Richtek RT7297B User Manual
RT7297B
®
DS7297B-02 September 2012 www.richtek.com
1
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Ordering Information
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.
Pin Configurations
(TOP VIEW)
Applications
Wireless AP/Router
Set-Top-Box
Industrial and Commerci al Low Power Systems
LCD Monitors a nd TVs
Green Electronics/Appliances
Point of Load Regulation of High-Performance DSPs
SOP-8 (Exposed Pad)
3A, 18V, 1.2MHz Synchronous Step-Down Converter
General Description
The RT7297B 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
RT7297B's current mode architecture and external
compensation allow the transient response to be
optimized over a wide input voltage range a nd loads. Cycleby-cycle current limit provides protection against shorted
outputs, and soft-start eliminate s input current surge during
start-up. The RT7297B 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 RT7297B is available in
an SOP-8 (Exposed Pad) pa ckage.
Features
±±
±±
±1.5% High Accuracy Reference Voltage
4.5V to 18V Input Voltage Range
3A Output Current
Integrated N-MOSFET Switches
Current Mode Control
Fixed Frequency Operation : 1.2MHz
Output Adjustable from 0.8V to 12V
Up to 95% Efficiency
Programmable Soft-Start
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Output Under Voltage Protection
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
BOOT
VIN
SW
GND
SS
EN
FB
COMP
GND
2
3
4
5
6
7
8
9
Package Type
SP : SOP-8 (Exposed Pad-Option 1)
RT7297B
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
H : UVP Hiccup
L : UVP Latch-Off
Marking Information
RT7297BxZSP : Product Number
x : H or L
YMDNN : Date Code
RT7297Bx
ZSPYMDNN
RT7297B
2
DS7297B-02 September 2012www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Functional Pin Description
Pin No. Pin Name Pin Function
1 BOOT
Bootstrap for High Side Gate Driver. Connect a 0.1μF or greater ceramic
capacitor from BOOT to SW pins.
2 VIN
Input Supply Voltage, 4.5V to 18V. Must bypass with a suitable large ceramic
capacitor.
3 SW Switch Node. Connect this pin to an external L-C filter.
4,
9 (Exposed Pad)
GND
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
5 FB
Feedback Input. It is used to regulate the output of the converter to a set value
via an external resistive voltage divider.
6 COMP
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.
7 EN
Enable Input Pin. A logic high enables the converter; a logic low forces the IC
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.
8 SS
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.
Typical Application Circuit
V
OUT
(V) R1 (kΩ) R2 (kΩ) RC (kΩ) CC (nF) L (μH) C
OUT
(μF)
8 27 3 51 2.2 10 22 x 2
5 62 1 1.8 33 2.2 6.8 22 x 2
3.3 75 24 22 2.2 3.6 22 x 2
2.5 25.5 12 16 2.2 3.6 22 x 2
1.5 10.5 12 10 2.2 2 22 x 2
1.2 12 24 8.2 2.2 2 22 x 2
1 3 12 6.8 2.2 2 22 x 2
Table 1. Suggested Components Selection
VIN
EN
GND
BOOT
FB
SW
7
5
2
3
1
L
3.6µH
0.1µF
22µF x 2
R1
75k
R2
24k
V
OUT
3.3V
10µF x 2
V
IN
4.5V to 18V
RT7297B
SS
8
C
SS
COMP
C
C
2.2nF
R
C
22k
C
P
Open
6
4, 9 (Exposed Pad)
C
BOOT
C
IN
0.1µF
C
OUT
R
EN
100k
RT7297B
3
DS7297B-02 September 2012 www.richtek.com
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Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Function Block Diagram
+
-
+
-
+
-
UV
Comparator
Oscillator
Foldback
Control
0.4V
Internal
Regulator
+
-
Shutdown
Comparator
Current Sense
Amplifier
BOOT
VIN
GND
SW
FB
EN
COMP
V
CC
6µA
Slope Comp
Current
Comparator
+
-
EA
0.8V
SRQ
Q
SS
+
-
1.2V
Lockout
Comparator
V
CC
+
Ω90m
Ω110m
V
A
2.5V
V
A
R
SENSE
5kΩ
RT7297B
4
DS7297B-02 September 2012www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Electrical Characteristics
(V
IN
= 12V, TA = 25°C, unless otherwise specified)
Absolute Maximum Ratings (Note 1)
Supply Input V oltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 20V
Switch Voltage, SW ------------------------------------------------------------------------------------------------ −0.3V to (V
IN
+ 0.3V)
V
BOOT
− VSW---------------------------------------------------------------------------------------------------------- −0.3V to 6V
Other Pins Voltage -------------------------------------------------------------------------------------------------- −0.3V to 20V
Power Dissipation, P
D
@ T
A
= 25°C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------- 1.333W
Pa ck age Thermal Resista nce (Note 2)
SOP-8 (Exposed Pad), θJA---------------------------------------------------------------------------------------- 75°C/W
SOP-8 (Exposed Pad), θJC--------------------------------------------------------------------------------------- 15°C/W
Lead Temperature (Soldering, 10 sec.)------------------------------------------------------------------------- 260°C
Junction T emperature----------------------------------------------------------------------------------------------- 150°C
Storage T emperature Range -------------------------------------------------------------------------------------- −65°C to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model)---------------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions (Note 4)
Supply Input V oltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 18V
Junction T emperature Range-------------------------------------------------------------------------------------- − 40°C to 125°C
Ambient T emperature Range-------------------------------------------------------------------------------------- − 40°C to 85°C
Parameter Symbol Test Conditions Min Typ Max Unit
Shutdown Supply Current V
EN
= 0V -- 0.5 3 μA
Supply Current V
EN
= 3V, VFB = 0.9V -- 0.8 1.2 mA
Reference Voltage V
REF
4.5V ≤ VIN ≤ 18V 0.788 0.8 0.812 V
Error Amplifier
Transconductance
G
EA
ΔIC = ±10μA -- 940 -- μA/V
High Side Switch
On-Resistance
R
DS(ON)1
-- 110 -- mΩ
Low Side Switch
On-Resistance
R
DS(ON)2
-- 90 -- mΩ
High Side Switch Leakage
Current
V
EN
= 0V, VSW = 0V -- 0 10 μA
Upper Switch Current Limit Min. Duty Cycle, V
BOOT
− VSW = 4.8V -- 5.1 -- A
COMP to Current Sense
Transconductance
G
CS
-- 4.7 -- A/V
Oscillation Frequency f
OSC1
1 1.2 1.4 MHz
Short Circuit Oscillation
Frequency
f
OSC2
VFB = 0V -- 270 -- kHz
Maximum Duty Cycle D
MAX
VFB = 0.7V -- 78 -- %
Minimum On Time tON -- 100 -- ns
RT7297B
5
DS7297B-02 September 2012 www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Parameter Symbol Test Conditions Min Typ Max Unit
Logic-High VIH 2.7 -- 18
EN Input Threshold
Voltage
Logic-Low V
IL
-- -- 0.4
V
Input Under Volt age Lockout
Threshold
V
UVLO
VIN Rising 3.8 4.2 4.5 V
Input Under Volt age Lockout
Hysteresis
ΔV
UVLO
-- 320 -- mV
Soft-Start Current ISS V
SS
= 0V -- 6 -- μA
Soft-Start Period tSS C
SS
= 0.1μF -- 13.5 -- ms
Thermal Shutdown TSD -- 150 -- °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. θ
JA
is measured at T
A
= 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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