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PDF RT7257G Data sheet ( Hoja de datos )
| Número de pieza | RT7257G | |
| Descripción | Synchronous Step-Down Converter | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT7257G
3A, 18V, 800kHz Synchronous Step-Down Converter
General Description
The RT7257G 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
RT7257G's current mode architecture and external
compensation allow the transient response to be
optimized over a wide input voltage range and loads.
Cycle-by-cycle current limit provides protection against
shorted outputs, and soft-start eliminates input current
surge during start-up. The RT7257G also provides under
voltage protection and thermal shutdown protection. The
low current (<3μA) shutdown mode provides output
disconnection, enabling easy power management in
battery-powered systems. The RT7257G is available in
an SOP-8 (Exposed Pad) package.
Marking Information
RT7257Gx
ZSPYMDNN
RT7257GxZSP : Product Number
x : H or L
YMDNN : Date Code
Features
z ±1.5% High Accuracy Reference Voltage
z 4.5V to 18V Input Voltage Range
z 3A Output Current
z Integrated N-MOSFET Switches
z Current Mode Control
z Fixed Frequency Operation : 800kHz
z Output Adjustable from 0.8V to 12V
z Up to 95% Efficiency
z Programmable Soft-Start
z Stable with Low ESR Ceramic Output Capacitors
z Cycle-by-Cycle Current Limit
z Input Under Voltage Lockout
z Output Under Voltage Protection
z Thermal Shutdown Protection
z RoHS Compliant and Halogen Free
Applications
z Wireless AP/Router
z Set-Top-Box
z Industrial and Commercial Low Power Systems
z LCD Monitors and TVs
z Green Electronics/Appliances
z Point of Load Regulation of High-Performance DSPs
Simplified Application Circuit
VIN VIN BOOT
CIN RT7257G
CBOOT
L
SW VOUT
REN
EN
R1
SS FB
COUT
CSS
CC RC
R2
GND
COMP
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7257G-01 September 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
Free Datasheet http://www.datasheet4u.com/
1 page  
RT7257G
Parameter
EN Input Threshold Logic-High
Voltage
Logic-Low
Input Under Voltage Lockout
Threshold
Input Under Voltage Lockout
Hysteresis
Soft-Start Current
Soft-Start Period
Thermal Shutdown
Symbol
VIH
VIL
VUVLO
ΔVUVLO
ISS
tSS
TSD
Test Conditions
VIN Rising
VSS = 0V
CSS = 0.1μF
Min Typ Max Unit
2.7 -- 18
V
-- -- 0.4
3.8 4.2 4.5
V
-- 320 --
-- 6 --
-- 13.5 --
-- 150 --
mV
μA
ms
°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.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7257G-01 September 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
Free Datasheet http://www.datasheet4u.com/
5 Page 
RT7257G
Under Voltage Protection
Hiccup Mode
For the RT7257GH, it provides Hiccup Mode Under Voltage
Protection (UVP). When the VFB voltage drops below 0.4V,
the UVP function will be triggered to shut down switching
operation. If the UVP condition remains for a period, the
RT7257GH will retry automatically. When the UVP
condition is removed, the converter will resume operation.
The UVP is disabled during soft-start period.
Hiccup Mode
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short
Time (50ms/Div)
Figure 5. Hiccup Mode Under Voltage Protection
Latch-Off Mode
For the RT7257GL, it provides Latch-Off Mode Under
Voltage Protection (UVP). When the FB voltage drops
below half of the feedback reference voltage, VFB, UVP
will be triggered and the RT7257GL will shut down in Latch-
Off Mode. In shutdown condition, the RT7257GL can be
reset by EN pin or power input VIN.
Latch-Off Mode
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short
Time (250μs/Div)
Figure 6. Latch-Off Mode Under Voltage Protection
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7257G-01 September 2012
Over Temperature Protection
The RT7257G features an Over Temperature Protection
(OTP) circuitry to prevent from overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature exceeds
150°C. Once the junction temperature cools down by
approximately 20°C, the converter will resume operation.
To maintain continuous operation, the maximum junction
temperature should be lower than 125°C.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
ΔIL
=
⎡
⎢⎣
VOUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN
⎤
⎥⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
For the ripple current selection, the value of ΔIL= 0.24(IMAX)
will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
ripple current stays below the specified maximum, the
inductor value should be chosen according to the following
equation :
L=
⎡
⎢⎣
f
×
VOUT
ΔIL(MAX)
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT ⎤
VIN(MAX) ⎥⎦
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit. Please
see Table 2 for the inductor selection reference.
Table 2. Suggested Inductors for Typical
Application Circuit
C ompo nen t
Supplier
Ser ie s
D ime nsion s
(mm)
TDK
VLF10045 10 x 9.7 x 4.5
TDK
SLF12565 12.5 x 12.5 x 6.5
TAIYO
YUDEN
NR8040
8x8x4
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
Free Datasheet http://www.datasheet4u.com/
11 Page | ||
| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet RT7257G.PDF ] | |
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