RT2872

®
RT2872
3A, 36V, Synchronous Step-Down Converter
General Description
Features
The RT2872 is a high efficiency, current-mode
synchronous step-down DC/DC converter that can deliver
up to 3A output current over a wide input voltage range
from 4.5V to 36V. The device integrates 105mΩ high-side
and 80mΩ low-side MOSFETs to achieve high conversion
efficiency. The current-mode control architecture supports
fast transient response and simple external compensation.
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4.5V to 36V Input Voltage Range
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3A Output Current
Internal N-MOSFETs
Current Mode Control
Frequency Operation : 300kHz to 1MHz
Adjustable Output Voltage from 0.8V to 30V
High Efficiency Up to 95%
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Current Limit
Input Under-Voltage Lockout
Output Under-Voltage Protection
Thermal Shutdown
AEC-Q100 Grade 3 Certification
RoHS Compliant and Halogen Free
A cycle-by-cycle current limit function provides protection
against shorted output and an internal soft-start eliminates
input current surge during start-up. The RT2872 provides
complete protection functions such as input under-voltage
lockout, output under-voltage protection, over-current
protection and thermal shutdown.
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The RT2872 is available in the thermal enhanced SOP-8
(Exposed Pad) package.
Applications
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Ordering Information
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RT2872
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Package Type
SP: SOP-8 (Exposed Pad-Option 2)
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Lead Plating System
G : Green (Halogen Free and Pb Free)
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Note :
Richtek products are :
`
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
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Suitable for use in SnPb or Pb-free soldering processes.
Point of Load Regulator in Distributed Power Systems
Digital Set Top Boxes
Broadband Communications
Vehicle Electronics
Automotive Audio, Navigation, and Information Systems
Enterprise Datacom Platforms Point of Load (POL)
Industrial Grade General Purpose Point of Load
Marking Information
RT2872GSP : Product Number
RT2872
GSPYMDNN
YMDNN : Date Code
Simplified Application Circuit
BOOT
VIN
VIN
CIN
RT2872
CB
L
VOUT
SW
R1
RT
GND
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2872-03
October 2013
COUT
FB
RT
CC
COMP
RC
R2
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1
RT2872
Pin Configurations
(TOP VIEW)
8
SW
BOOT
2
EN
3
GND
4
GND
VIN
7
RT
6
COMP
5
FB
9
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
SW
Switch Node. Connect to external L-C filter.
2
BOOT
Bootstrap Supply for the High-Side MOSFET. Connect a 100nF or greater
capacitor between the BOOT and SW pins.
3
EN
Enable Control Input. A logic-high enables the converter; a logic-low forces the
device into shutdown mode.
GND
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum thermal dissipation.
5
FB
Feedback Voltage Input. This pin is used to set the output voltage of the
converter to regulate to the desired value via an resistive divider.
6
COMP
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a R-C network from the COMP to GND. In some cases, an additional
capacitor from COMP to GND is required.
7
RT
Switching Frequency Setting. Connect an external resistor to set the switching
frequency from 300kHz to 1MHz.
8
VIN
Power Input. The input voltage range is from 4.5V to 36V. Must bypass with a
suitable large ceramic capacitor at this pin.
4,
9 (Exposed Pad)
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is a registered trademark of Richtek Technology Corporation.
DS2872-03
October 2013
RT2872
Function Block Diagram
VIN
VCC
Internal
Regulator
Shutdown
Comparator
1.2V
Oscillator
VA VCC
-
5kΩ
EN
Slope Comp
Foldback
Control
+
0.4V
Lockout
Comparator
+
1.7V
+
+
Current
Comparator
3.6V
0.8V
RT
RSENSE
VA
BOOT
UV
UV
Comparator
SS
Current Sense
Amplifier
+
-
S
Q
R
Q
SW
GND
+
+ EA
-
FB
COMP
Operation
The RT2872 is a constant frequency, current-mode
synchronous step-down converter. In normal operation,
the high-side N-MOSFET is turned on when the S-R latch
is set by the oscillator and is turned off when the current
comparator resets the S-R latch. While the high-side
N-MOSFET is turned off, the low-side N-MOSFET is turned
on to conduct the inductor current until next cycle begins.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (VFB) with the internal 0.8V reference.
When the load current increases, it causes a drop in the
feedback voltage relative to the reference, and then the
error amplifier's output voltage rises to allow higher inductor
current to match the load current.
Oscillator
The oscillator frequency can be set by using an external
resister RT. Oscillator frequency range is from 300kHz to
1MHz.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for highside gate driver.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2872-03
October 2013
Enable
The converter is turned on when the EN pin is higher than
2V. When the EN pin is lower than 0.4V, the converter will
enter shutdown mode and reduce the supply current to
0.5μA.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build a soft-start ramp voltage. The FB voltage will track
the internal ramp voltage during soft-start interval. The
typical soft-start time is 2ms.
UV Comparator
If the feedback voltage is lower than 0.4V, the UV
Comparator will go high to turn off the high-side MOSFET.
The output under voltage protection is designed to operate
in hiccup mode. When the UV condition is removed, the
converter will resume switching.
Thermal Shutdown
The over-temperature protection function will shut down
the switching operation when the junction temperature
exceeds 150°C. Once the junction temperature cools
down by approximately 20°C, the converter will
automatically resume switching.
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3
RT2872
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VIN -----------------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------BOOT Pin ------------------------------------------------------------------------------------------------------------EN Pin (with REN (150kΩ to 600kΩ) to VIN) ----------------------------------------------------------------SW Voltage (t < 10ns) --------------------------------------------------------------------------------------------EN Pin -----------------------------------------------------------------------------------------------------------------Other Pins ------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
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) ----------------------------------------------------------------------------------------
Recommended Operating Conditions
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−0.3V to 40V
−0.3V to (VIN + 0.3V)
−0.3V to 46.3V
−0.3V to 40V
−5V to 46.3V
−0.3V to 3.6V
−0.3V to 40V
2.041W
49°C/W
8°C/W
260°C
150°C
−65°C to 150°C
2kV
(Note 4)
Supply Input Voltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 36V
Junction Temperature Range -------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 12V, CIN = 20μF, TA = −40°C to 85°C, unless otherwise specified)
Parameter
Symbol
Shutdown Supply Current
Test Conditions
Min
Typ
Max
Unit
VEN = 0V
--
--
10
μA
--
1
1.3
mA
0.784
0.8
0.816
V
Quiescent Current
IQ
VEN = 3V, VFB = 0.9V
Feedback Reference Voltage
VREF
4.5V ≤ VIN ≤ 36V
Switch
On-Resistance
High-Side
RDS(ON)1
--
105
190
Low-Side
RDS(ON)2
--
80
145
4.25
5
5.75
A
--
1.7
--
A
RT = 191kΩ
264
300
336
RT = 113kΩ
440
500
560
RT = 51kΩ
880
1000
1120
High-Side Switch Current Limit
Range
UOC
Low-Side Switch Current Limit
Oscillation Frequency
From Drain to Source
fOSC1
mΩ
kHz
Short-Circuit Oscillation
Frequency
fOSC2
VFB = 0V, RT = 113kΩ
--
50
--
kHz
Maximum Duty Cycle
DMAX
VFB = 0.7V
--
95
--
%
Minimum On-Time
tON
--
100
120
ns
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is a registered trademark of Richtek Technology Corporation.
DS2872-03
October 2013
RT2872
Parameter
EN Input Voltage
Symbol
Test Conditions
Min
Typ
Max
Unit
Logic-High
VIH
2
--
3.3
Logic-Low
VIL
--
--
0.4
3.7
4.2
4.5
V
V
Input Under-Voltage Lockout
Threshold
VUVLO
Input Under-Voltage Lockout
Hysteresis
ΔVUVLO
--
250
--
mV
Thermal Shutdown Threshold
T SD
--
150
--
°C
Thermal Shutdown Hysteresis
ΔTSD
--
25
--
°C
COMP to Current Sense
Trans-conductance
GCS
--
4.1
--
A/V
--
950
--
μA/V
VIN Rising
ΔICOMP = ±10μA
Error Amplifier Trans-conductance GEA
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. The PCB copper area with exposed pad is 70mm2.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2872-03
October 2013
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RT2872
Typical Application Circuit
8
VIN
4.5V to 36V
VIN
RT2872
2
BOOT
CIN
10µF x 2
Enable
REN
SW 1
3
7
RT
113k
4, 9 (Exposed Pad)
CB
100nF
L
VOUT
R1
EN
FB 5
RT
GND
COMP
6
COUT
CC
RC
R2
Table 1. Suggested Component Values
VOUT (V)
R1 (kΩ)
R2 (kΩ)
RC (kΩ)
L (μH)
CC (nF)
COUT (μF)
12
47
3.35
47
10
2.7
22 x 2
8
27
3
36
8.2
2.7
22 x 2
5
62
11.8
24
6.8
2.7
22 x 2
3.3
75
24
16
4.7
2.7
22 x 2
2.5
25.5
12
12
3.6
2.7
22 x 2
1.2
30
60
6.8
2.2
2.7
22 x 2
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is a registered trademark of Richtek Technology Corporation.
DS2872-03
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RT2872
Typical Operating Characteristics
Efficiency vs. Output Current
Reference Voltage vs. Input Voltage
100
0.810
90
0.808
VIN =
VIN =
VIN =
VIN =
VIN =
70
60
50
Reference Voltage (V)
Efficiency (%)
80
5V
12V
24V
30V
36V
40
30
20
0.805
0.803
0.800
0.798
0.795
0.793
10
VOUT = 3.3V, RT = 113k
VIN = 4.5V to 36V, IOUT = 0A, RT = 113k
0
0.790
0
0.5
1
1.5
2
2.5
3
4
8
12
16
20
24
28
32
36
Input Voltage (V)
Output Current (A)
Reference vs. Temperature
Output Voltage vs. Output Current
3.300
0.810
3.290
0.805
Output Voltage (V)
Reference Voltage (V)
3.295
0.800
VIN =
VIN =
VIN =
VIN =
0.795
4.5V
12V
24V
36V
3.285
3.280
VIN =
VIN =
VIN =
VIN =
VIN =
3.275
3.270
3.265
5V
12V
24V
30V
36V
3.260
VOUT = 3.3V, IOUT = 0A, RT = 113k
0.790
3.255
VOUT = 3.3V, RT = 113k
3.250
-50
-25
0
25
50
75
100
0
125
0.5
Temperature (°C)
Switching Frequency vs. Input Voltage
2
2.5
3
Switching Frequency vs. Temperature
515
Switching Frequency (kHz)1
Switching Frequency (kHz)1
1.5
600
520
510
505
500
495
490
485
VOUT = 3.3V, IOUT = 0A, RT = 113k
480
580
560
540
520
500
480
VIN =
VIN =
VIN =
VIN =
460
440
420
4.5V
12V
24V
36V
VOUT = 3.3V, IOUT = 0A, RT = 113k
400
4
8
12
16
20
24
28
32
Input Voltage (V)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2872-03
1
Output Current (A)
October 2013
36
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT2872
Frequency vs. RT
Current Limit vs. Temperature
1200
8
1100
1000
Frequency (kHz)1
Inductor Current (A)
7
6
5
VIN =
VIN =
VIN =
VIN =
4
3
36V
24V
12V
4.5V
900
800
700
600
500
400
300
VIN = 12V, IOUT = 0A
200
2
-50
-25
0
25
50
75
100
50
125
Load Transient Response
95
110 125 140 155 170 185 200
Load Transient Response
VOUT
(200mV/Div)
VOUT
(200mV/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V,
IOUT = 3A to 1.5A, RT = 113k
VIN = 12V, VOUT = 3.3V,
IOUT = 0A to 3A, RT = 113k
Time (100μs/Div)
Time (100μs/Div)
Switching
Switching
VOUT
(5mV/Div)
VOUT
(5mV/Div)
VSW
(5V/Div)
VSW
(5V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.5A, RT = 113k
Time (1μs/Div)
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8
80
RT (kΩ )
Temperature (°C)
IL
(1A/Div)
65
IL
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A, RT = 113k
Time (1μs/Div)
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DS2872-03
October 2013
RT2872
Power Off from EN
Power On from EN
VEN
(2V/Div)
VEN
(2V/Div)
VOUT
(2V/Div)
VOUT
(2V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A, RT = 113k
VIN = 12V, VOUT = 3.3V, IOUT = 3A, RT = 113k
Time (2.5ms/Div)
Time (2.5ms/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, RT = 113k
Time (5ms/Div)
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DS2872-03
October 2013
VIN = 12V, VOUT = 3.3V, IOUT = 3A, RT = 113k
Time (5ms/Div)
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RT2872
Application Information
Output Voltage Setting
Chip Enable Operation
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT2872 quiescent current drops to lower than
3μA. Driving the EN pin high (>2.5V, <3.3V) 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).
VOUT
R1
FB
RT2872
R2
GND
REN must be chose between 150kΩ to 600kΩ, which is
to avoid huge leak current into chip.
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 ⎠
where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT and SW pins. 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
RT2872. Note that the external boot voltage must be lower
than 5.5V
EN
REN
VIN
EN
RT2872
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 2.5V
is available, as shown in Figure 4. In this case, a 300kΩ
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.
VIN
REN
300k
EN
EN
Q1
RT2872
GND
Figure 4. Digital Enable Control Circuit
5V
Under-Voltage Protection
Hiccup Mode
BOOT
100nF
RT2872
SW
Figure 2. External Bootstrap Diode
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The RT2872 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
RT2872 will retry automatically. When the UVP condition
is removed, the converter will resume operation. The UVP
is disabled during soft-start period.
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RT2872
Hiccup Mode
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.
VOUT
(2V/Div)
Table 2. Suggested Inductors for Typical
Application Circuit
ILX
(2A/Div)
IOUT = Short
Time (50ms/Div)
Figure 5. Hiccup Mode Under-Voltage Protection
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
Over-Temperature Protection
CIN and COUT Selection
The RT2872 features an Over-Temperature Protection
(OTP) circuitry to prevent overheat 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.
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
approximate RMS current equation is given :
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 ⎤⎥
VIN ⎦
⎣ f ×L ⎦ ⎣
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 − VIN(MAX) ⎥
f
I
×
Δ
L(MAX)
⎣
⎦ ⎣
⎦
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DS2872-03
October 2013
V
IRMS = IOUT(MAX) OUT
VIN
VIN
−1
VOUT
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 ⎥⎦
⎣
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RT2872
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.
Thermal Considerations
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 :
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 of
RT2872, 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 :
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 6, 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 6.a), θJA is 75°C/W.
Adding copper area of pad under the SOP-8 (Exposed
Pad) (Figure 6.b) reduces the θJA to 64°C/W. Even further,
increasing the copper area of pad to 70mm2 (Figure 6.e)
reduces the θJA to 49°C/W.
The maximum power dissipation depends on operating
ambient temperature for fixed T J(MAX) and thermal
resistance θJA. The Figure 7 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
2.0
Power Dissipation (W)
The output ripple will be the highest at the maximum input
1.8
Copper Area
70mm2
50mm2
30mm2
10mm2
Min.Layout
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 7. Derating Curve of Maximum Power Dissipation
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)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
is a registered trademark of Richtek Technology Corporation.
DS2872-03
October 2013
RT2872
Layout Considerations
For best performance of the RT2872, the following layout
guidelines must be strictly followed.
(a) Copper Area = (2.3 x 2.3) mm2, θJA = 75°C/W
`
Input capacitor must be placed as close to the IC as
possible.
`
SW should be connected to inductor by wide and short
trace. Keep sensitive components away from this trace.
`
The RT resistor, compensator and feedback components
must be connected as close to the device as possible.
(b) Copper Area = 10mm2, θJA = 64°C/W
(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 6. Thermal Resistance vs. Copper Area Layout
Design
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS2872-03
October 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT2872
Input capacitor must be placed
as close to the IC as possible.
VOUT
VIN
COUT
SW should be connected to
inductor by wide and short trace.
Keep sensitive components
away from this trace and CBOOT.
RS*
L
CIN
The RT resistor must be connected
as close to the device as possible.
Keep sensitive components away.
CS*
RT
CBOOT
VIN
8
SW
REN
BOOT
2
EN
3
GND
4
GND
VIN
7
RT
6
COMP
5
FB
9
CC
R1
VOUT
RC
CP
R2
The REN component
must be connected.
GND
The Compensator and feedback
components must be connected as
close to the device as possible.
* : Option
Figure 8. PCB Layout Guide
Table 3. Suggested Capacitors for CIN and COUT
Location
Component Supplier
Part No.
Capacitance (μF)
Case Size
CIN
MURATA
GRM32ER71H475K
4.7
1206
CIN
TAIYO YUDEN
UMK325BJ475MM-T
4.7
1206
CIN
MURATA
GRM31CR61E106K
10
1206
CIN
TDK
C3225X5R1E106K
10
1206
CIN
TAIYO YUDEN
TMK316BJ106ML
10
1206
C OUT
MURATA
GRM31CR60J476M
47
1206
C OUT
TDK
C3225X5R0J476M
47
1210
C OUT
MURATA
GRM32ER71C226M
22
1210
C OUT
TDK
C3225X5R1C22M
22
1210
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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14
is a registered trademark of Richtek Technology Corporation.
DS2872-03
October 2013
RT2872
Outline Dimension
H
A
M
EXPOSED THERMAL PAD
(Bottom of Package)
Y
J
X
B
F
C
I
D
Dimensions In Millimeters
Symbol
Dimensions In Inches
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
14F, No. 8, Tai Yuen 1st 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.
DS2872-03
October 2013
www.richtek.com
15