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
(TOPVIEW)
Applications
WirelessAP/Router
Set-Top-Box
Industrial and Commercial Low Power Systems
LCDMonitorsandTVs
GreenElectronics/Appliances
PointofLoadRegulationof High-PerformanceDSPs
SOP-8 (Exposed Pad)
3A, 18V, 1.2MHz Synchronous Step-Down Converter
General Description
TheRT7297Bisahighefficiency,monolithicsynchronous
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
optimizedoverawideinputvoltagerangeandloads.Cycle-
by-cyclecurrentlimitprovides protection against shorted
outputs,andsoft-starteliminatesinputcurrentsurgeduring
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) package.
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
45
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
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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 100kΩpull 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
VOUT (V) R1 (kΩ) R2 (kΩ) RC(kΩ) CC(nF) L (μH) COUT (μF)
8 27 3 51 2.2 10 22 x 2
5 62 11.8 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
1L
3.6µH
0.1µF
22µF x 2
R1
75k
R2
24k
VOUT
3.3V
10µF x 2
VIN
4.5V to 18V RT7297B
SS
8
CSS
COMP
CC
2.2nF RC
22k
CP
Open
6
4, 9 (Exposed Pad)
CBOOT
CIN
0.1µF
COUT
REN 100k

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Function Block Diagram
+
-
+
-
+
-
UV
Comparator
Oscillator
Foldback
Control
0.4V
Internal
Regulator
+
-
Shutdown
Comparator
Current Sense
Amplifier
BOOT
VIN
GND
SW
FB
EN
COMP
VCC
6µA
Slope Comp
Current
Comparator
+
-EA
0.8V
S
R
Q
Q
SS
+
-
1.2V
Lockout
Comparator
VCC
+
Ω90m
Ω110m
VA
2.5V
VARSENSE
5kΩ

RT7297B
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Electrical Characteristics
(VIN = 12V, TA= 25°C, unless otherwise specified)
Absolute Maximum Ratings (Note 1)
Supply InputVoltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 20V
Switch Voltage, SW ------------------------------------------------------------------------------------------------ −0.3Vto (VIN + 0.3V)
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
 Package Thermal 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 InputVoltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 18V
JunctionTemperatureRange-------------------------------------------------------------------------------------- −40°C to125°C
AmbientTemperatureRange-------------------------------------------------------------------------------------- −40°C to 85°C
Parameter Symbol Test Conditions Min Typ Max Unit
Shutdown Supply Current VEN = 0V -- 0.5 3 μA
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 ΔIC= ±10μA -- 940 -- μA/V
High Side Switch
On-Resistance RDS(ON)1 -- 110 -- mΩ
Low Side Switch
On-Resistance RDS(ON)2 -- 90 -- mΩ
High Side Switch Leakage
Current V
EN = 0V, VSW = 0V -- 0 10 μA
Upper Switch Current Limit Min. Duty Cycle, VBOOT − VSW = 4.8V -- 5.1 -- A
COMP to Current Sense
Transconductance GCS -- 4.7 -- A/V
Oscillation Frequency fOSC1 1 1.2 1.4 MHz
Short Circuit Oscillation
Frequency fOSC2 V
FB = 0V -- 270 -- kHz
Maximum Duty Cycle DMAX V
FB = 0.7V -- 78 -- %
Minimum On Time tON -- 100 -- ns

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Parameter Symbol Test Conditions Min Typ Max Unit
Logic-High VIH 2.7 -- 18
EN Input Threshold
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 ΔVUVLO -- 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. 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 Operating Characteristics
Efficiency vs. Load Current
0
10
20
30
40
50
60
70
80
90
100
00.511.522.53
Load Current (A)
Efficiency (%)
VOUT = 3.3V
VIN = 12V
VIN = 17V
Output Voltage vs. Temperature
3.26
3.27
3.28
3.29
3.30
3.31
3.32
3.33
3.34
-50 -25 0 25 50 75 100 125
Temperature (°C)
Output Voltage (V)
VIN = 12V, VOUT = 3.3V
Output Voltage vs. Load Current
3.20
3.22
3.24
3.26
3.28
3.30
3.32
3.34
3.36
3.38
3.40
0 0.5 1 1.5 2 2.5 3
Load Current (A)
Output Voltage (V)
VOUT = 3.3V
VIN = 12V
VIN = 17V
Switching Frequency vs. Temperature
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17
1.18
1.19
1.20
1.21
1.22
1.23
1.24
-50 -25 0 25 50 75 100 125
Temperature (°C)
Switching Frequency (kHz)1
VOUT = 3.3V, IOUT = 0A
VIN = 17V
VIN = 12V
Switching Frequency vs. Input Voltage
1.16
1.17
1.18
1.19
1.20
1.21
1.22
1.23
1.24
1.25
1.26
4.5 7 9.5 12 14.5 17
Input Voltage (V)
Switching Frequency (MHz)1
VIN = 4.5V to 17V, VOUT = 3.3V, IOUT = 0A
Output Voltage vs. Input Voltage
3.26
3.27
3.28
3.29
3.30
3.31
3.32
3.33
3.34
4 6 8 1012141618
Input Voltage (V)
Output Voltage (V)
VIN = 4.5V to 17V

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Current Limit vs. Temperature
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
-50-25 0 25 50 75100125
Temperature (°C)
Current Limit (A)
VIN = 12V, VOUT = 3.3V
Current Limit vs. Input Voltage
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
4.5 7 9.5 12 14.5 17
Input Voltage (V)
Current Limit (A)
VIN = 4.5V to 17V, VOUT = 3.3V
Time (100μs/Div)
Load Transient Response
VOUT
(100mV/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0A to 3A
Time (100μs/Div)
Load Transient Response
VOUT
(100mV/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.5A to 3A
Time (500ns/Div)
Switching
VSW
(10V/Div)
VOUT
(5mV/Div)
IL
(2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
Time (500ns/Div)
Switching
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VSW
(10V/Div)
VOUT
(5mV/Div)
IL
(2A/Div)

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Power On from VIN
Time (10ms/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
IL
(2A/Div)
VOUT
(2V/Div)
VIN
(5V/Div)
Power Off from VIN
Time (10ms/Div)
IL
(2A/Div)
VOUT
(2V/Div)
VIN
(5V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Power On from EN
Time (10ms/Div)
VOUT
(2V/Div)
VEN
(5V/Div)
IL
(2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 3A
Power Off from EN
Time (10ms/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VOUT
(2V/Div)
VEN
(5V/Div)
IL
(2A/Div)

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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
Connect a 100nF low ESR ceramic capacitor between
the BOOT pin and SW pin. This capacitor provides the
gatedrivervoltage forthehighsideMOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvementwheninputvoltageislowerthan5.5Vorduty
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
RT7297B. 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 shutdown the device. During shutdown
mode,theRT7297B quiescentcurrentdropstolower than
3μA. Driving the EN pin high (>2.7V, <18V) will turn on
the device again. For external timing control, the EN pin
canalsobeexternallypulled highbyaddingaREN resistor
and CEN capacitor from the VINpin (see Figure 3).
SW
BOOT
5V
RT7297B 0.1µF
RT7297B
GND
FB
R1
R2
VOUT
Soft-Start
The RT7297B 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
beprogrammedbytheexternalcapacitorbetweenSS and
GND. An internal current source ISS (6μA) charges an
external capacitor to build a soft-start ramp voltage. The
VFB voltagewilltrackthe internalramp voltage duringsoft-
start interval. The typical soft-start time is calculated as
follows :
RT7297B
EN
GND
VIN REN
CEN
EN
An external MOSFET can be added to implement digital
controlontheENpin when nosystemvoltageabove1.8V
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.
RT7297B
EN
GND
100k
VIN
REN
Q1
EN
Figure3.EnableTimingControl
Figure 4.Digital Enable Control Circuit
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
μ
μμ
×
× ≒

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OUT OUT
LIN
VV
I = 1
fL V
⎡⎤⎡ ⎤
Δ×−
⎢⎥⎢ ⎥
×
⎣⎦⎣ ⎦
Under Voltage Protection
Hiccup Mode
FortheRT7297BH,itprovidesHiccupMode UnderVoltage
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 RT7297BH will retry automatically. When the
UVP condition is removed, the converter will resume
operation. The UVP is disabled during Soft-Start period.
Having a lower ripple current reduces not only the ESR
lossesintheoutputcapacitors butalsotheoutputvoltage
ripple.Highfrequencywithsmall ripplecurrent canachieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
Fortheripplecurrentselection,the valueofΔIL=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
inductorvalueshould bechosenaccordingto thefollowing
equation :
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Supplier Series Dimensions (mm)
TDK VLF10045 10 x 9.7 x 4.5
TDK SLF12565 12.5 x 12.5x 6.5
TAIYO
YUDEN NR8040 8 x 8 x 4
OUT OUT
L(MAX) IN(MAX)
VV
L = 1
fI V
⎡⎤⎡ ⎤
×−
⎢⎥⎢ ⎥
×Δ
⎣⎦⎣ ⎦
Theinductor'scurrentrating(causeda40°Ctemperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
begreaterthanthe short circuit peak current limit.Please
see Table 2 for the inductor selection reference.
Figure 5. Hiccup Mode Under Voltage Protection
Figure6.Latch-OffModeUnderVoltageProtection
Latch-Off Mode
For the RT7297BL, it provides Latch-Off Mode Under
Voltage Protection (UVP). When the FB voltage drops
below half of the feedback reference voltage, VFB, UVP
willbetriggered andtheRT7297BLwillshutdownin Latch-
Off Mode. In shutdown condition, the RT7297BL can be
reset by EN pin or power input VIN.
Over Temperature Protection
The RT7297B 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
approximately20°C,theconverterwillresumeoperation.
Tomaintaincontinuousoperation,themaximumjunction
temperatureshouldbelowerthan125°C.
Inductor Selection
Theinductorvalueandoperating frequencydeterminethe
ripple current according to a specific input and output
voltage. The ripple current ΔILincreases with higher VIN
anddecreases with higher inductance.
Time (50ms/Div)
Hiccup Mode
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short
Time (250μs/Div)
Latch-Off Mode
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short