek.com
5
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