RICHTEK RT7257C

®
RT7257C
3A, 18V, 340kHz Synchronous Step-Down Converter
General Description
Features
The RT7257C 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
RT7257C'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 RT7257C 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 RT7257C is available in
an SOP-8 (Exposed Pad) package.
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Ordering Information
RT7257C
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
H : UVP Hiccup
L : UVP Latch-Off
±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 : 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
Applications
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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
Note :
Richtek products are :
`
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
(TOP VIEW)
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
8
BOOT
VIN
2
SW
GND
3
GND
EN
6
COMP
5
FB
9
4
SS
7
SOP-8 (Exposed Pad)
RT7257CxZSP : Product Number
RT7257Cx
ZSPYMDNN
x : H or L
YMDNN : Date Code
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7257C-02
September 2012
is a registered trademark of Richtek Technology Corporation.
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1
RT7257C
Typical Application Circuit
2
VIN
4.5V to 18V
CIN
10µF x 2
BOOT
VIN
RT7257C
SW 3
Chip Enable
7 EN
8 SS
CSS
0.1µF
1
4, 9 (Exposed Pad)
GND
CBOOT
L
0.1µF 10µH
VOUT
3.3V
R1
75k
COUT
22µF x 2
FB 5
COMP
6
CC
4.7nF
RC
12k
R2
24k
CP
Open
Table 1. Suggested 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
RC (kΩ)
24
18
12
8.2
3.6
3
2.7
CC (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
COUT (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.
Pin Name
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 18V. Must bypass with a suitable large ceramic
capacitor.
1
BOOT
2
VIN
3
SW
Switch Node. Connect this pin to an external L-C filter.
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
8
SS
4,
9 (Exposed Pad)
Enable Input. A logic high enables the converter; a logic low forces the
RT7257C 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.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS7257C-02
September 2012
RT7257C
Function Block Diagram
VIN
Internal
Regulator
Oscillator
Slope Comp
Shutdown
Comparator VA VCC
1.2V
+
Foldback
Control
-
5kΩ
EN
Current Sense
Amplifier
+
-
VA
RSENSE
0.4V
+
UV
Comparator
Lockout
Comparator
+
1.8V
BOOT
+
Current
Comparator
VCC
S
Q
110mΩ
R
Q
90mΩ
SW
GND
6µA
0.8V
SS
+
+EA
-
FB
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7257C-02
September 2012
COMP
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3
RT7257C
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VIN -----------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------VBOOT − VSW ---------------------------------------------------------------------------------------------------------Other Pins Voltage ------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
−0.3V to 20V
−0.3V to (VIN + 0.3V)
−0.3V to 6V
−0.3V to 20V
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad), θJA ---------------------------------------------------------------------------------------SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ----------------------------------------------------------------------------------------
1.333W
Recommended Operating Conditions
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75°C/W
15°C/W
260°C
150°C
−65°C to 150°C
2kV
(Note 4)
Supply Input Voltage, VIN ------------------------------------------------------------------------------------------ 4.5V to 18V
Junction Temperature Range -------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Shutdown Supply Current
VEN = 0V
--
0.5
3
μA
Supply Current
VEN = 3 V, VFB = 0.9V
--
0.8
1.2
mA
0.788
0.8
0.812
V
--
940
--
μA/V
RDS(ON)1
--
110
--
mΩ
RDS(ON)2
--
90
--
mΩ
VEN = 0V, VSW = 0V
--
0
10
μA
Min. Duty Cycle, VBOOT − VSW = 4.8V
--
5.1
--
A
VREF
4.5V ≤ VIN ≤ 18V
GEA
ΔIC = ±10μA
Reference 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
GCS
--
4.7
--
A/V
fOSC1
300
340
380
kHz
Short Circuit Oscillation
Frequency
fOSC2
VFB = 0V
--
100
--
kHz
Maximum Duty Cycle
DMAX
VFB = 0.7V
--
93
--
%
Minimum On Time
tON
--
100
--
ns
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is a registered trademark of Richtek Technology Corporation.
DS7257C-02
September 2012
RT7257C
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Logic-High
VIH
2
--
18
Logic-Low
Input Under Voltage Lockout
Threshold
Input Under Voltage Lockout
Hysteresis
Soft-Start Current
VIL
--
--
0.4
3.8
4.2
4.5
V
--
320
--
mV
ISS
VSS = 0V
--
6
--
μA
Soft-Start Period
tSS
CSS = 0.1μF
--
13.5
--
ms
Thermal Shutdown
TSD
--
150
--
°C
EN Input Threshold
Voltage
VUVLO
VIN Rising
ΔVUVLO
V
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.
DS7257C-02
September 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT7257C
Typical Operating Characteristics
Efficiency vs. Load Current
Output Voltage vs. Input Voltage
100
3.34
90
3.33
VIN = 4.5V
VIN = 12V
VIN = 17V
70
60
3.32
Output Voltage (V)
Efficiency (%)
80
50
40
30
3.31
3.30
3.29
3.28
20
3.27
10
VOUT = 3.3V
0
0.01
VIN = 4.5V to 23V, VOUT = 3.3V, IOUT = 0.5A
3.26
0.1
1
10
4
6
8
Load Current (A)
10
12
14
16
18
Input Voltage (V)
Output Voltage vs. Temperature
Output Voltage vs. Load Current
3.34
3.380
3.354
3.32
Output Voltage (V)
Output Voltage (V)
3.33
3.31
3.30
3.29
3.28
3.328
VIN = 4.5V
VIN = 12V
VIN = 17V
3.302
3.276
3.27
VOUT = 3.3V
VIN = 12V, VOUT = 3.3V, IOUT = 0.5A
3.26
3.250
-50
-25
0
25
50
75
100
125
0
0.5
1
Temperature (°C)
Switching Frequency vs. Input Voltage
2
2.5
3
Switching Frequency vs. Temperature
380
370
370
Switching Frequency (kHz)1
Switching Frequency (kHz)1
1.5
Load Current (A)
360
350
340
330
320
310
360
350
340
330
320
310
VOUT = 3.3V, IOUT = 0.5A
300
VIN = 12V, VOUT = 3.3V, IOUT = 0.5A
300
4.5
7
9.5
12
14.5
Input Voltage (V)
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17
-50
-25
0
25
50
75
100
125
Temperature (°C)
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DS7257C-02
September 2012
RT7257C
Load Transient Response
Current Limit vs. Temperature
8
Current Limit (A)
7
VOUT
(100mV/Div)
6
5
4
IOUT
(2A/Div)
3
VIN = 12V, VOUT = 3.3V
VIN = 12V, VOUT = 3.3V, IOUT = 0.2A to 3A
2
-50
-25
0
25
50
75
100
Time (250μs/Div)
125
Temperature (°C)
Load Transient Response
Switching
VOUT
(10mV/Div)
VOUT
(100mV/Div)
VSW
(10V/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.5A to 3A
IL
(2A/Div)
Time (250μs/Div)
Time (1μs/Div)
Power On from VIN
Power Off from VIN
VIN
(5V/Div)
VIN
(5V/Div)
VOUT
(2V/Div)
VOUT
(2V/Div)
IL
(2A/Div)
IL
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7257C-02
VIN = 12V, VOUT = 3.3V, IOUT = 3A
September 2012
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
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7
RT7257C
Power Off from EN
Power On from EN
VEN
(5V/Div)
VEN
(5V/Div)
VOUT
(2V/Div)
VOUT
(2V/Div)
IL
(2A/Div)
IL
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
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VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
is a registered trademark of Richtek Technology Corporation.
DS7257C-02
September 2012
RT7257C
Application Information
Output Voltage Setting
Soft-Start
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The RT7257C 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
be programmed by the external capacitor between SS and
GND. An internal current source ISS (6μA) charges an
external capacitor to build a soft-start ramp voltage. The
VFB voltage will track the internal ramp voltage during softstart interval. The typical soft-start time is calculated as
follows :
0.8 × CSS
Soft-Start time tSS =
, if CSS capacitor
ISS
VOUT
R1
FB
RT7257C
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT = VREF ⎛⎜ 1+ R1 ⎞⎟
⎝ R2 ⎠
is 0.1μF, then soft-start time =
0.8 × 0.1μ
≒ 13.5ms
6μ
Chip Enable Operation
Where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
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
RT7257C. Note that the external boot voltage must be
lower than 5.5V.
5V
BOOT
RT7257C
0.1µF
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT7257C quiescent current drops to lower than
3μA. Driving the EN pin high (>1.8V, < 18V) will turn on
the device again. For external timing control, the EN pin
can also be externally pulled high by adding a REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
EN
VIN
EN
RT7257C
CEN
GND
Figure 3. Enable Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 1.8V
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.
SW
VIN
Figure 2. External Bootstrap Diode
REN
EN
REN
100k
EN
Q1
RT7257C
GND
Figure 4. Digital Enable Control Circuit
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7257C-02
September 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
9
RT7257C
Under Voltage Protection
Over Temperature Protection
Hiccup Mode
For the RT7257CH, 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
RT7257CH will retry automatically. When the UVP
condition is removed, the converter will resume operation.
The UVP is disabled during soft-start period.
Hiccup Mode
The RT7257C 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
V
V
ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥
f
×
L
VIN ⎦
⎣
⎦ ⎣
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short
Time (50ms/Div)
Figure 5. Hiccup Mode Under Voltage Protection
Latch-Off Mode
For the RT7257CL, 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 RT7257CL will shutdown in LatchOff Mode. In shutdown condition, the RT7257CL can be
reset by EN pin or power input 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 :
⎡ VOUT ⎤ ⎡
VOUT ⎤
L =⎢
× ⎢1 −
⎥
⎥
f
I
V
×
Δ
L(MAX) ⎦ ⎣
IN(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.
Latch-Off Mode
VOUT
(2V/Div)
Table 2. Suggested Inductors for Typical
Application Circuit
ILX
(2A/Div)
IOUT = Short
Time (250μs/Div)
Figure 6. Latch-Off Mode Under Voltage Protection
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Component
Supplier
Series
Dimensions
(mm)
TDK
VLF10045
10 x 9.7 x 4.5
TDK
TAIYO
YUDEN
SLF12565
12.5 x 12.5 x 6.5
NR8040
8x8x4
is a registered trademark of Richtek Technology Corporation.
DS7257C-02
September 2012
RT7257C
CIN and COUT Selection
Thermal Considerations
The input capacitance, C IN, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
V
VIN
IRMS = IOUT(MAX) OUT
−1
VIN
VOUT
For continuous operation, do not exceed the maximum
operation junction temperature 125°C. The maximum
power dissipation depends on the thermal resistance of
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient.
The maximum power dissipation can be calculated by
following formula :
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst case condition is
commonly used for design because even significant
deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design.
For the input capacitor, two 10μF low ESR ceramic
capacitors are suggested. For the suggested capacitor,
please refer to Table 3 for more details.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response as described in a later section.
The output ripple, ΔVOUT , is determined by :
1
⎤
ΔVOUT ≤ ΔIL ⎡⎢ESR +
8fCOUT ⎥⎦
⎣
The output ripple will be the highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Higher values,
lower cost ceramic capacitors are now becoming available
in smaller case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regulator
applications. However, care must be taken when these
capacitors are used at input and output. When a ceramic
capacitor is used at the input and the power is supplied
by a wall adapter through long wires, a load step at the
output can induce ringing at the input, VIN. At best, this
ringing can couple to the output and be mistaken as loop
instability. At worst, a sudden inrush of current through
the long wires can potentially cause a voltage spike at
VIN large enough to damage the part.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7257C-02
September 2012
PD(MAX) = (TJ(MAX) − TA ) / θJA
Where T J(MAX) is the maximum operation junction
temperature , TA is the ambient temperature and the θJA is
the junction to ambient thermal resistance.
For recommended operating conditions specification, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance θJA is layout dependent. For
SOP-8 (Exposed Pad) package, the thermal resistance
θJA is 75°C/W on the standard JEDEC 51-7 four-layers
thermal test board. The maximum power dissipation at
TA = 25°C can be calculated by following formula :
P D(MAX) = (125°C − 25°C) / (75°C/W) = 1.333W
(min.copper area PCB layout)
P D(MAX) = (125°C − 25°C) / (49°C/W) = 2.04W
(70mm2copper area PCB layout)
The thermal resistance θJA of SOP-8 (Exposed Pad) is
determined by the package architecture design and the
PCB layout design. However, the package architecture
design had been designed. If possible, it's useful to
increase thermal performance by the PCB layout copper
design. The thermal resistance θJA can be decreased by
adding copper area under the exposed pad of SOP-8
(Exposed Pad) package.
As shown in Figure 7, the amount of copper area to which
the SOP-8 (Exposed Pad) is mounted affects thermal
performance. When mounted to the standard
SOP-8 (Exposed Pad) pad (Figure 7.a), θJA is 75°C/W.
Adding copper area of pad under the SOP-8 (Exposed
Pad) (Figure 7.b) reduces the θJA to 64°C/W. Even further,
increasing the copper area of pad to 70mm2 (Figure 7.e)
reduces the θJA to 49°C/W.
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RT7257C
The maximum power dissipation depends on operating
ambient temperature for fixed T J(MAX) and thermal
resistance θJA. The Figure 8 of derating curves allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation allowed.
2.2
Four-Layer PCB
Power Dissipation (W)
2.0
(a) Copper Area = (2.3 x 2.3) mm2, θJA = 75°C/W
1.8
Copper Area
70mm2
50mm2
30mm2
10mm2
Min.Layout
1.6
1.4
1.2
1.0
0.8
0.6
0.4
(b) Copper Area = 10mm2, θJA = 64°C/W
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 8. Derating Curve of Maximum Power Dissipation
(c) Copper Area = 30mm2 , θJA = 54°C/W
(d) Copper Area = 50mm2 , θJA = 51°C/W
(e) Copper Area = 70mm2 , θJA = 49°C/W
Figure 7. Thermal Resistance vs. Copper Area Layout
Design
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is a registered trademark of Richtek Technology Corporation.
DS7257C-02
September 2012
RT7257C
Layout Consideration
`
SW node is with high frequency voltage swing and
should be kept at small area. Keep analog components
away from the SW node to prevent stray capacitive noise
pick-up.
`
Connect feedback network behind the output capacitors.
Keep the loop area small. Place the feedback
components near the RT7257C.
`
An example of PCB layout guide is shown in Figure 9
for reference.
Follow the PCB layout guidelines for optimal performance
of the RT7257C.
`
`
Keep the traces of the main current paths as short and
wide as possible.
Put the input capacitor as close as possible to the device
pins (VIN and GND).
VIN
GND
SW GND
VIN
CBOOT
Input capacitor must
be placed as close
to the IC as possible.
8
BOOT
L
VOUT
REN
CSS
CIN
VIN
2
SW
3
GND
4
GND
CC
SS
7
EN
6
COMP
5
FB
9
The feedback components
must be connected as close
to the device as possible.
CP
RC
R1
R2
COUT
VOUT
GND
SW node is with high frequency voltage swing and should
be kept at small area. Keep analog components away from
the SW node to prevent stray capacitive noise pick-up
Figure 9. PCB Layout Guide
Table 3. Suggested Capacitors for CIN and COUT
Location
Component Supplier
Part No.
Capacitance (μF)
Case Size
CIN
MURATA
GRM31CR61E106K
10
1206
CIN
TDK
C3225X5R1E106K
10
1206
CIN
TAIYO YUDEN
TMK316BJ106ML
10
1206
COUT
MURATA
GRM31CR60J476M
47
1206
COUT
TDK
C3225X5R0J476M
47
1210
COUT
MURATA
GRM32ER71C226M
22
1210
COUT
TDK
C3225X5R1C22M
22
1210
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7257C-02
September 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT7257C
Outline Dimension
H
A
M
EXPOSED THERMAL PAD
(Bottom of Package)
Y
J
X
B
F
C
I
D
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
4.801
5.004
0.189
0.197
B
3.810
4.000
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.510
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.170
0.254
0.007
0.010
I
0.000
0.152
0.000
0.006
J
5.791
6.200
0.228
0.244
M
0.406
1.270
0.016
0.050
X
2.000
2.300
0.079
0.091
Y
2.000
2.300
0.079
0.091
X
2.100
2.500
0.083
0.098
Y
3.000
3.500
0.118
0.138
Option 1
Option 2
8-Lead SOP (Exposed Pad) Plastic Package
Richtek Technology Corporation
5F, No. 20, Taiyuen Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
www.richtek.com
14
DS7257C-02
September 2012