Richtek RT7247A User manual

RT7247A
®
DS7247A-03 October 2016 www.richtek.com
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Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
©
Design
Tools Sample &
Buy
Applications
WirelessAP/Router
Set-Top-Box
Industrial and Commercial Low Power Systems
LCDMonitorsandTVs
GreenElectronics/Appliances
PointofLoadRegulationof High-PerformanceDSPs
2A, 18V, 340kHz Synchronous Step-Down Converter
General Description
TheRT7247Aisahighefficiency,monolithicsynchronous
step-down DC/DC converter that can deliver up to 2A
output current from a 4.5V to 18V input supply. The
RT7247A's current mode architecture and external
compensation allow the transient response to be
optimized over a wide input range and loads. Cycle-by-
cycle current limit provides protection against shorted
outputs,and soft-starteliminatesinputcurrentsurgeduring
start-up. The RT7247A 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 RT7247A is available in
an SOP-8 (Exposed Pad) package.
Features


±±
±±
±1.5% High Accuracy Reference Voltage


4.5V to 18V Input Voltage Range


2A Output Current


Integrated N-MOSFET Switches


Current Mode Control


Fixed Frequency Operation : 340kHz


Output Adjustable from 0.8V to 15V


Stable with Low ESR Ceramic Output Capacitors


Up to 95% Efficiency


Programmable Soft-Start


Cycle-by-Cycle Over Current Protection


Input Under Voltage Lockout


Output Under Voltage Protection


Thermal Shutdown Protection


RoHS Compliant and Halogen Free
Marking Information
RT7247Ax
GSPYMDNN
RT7247AxGSP : Product Number
x : H or L or N
YMDNN : Date Code
Simplified Application Circuit
VIN
EN
GND
BOOT
FB
SW L
R1
R2
VOUT
VIN RT7247A
SS
CSS COMP CCRC
CP
CBOOT
CIN
COUT
Chip Enable

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Copyright 2016 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 0.1F 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. A logic high enables the converter; a logic low forces the IC into
shutdown mode reducing the supply current to less than 3A. Attach this pin
to VIN with a 100k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 0.1F capacitor sets the
soft-start period to 13.5ms.
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.
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
RT7247A
Lead Plating System
G : Green (Halogen Free and Pb Free)
H : UVP Hiccup
L : UVP Latch-Off
N : UVP Disabled
Pin Configurations
(TOPVIEW)
SOP-8 (Exposed Pad)
BOOT
VIN
SW
GND
SS
EN
FB
COMP
GND
2
3
45
6
7
8
9

RT7247A
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Function Block Diagram
RSENSE
+
-
+
-
+
-
UV
Comparator
Oscillator
Foldback
Control
0.4V
Internal
Regulator
+
-
1.8V
Shutdown
Comparator
Current Sense
Amplifier
BOOT
VIN
GND
SW
FB
EN
COMP
VAVCC
6µA
Slope Comp
Current
Comparator
+
-EA
0.8V
S
R
Q
Q
SS
+
-
1.2V
Lockout
Comparator
VCC
+
150m
5k
VA
130m
Operation
Shutdown Comparator
Activate internal regulator once EN input level is larger
than the target level. Force IC to enter shutdown mode
when the ENinput level is lower than 0.4V.
Internal Regulator
Provide internal power for logic control and switch gate
drivers.
Lockout Comparator
Activate the Current Comparator, release lock-out logic,
and enable the switches as EN input level is larger than
lockout voltage. Otherwise, the switches still locks out.
Oscillator
The oscillator provides internal clock and controls the
converter'sswitchingfrequency.
Foldback Control
Dynamicallyadjusttheinternal clock. It provides a slower
frequency as a lower FB voltage.
UV Comparator
AsFBvoltageis lower than the UVvoltage,it will activate
a UV protect scheme.
ErrorAmp
TheoutputvoltageCOMPoftheerroramplifieris adjusted
comparing FB signal with the internal reference voltage
and SS signal.
Current Sense Amplifier
RSENSE detects the peak current of the high-side switch.
This signal is amplified by the Current Sense Amplifier
and added with a Slope Compensation. Then, it controls
the switches by comparing this signal with the COMP
voltage.

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Electrical Characteristics
(VIN = 12V, TA= 25°C, unless otherwise specified)
Absolute Maximum Ratings (Note 1)
Supply Input Voltage, VIN ------------------------------------------------------------------------------------------ −0.3V to 20V
Switch Voltage, SW ------------------------------------------------------------------------------------------------ −0.3V to (VIN + 0.3V)
<10ns ------------------------------------------------------------------------------------------------------------------ −5V to 25V
VBOOT −VSW ---------------------------------------------------------------------------------------------------------- −0.3V to 6V
Other Pins Voltage ------------------------------------------------------------------------------------------------- −0.3V to 20V
Power Dissipation, PD@ TA= 25°C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------- 1.333W
PackageThermal Resistance (Note 2)
SOP-8 (Exposed Pad), θJA ---------------------------------------------------------------------------------------- 75°C/W
SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------------- 15°C/W
LeadTemperature(Soldering,10sec.)------------------------------------------------------------------------- 260°C
JunctionTemperature----------------------------------------------------------------------------------------------- 150°C
StorageTemperatureRange -------------------------------------------------------------------------------------- −65°Cto150°C
ESD Susceptibility (Note 3)
HBM(HumanBodyModel)---------------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions (Note 4)
Supply Input Voltage, VIN ------------------------------------------------------------------------------------------ 4.5V to 18V
JunctionTemperatureRange-------------------------------------------------------------------------------------- −40°Cto 125°C
AmbientTemperatureRange-------------------------------------------------------------------------------------- −40°Cto85°C
Parameter Symbol Test Conditions Min Typ Max Unit
Shutdown Supply Current VEN = 0V -- 0.5 3 A
Supply Current VEN = 3V, VFB = 0.9V -- 0.8 1.2 mA
Reference Voltage VREF 4.5V VIN 18V 0.788 0.8 0.812 V
Error Amplifier
Transconductance GEA IC= ±10A -- 940 -- A/V
High Side Switch
On-Resistance RDS(ON)1 -- 150 -- m
Low Side Switch
On-Resistance RDS(ON)2 -- 130 -- m
High Side Switch Leakage
Current V
EN = 0V, VSW = 0V -- 0 10 A
Upper Switch Current Limit Min. Duty Cycle, VBOOT VSW = 4.8V -- 4 -- A
COMP to Current Sense
Transconductance GCS -- 3.7 -- A/V
Oscillation Frequency fOSC1 300 340 380 kHz
Short Circuit Oscillation
Frequency fOSC2 V
FB = 0V -- 100 -- kHz
Maximum Duty Cycle DMAX V
FB = 0.7V -- 93 -- %
Minimum On-Time tON -- 100 -- ns

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Parameter Symbol Test Conditions Min Typ Max Unit
Logic-High VIH 2 -- 18
EN Input Voltage Logic-Low VIL -- -- 0.4
V
Input Under Voltage Lockout
Threshold VUVLO V
IN Rising 3.8 4.2 4.5 V
Input Under Voltage Lockout
Hysteresis VUVLO -- 320 -- mV
Soft-Start Current ISS V
SS = 0V -- 6 -- A
Soft-Start Period tSS C
SS = 0.1F -- 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. 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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Typical Application Circuit
VOUT (V) R1 (k) R2 (k) RC(k) CC(nF) L (H) COUT (F)
8 27 3 27 3.3 22 22 x 2
5 62 11.8 20 3.3 15 22 x 2
3.3 75 24 13 3.3 10 22 x 2
2.5 25.5 12 9.1 3.3 6.8 22 x 2
1.5 10.5 12 4.7 3.3 3.6 22 x 2
1.2 12 24 3.6 3.3 3.6 22 x 2
1 3 12 3 3.3 3.6 22 x 2
Table 1. Suggested Components Selection
VIN
EN
GND
BOOT
FB
SW
7
5
2
3
1L
10µH
0.1µF
22µF x 2
R1
75k
R2
24k
VOUT
3.3V
10µF x 2
VIN
4.5V to 18V RT7247A
SS
8
CSS
COMP
CC
3.3nF RC
13k
CP
Open
6
4, 9 (Exposed Pad)
CBOOT
CIN
0.1µF
COUT
Chip Enable

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Typical Operating Characteristics
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Output Current (A)
Efficiency (%)
VOUT = 3.3V
VIN = 4.5V
VIN = 12V
VIN = 17V
Output Voltage vs. Input Voltage
3.27
3.28
3.29
3.30
3.31
3.32
3.33
4 6 8 1012141618
Input Voltage (V)
Output Voltage (V)
VOUT = 3.3V, IOUT = 1A
Reference Voltage vs. Temperature
0.75
0.76
0.77
0.78
0.79
0.80
0.81
0.82
0.83
0.84
0.85
-50-25 0 25 50 75100125
Temperature (°C)
Reference Voltage (V)
VOUT = 3.3V, IOUT = 1A
VIN = 4.5V
VIN = 12V
VIN = 17V
Output Voltage vs. Output Current
3.280
3.285
3.290
3.295
3.300
3.305
3.310
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Output Current (A)
Output Voltage (V)
VOUT = 3.3V
VIN = 4.5V
VIN = 12V
VIN = 17V
Switching Frequency vs. Input Voltage
300
310
320
330
340
350
360
370
380
369121518
Input Voltage (V)
Switching Frequency (kHz)1
VOUT = 3.3V, IOUT = 0A
Switching Frequency vs. Temperature
300
310
320
330
340
350
360
370
380
-50-25 0 25 50 75100125
Temperature (°C)
Switching Frequency (kHz)1
VOUT = 3.3V, IOUT = 0A
VIN = 4.5V
VIN = 12V
VIN = 17V

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Time (10ms/Div)
Power On from VIN
VOUT
(2V/Div)
VIN
(5V/Div)
IL
(2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A
Time (10ms/Div)
Power Off from VIN
VOUT
(2V/Div)
VIN
(5V/Div)
IL
(2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A
Time (2.5μs/Div)
Output Ripple Voltage
VOUT
(5mV/Div)
VSW
(5V/Div)
IL
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A
Time (100μs/Div)
Load Transient Response
VOUT
(100mV/Div)
IOUT
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1A to 2A
Current Limit vs. Temperature
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
-50-25 0 25 50 75100125
Temperature (°C)
Current Limit (A)
VOUT = 3.3V
VIN = 12V
VIN = 17V
Time (100μs/Div)
Load Transient Response
VOUT
(100mV/Div)
IOUT
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 0A to 2A

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Time (10ms/Div)
Power On from EN
VOUT
(2V/Div)
VEN
(5V/Div)
IL
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A
Time (10ms/Div)
Power Off from EN
VOUT
(2V/Div)
VEN
(5V/Div)
IL
(1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A

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Application Information
Output Voltage Setting
TheresistivedividerallowstheFBpin to sense the output
voltage as shown in Figure 1.
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divideraccordingto the followingequation:
OUT REF R1
V =V 1
R2




Where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connecta0.1μFlowESRceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
drivervoltageforthehighsideMOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvementwheninputvoltageis lowerthan5.5Vorduty
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
RT7247A. Note that the external boot voltage must be
lowerthan 5.5V
Figure 2. External BootstrapDiode
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode,theRT7247A quiescentcurrentdropstolower than
3μA.Driving the ENpin high (>2V, <18V) will turn on the
device again. For external timing control, the ENpin can
also be externally pulled high by adding a REN resistor
and CEN capacitor from theVINpin (see Figure 3).
Soft-Start
The RT7247A provides soft-start function. The soft-start
function is used to prevent large inrush current while
converter is being powered-up. The soft-start timing can
beprogrammedbytheexternalcapacitorbetweenSSand
GND. An internal current source ISS (6μA) charges an
external capacitor to build a soft-start ramp voltage. The
VFB voltagewilltracktheinternalramp voltage duringsoft-
start interval. The typical soft start time is calculated as
follows :
SS
SS SS
SS
0.8 C
Soft-Start time t = , if C capacitor
I0.8 0.1
is 0.1 F, then soft-start time = 13.5ms
6



 ≒
An external MOSFET can be added to implement digital
controlontheENpin when nosystemvoltageabove2.5V
is available, as shown in Figure 4. In this case, a 100kΩ
pull-up resistor, REN, is connected between VIN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
Figure3.EnableTimingControl
Figure 4.Digital Enable Control Circuit
RT7247A
GND
FB
R1
R2
VOUT
SW
BOOT
5V
RT7247A 100nF
RT7247A
EN
GND
VIN REN
CEN
EN
RT7247A
EN
GND
100k
VIN
REN
Q1
EN