Preliminary
RT7257E
3A, 17V, 340kHz Synchronous Step-Down Converter
General Description
The RT7257E is a high efficiency, monolithic synchronous
step-down DC/DC converter that can deliver up to 3A
output current from a 4.5V to 17V input supply. The
RT7257E's current mode architecture and external
compensation allow 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. The RT7257E provides thermal
shutdown protection. The low current (<3μA) shutdown
mode provides output disconnection, enabling easy power
management in battery-powered systems. The RT7257E
is available in an SOP-8 (Exposed Pad) package.
Features
±
1.5% High Accuracy Feedback Voltage
4.5V to 17V Input Voltage Range
3A Output Current
Integrated N-MOSFET Switches
Current Mode Control
Fixed Frequency Operation : 340kHz
Output Adjustable from 0.8V to 15V
Up to 95% Efficiency
Programmable Soft-Start
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
Ordering Information
RT7257EN
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
Z : ECO (Ecological Element with
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.
Applications
Wireless AP/Router
Set-Top-Box
Industrial and Commercial Low Power Systems
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation of High-Performance DSPs
Pin Configurations
(TOP VIEW)
BOOT
VIN
2
3
4
8
7
SS
EN
COMP
FB
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
SW
GND
GND
9
6
5
SOP-8 (Exposed Pad)
DS7257E-P00_CN
October 2011
www.richtek.com
1
RT7257E
Typical Application Circuit
V
IN
4.5V to 17V
Chip Enable
2
C
IN
10µF x 2
Preliminary
VIN
BOOT
RT7257E
1
C
BOOT
L
100nF 10µH
R1
75k
V
OUT
3.3V
C
OUT
22µF x 2
SW 3
7 EN
8 SS
FB 5
COMP
6
C
C
4.7nF
R
C
12k
R2
24k
C
SS
0.1µF
4, 9 (Exposed Pad)
GND
C
P
Open
Table 1. Recommended Components Selection
V
OUT
(V)
8
5
3.3
2.5
1.5
1.2
1
R1 (kΩ)
27
62
75
25.5
10.5
12
3
R2 (kΩ)
3
11.8
24
12
12
24
12
R
C
(kΩ)
24
18
12
8.2
3.6
3
2.7
C
C
(nF)
4.7
4.7
4.7
4.7
4.7
4.7
4.7
L (μH)
22
15
10
6.8
3.6
3.6
3.6
C
OUT
(μF)
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
Functional Pin Description
Pin No.
1
2
3
4,
9 (Exposed Pad)
5
6
Pin Name
BOOT
VIN
SW
GND
FB
COMP
Pin Function
Bootstrap for High Side Gate Driver. Connect a 0.1μF or greater ceramic
capacitor from BOOT to SW pins.
Input Supply Voltage, 4.5V to 17V. Must bypass with a suitably large ceramic
capacitor.
Switch Node. Connect this pin to an external L-C filter.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Feedback Input. It is used to regulate the output of the converter to a set value
via an internal resistive voltage divider.
Compensation Node. COMP is used to compensate the regulation control
loop. Connect a series RC network from COMP to GND. In some cases, an
additional capacitor from COMP to GND is required.
Enable Input. A logic high enables the converter; a logic low forces the
RT7257E into shutdown mode reducing the supply current to less than 3μA.
Attach this pin to VIN with a 100kΩ pull up resistor for automatic startup.
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor
from SS to GND to set the soft-start period. A 0.1μF capacitor sets the
soft-start period to 13.5ms.
7
EN
8
SS
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2
DS7257E-P00_CN
October 2011
Preliminary
Function Block Diagram
VIN
Internal
Regulator
Shutdown
Comparator VA V
CC
1.2V
+
-
Lockout
Comparator
-
1.8V
+
V
CC
6µA
SS
0.8V
+
+EA
-
0.4V
Oscillator
RT7257E
Foldback
Control
+
Current Sense
Amplifier
Slope Comp
+
-
VA
BOOT
S
+
-
Current
Comparator
R
Q
Q
110mΩ
5k
EN
-
UV
Comparator
SW
90mΩ
GND
FB
COMP
DS7257E-P00_CN
October 2011
www.richtek.com
3
RT7257E
Absolute Maximum Ratings
Preliminary
(Note 1)
−0.3V
to 20V
−0.3V
to (V
IN
+ 0.3V)
−0.3V
to 6V
−0.3V
to 20V
Supply Input Voltage, V
IN
------------------------------------------------------------------------------------------
Switch Voltage, SW ------------------------------------------------------------------------------------------------
V
BOOT
−
V
SW
----------------------------------------------------------------------------------------------------------
Other Pins Voltage -------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------- 1.333W
Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad),
θ
JA
---------------------------------------------------------------------------------------- 75°C/W
SOP-8 (Exposed Pad),
θ
JC
--------------------------------------------------------------------------------------- 15°C/W
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------- 260°C
Junction Temperature ----------------------------------------------------------------------------------------------- 150°C
Storage Temperature Range --------------------------------------------------------------------------------------
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------- 2kV
MM (Machine Mode) ------------------------------------------------------------------------------------------------ 200V
Recommended Operating Conditions
(Note 4)
Supply Input Voltage, V
IN
------------------------------------------------------------------------------------------ 4.5V to 17V
Junction Temperature Range --------------------------------------------------------------------------------------
−40°C
to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------
−40°C
to 85°C
Electrical Characteristics
(V
IN
= 12V, T
A
= 25°C, unless otherwise specified)
Parameter
Shutdown Supply Current
Supply Current
Feedback Voltage
Error Amplifier
Transconductance
High Side Switch
On-Resistance
Low Side Switch
On-Resistance
High Side Switch Leakage
Current
Upper Switch Current Limit
COMP to Current Sense
Transconductance
Oscillation Frequency
Short Circuit Oscillation
Frequency
Maximum Duty Cycle
Minimum On Time
Symbol
V
EN
= 0V
Test Conditions
Min
--
--
0.788
--
--
--
Typ
0.5
0.8
0.8
940
110
90
0
5.1
4.7
340
100
93
100
Max
3
1.2
0.812
--
--
--
10
--
--
380
--
--
--
Unit
μA
mA
V
μA/V
mΩ
mΩ
μA
A
A/V
kHz
kHz
%
ns
V
EN
= 3 V, V
FB
= 0.9V
V
FB
G
EA
R
DS(ON)1
R
DS(ON)2
V
EN
= 0V, V
SW
= 0V
Min. Duty Cycle, V
BOOT
−
V
SW
= 4.8V
G
CS
f
OSC1
f
OSC2
D
MAX
t
ON
V
FB
= 0V
V
FB
= 0.7V
4.5V
≤
V
IN
≤
17V
ΔI
C
= ±10μA
--
--
--
300
--
--
--
To be continued
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4
DS7257E-P00_CN
October 2011
Preliminary
Parameter
EN Input Threshold
Voltage
Logic-High
Logic-Low
Symbol
V
IH
V
IL
V
UVLO
ΔV
UVLO
I
SS
t
SS
T
SD
V
SS
= 0V
C
SS
= 0.1μF
V
IN
Rising
Test Conditions
Min
2
--
3.8
--
--
--
--
Typ
--
--
4.2
320
6
RT7257E
Max
5.5
0.4
4.5
--
--
--
--
Unit
V
V
mV
μA
ms
°C
Input Under Voltage Lockout Threshold
Input Under Voltage Lockout Hysteresis
Soft-Start Current
Soft-Start Period
Thermal Shutdown
13.5
150
Note 1.
Stresses listed as the above
"Absolute
Maximum Ratings" may cause permanent damage to the device. These are for
stress ratings. 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 for extended
periods may remain possibility to affect device reliability.
Note 2.
θ
JA
is measured in natural convection at T
A
= 25°C on a high effective thermal conductivity four-layer test board of
JEDEC 51-7 thermal measurement standard. The measurement case position of
θ
JC
is on 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.
DS7257E-P00_CN
October 2011
www.richtek.com
5
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