Richtek C3225X5R1C226M 3a, 24v, 3mhz step-down converter Datasheet

®
RT8280
3A, 24V, 3MHz Step-Down Converter
General Description
The RT8280 is a high voltage buck converter that can
support an input voltage range from 4.5V to 24V with output
current up to 3A. Current mode operation provides fast
transient response and eases loop stabilization.
The chip provides protection functions such as cycle-bycycle current limiting and thermal shutdown protection.
In shutdown mode, the regulator only draws 25μA of
supply current. The RT8280 is available in a SOP-8
(Exposed Pad) package.
Features
Wide Operating Input Range : 4.5V to 24V
Adjustable Output Voltage Range : 0.8V to 15V
Output Current up to 3A
25μ
μA Low Shutdown Current
High Efficiency up to 90% at 2.2MHz
Programmable Frequency : 220kHz to 3MHz
Internal Soft-Start
Stable with Low ESR Output Ceramic Capacitors
Thermal Shutdown Protection
Cycle-By-Cycle Over Current Protection
RoHS Compliant and Halogen Free
Ordering Information
RT8280
Applications
Package Type
SP : SOP-8 (Exposed Pad-Option 1)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
`
DSL Modem for ADSL2+ Standard
Distributed Power Systems
Pre-Regulator for Linear Regulators
Pin Configurations
(TOP VIEW)
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
RT8280GSP : Product Number
RT8280
GSPYMDNN
YMDNN : Date Code
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8280-02 March 2012
BOOT
VIN
2
SW
GND
3
4
GND
9
8
RT
7
EN
6
COMP
5
FB
SOP-8 (Exposed Pad)
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RT8280
Typical Application Circuit
2
VIN
4.5V to 24V
CIN
10µF
Chip Enable
VIN
BOOT
RT8280
7 EN
1
SW 3
CBOOT
L
10nF 2.2µH
B330A
R1
31.6k
8 RT
RRT
24k
4,
9 (Exposed Pad)
VOUT
3.3V/3A
FB 5
GND
COMP
6
CC
1.8nF
RC
24k
COUT
22µF
R2
10k
NC
CP
VOUT (V)
10
8
5
3.3
2.5
1.8
1.5
1.2
Table 1. Recommended Component Selection for fSW = 2.2MHz
R1 (kΩ)
R2 (kΩ)
RC (kΩ)
C C (nF)
L (μH)
115
10
68
0.82
8.2
91
10
51
1
6.8
52.3
10
36
1.2
4.7
31.6
10
24
1.8
2.2
21.5
10
18
2.2
2
12.4
10
13
2.2
1.5
8.87
10
12
2.2
1.5
4.99
10
9
1.8
1
COUT (μF)
22
22
22
22
22
22
22
22
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
BOOT
Bootstrap Power. BOOT supplies the drive for the high side N-MOSFET switch.
Connect a 10nF or greater capacitor from SW to BOOT to power the high side
switch.
2
VIN
Supply Input. VIN supplies the power to the IC, as well as the step-down
converter switches. Drive VIN with a 4.5V to 24V power source. Bypass VIN to
GND with a suitably large capacitor to eliminate noise on the input to the IC.
3
SW
Switch Node. SW is the switching node that supplies power to the output.
Connect the output LC filter from SW to the output load. Note that a capacitor is
required from SW to BOOT to power the high side switch.
4,
9 (Exposed Pad)
GND
5
FB
6
COMP
7
EN
8
RT
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Feedback Input. FB senses the output voltage to regulate. Drive FB with a
resistive voltage divider from the output voltage.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND to compensate the regulation
control loop. In some cases, an additional capacitor from COMP to GND is
required.
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN
higher than 1.4V to turn on the regulator, lower than 0.4V to turn off. For
automatic startup, leave EN unconnected.
Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the
switching frequency.
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DS8280-02 March 2012
RT8280
Function Block Diagram
VIN
Internal
Regulator
1µA
Oscillator
-
VA VCC
EN
10k
Foldback
Control
0.4V
-
0.8V
+
-
EA
VA
BOOT
+
UV
Comparator
+
Shutdown
Comparator
1V
3V
Current Sense
Slope Comp Amplifier
+
S
Q
R
Q
SW
+
Current
Comparator
+
GND
0.8V
-
FB
COMP RT
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RT8280
Absolute Maximum Ratings
(Note 1)
VIN ---------------------------------------------------------------------------------------------------------------- −0.3V to 26V
SW --------------------------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V)
BOOT ----------------------------------------------------------------------------------------------------------- (SW − 0.3V) to (SW + 6V)
All Other Pins ------------------------------------------------------------------------------------------------- −0.3V to 6V
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 Mode) --------------------------------------------------------------------------------MM (Machine Mode) -----------------------------------------------------------------------------------------
Recommended Operating Conditions
1.333W
75°C/W
15°C/W
260°C
150°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VIN ----------------------------------------------------------------------------------- 4.5V to 24V
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
4.5V ≤ VIN ≤ 24V
Min
Typ
Max
Unit
0.784
0.8
0.816
V
Feedback Reference Voltage
VFB
Upper Switch On Resistance
RDS(ON)1
--
0.11
--
Ω
Lower Switch On Resistance
RDS(ON)2
--
10
--
Ω
Upper Switch Leakage
ILEAK
--
0
10
μA
Current Limit
ILIM
--
5
--
A
--
3.8
--
A/V
Current Sense
Transconductance
Output Current to Comp Pin
Voltage
Error Amplifier
Transconductance
Oscillator Frequency
VEN = 0V, VSW = 0V
Duty = 90%,
VBOOT − VSW = 4.8V
gCS
gEA
ΔIC = ±10μA
--
920
--
μA/V
fSW
RRT = 24kΩ
--
2.2
--
MHz
VFB = 0V, RRT = 24kΩ
--
230
--
kHz
VUVLO
--
4.2
--
V
ΔVUVLO
--
430
--
mV
--
65
--
%
--
70
--
ns
Short Circuit Frequency
Under Voltage Lockout
Threshold Rising
Under Voltage Lockout
Threshold Hysteresis
Maximum Duty Cycle
DMAX
Minimum On Time
tON
VFB = 0.7V, RRT = 24kΩ
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DS8280-02 March 2012
RT8280
Parameter
EN Threshold
Voltage
Symbol
Test Conditions
Min
Typ
Max
Unit
Logic-High
VIH
1.4
--
5.5
Logic-Low
VIL
--
--
0.4
--
1
--
μA
Enable Pull Up Current
V
Quiescent Current
IQ
VEN = 2V, VFB = 1V
--
0.8
1
mA
Shutdown Current
ISHDN
VEN = 0V
--
25
--
μA
--
150
--
°C
Thermal Shutdown
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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DS8280-02 March 2012
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RT8280
Typical Operating Characteristics
Efficiency vs. Output Current
100
90
90
80
80
70
70
Efficiency (%)
Efficiency (%)
Efficiency vs. Output Current
100
60
50
40
30
20
VIN = 12V
VIN = 19V
60
50
40
30
20
10
VIN = 12V, VOUT = 3.3V, fSW = 2.2MHz
10
0
0.0
0.5
1.0
1.5
2.0
2.5
VOUT = 5V, fSW = 2.2MHz
0
3.0
0.0
0.5
1.0
Output Current (A)
1.5
2.0
2.5
3.0
Output Current (A)
Output Voltage vs. Output Current
Output Voltage vs. Input Voltage
5.30
3.35
5.25
3.34
5.15
Output Voltage (V)
Output Voltage (V)
5.20
5.10
VIN = 19V
5.05
5.00
VIN = 12V
4.95
4.90
4.85
3.33
3.32
3.31
3.30
4.80
IOUT = 0A
4.75
3.29
4.70
0.0
0.5
1.0
1.5
2.0
2.5
3
3.0
6
9
15
18
21
24
Input Voltage (V)
Output Current (A)
Reference Voltage vs. Temperature
Quiescent Current vs. Temperature
0.810
0.90
0.805
0.85
Quiescent Current (mA)
Reference Voltage (V)
12
0.800
0.795
0.790
0.785
0.80
0.75
0.70
0.65
VIN = 12V, IOUT = 0A
VIN = 12V, RRT = Open
0.60
0.780
-50
-25
0
25
50
75
100
Temperature (°C)
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125
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT8280
Current Limit vs. Temperature
Current Limit vs. Duty Cycle
6.0
6.50
6.25
5.5
5.75
Peak Current (A)
Peak Current (A)
6.00
fSW = 2.2MHz
5.50
5.25
fSW = 400kHz
5.00
4.75
4.50
4.25
5.0
4.5
4.0
3.5
4.00
3.75
VIN = 12V, VOUT = 3.3V, fSW = 2.2MHz
3.0
3.50
0
20
40
60
80
-50
100
-25
0
25
50
75
Duty Cycle (%)
Temperature (°C)
Switching Frequency vs. Temperature
Output Ripple Voltage
100
125
Switching Freq. (MHz)1
2.4
V OUT
(10mV/Div)
2.3
2.2
VSW
(5V/Div)
2.1
2.0
1.9
VIN = 12V, VOUT = 3.3V, RRT = 24kΩ
IL
(2A/Div)
1.8
-50
-25
0
25
50
75
100
125
VIN = 12V, VOUT = 3.3V
lOUT = 3A, fSW = 2.2MHz
Time (200ns/Div)
Temperature (°C)
Load Transient Response
VIN = 12V, VOUT = 3.3V
lOUT = 0A to 3A, fSW = 2.2MHz
Load Transient Response
VOUT
(200mV/Div)
VOUT
(200mV/Div)
IOUT
(1A/Div)
IOUT
(1A/Div)
Time (100μs/Div)
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VIN = 12V, VOUT = 3.3V
lOUT = 1.5A to 3A, fSW = 2.2MHz
Time (100μs/Div)
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RT8280
Power On from EN
Power off from EN
VIN = 12V, VOUT = 3.3V
lOUT = 3A, fS = 2.2MHz
VEN
(2V/Div)
VEN
(2V/Div)
VOUT
(1V/Div)
VOUT
(1V/Div)
IL
(2V/Div)
VIN = 12V, VOUT = 3.3V
lOUT = 3A, fSW = 2.2MHz
Time (400μs/Div)
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IL
(2V/Div)
Time (40μs/Div)
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RT8280
Application Information
The RT8280 is an asynchronous high voltage buck
converter that supports an input voltage range from 4.5V
to 24V with output current up to 3A.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense
the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8280
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT = VFB ⎛⎜ 1+ R1 ⎞⎟
⎝ R2 ⎠
where VFB is the feedback reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 10nF 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 voltage source and the BOOT
pin for efficiency improvement when input voltage is lower
than 5.5V or duty cycle is higher than 65% .The bootstrap
diode can be a low cost one such as IN4148 or BAT54.
The external voltage source must be fixed at 5V and can
be provided from the system or the output of the RT8280.
Note that the external boot voltage must be lower than
5.5V.
5V
BOOT
RT8280
10nF
Operating Frequency
Selection of the operating frequency is a trade off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequency improves efficiency by
reducing internal gate charge and switching losses, but
requires larger inductance and/or capacitance to maintain
low output ripple voltage. The operating frequency of the
RT8280 is determined by an external resistor that is
connected between the RT pin and ground. The value of
the resistor sets the ramp current that is used to charge
and discharge an internal timing capacitor within the
oscillator. Selection of the RT resistor value can be
determined by examining the curve below in Figure3.
Although frequencies as high as 3MHz are available, the
minimum on-time of the RT8280 imposes a limit on the
operating duty cycle. Figure 4 shows the examples of
minimum on-time constraint for output voltages 3.3V and
1.8V. It is recommended to operate the RT8280 in the
region under the corresponding Vout curve.
Except the minimum on-time constraint, the limit of
maximum duty also needs to be considered. In ideal case,
the duty cycle of the RT8280 can be calculated by below
equation, But in practical case it will be higher than the
calculation result since all the components in a converter
circuit are not ideal. Figure 5 shows an example for the
limit of maximum duty. With 5V input voltage, the 3.3V
output voltage of the RT8280 becomes out of regulation
when the output current is increased. However, when the
input voltage is changed to 12V, the 3.3V output voltage
of the RT8280 remains in regulation even with 3A output
current. According to equation below, the duty cycle is
0.67 for the RT8280 operated with 5V input voltage and
3.3V output voltage in 2.2MHz switching frequency. The
ideal case duty cycle calculation is already over the limit
of maximum duty (65%). Thus, it is obvious that the
RT8280 can't support 3A output current in such
conditions :
Duty Cycle = 1 − 0.15 x fSW
(MHz)
SW
Figure 2. External Bootstrap Diode
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RT8280
Switching Frequency vs. RRT
Switching Frequency (kHz)1
3000
Chip Enable Operation
2750
2500
2250
2000
1750
1500
1250
1000
750
500
250
0
0
200
400
600
800
1000
RRRT
(kΩ)
RT (kΩ)
Figure 3. Switching Frequency vs. RRT
Minimum On-Time Constraint
30.0
27.5
Input Voltage (V)
25.0
22.5
Inductor Selection
20.0
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 :
VOUT = 3.3V
17.5
15.0
12.5
VOUT = 1.8V
10.0
7.5
V
V
ΔIL = ⎡⎢ OUT ⎤⎥ x ⎡⎢1− OUT ⎤⎥
f
x
L
VIN ⎦
⎣ SW
⎦ ⎣
5.0
2.5
0.0
1000
1400
1800
2200
2600
3000
Switching Frequency (kHz)
Figure 4. Minimum On-Time Constraint to Input Voltage
Output Voltage vs. Load Current
3.5
3.4
3.3
Output Voltage (V)
The EN pin is the enable input. Pull the EN pin low (<0.4V)
to shutdown the device. During shutdown mode, the
RT8280 quiescent current drops to lower than 25μA. Drive
the EN pin high (>1.4V, < 5.5V), to turn on the device. If
the EN pin is open, it will be pulled high by the internal
circuit. For external timing control (e.g.RC), the EN pin
can also be externally pulled high by adding a 100kΩ or
greater resistor from the VIN pin (see Figure 6). In some
cases, the output voltage of the RT8280 may still be under
UVP threshold when soft-start finishes. Then the RT8280
will restart again and the output voltage of the RT8280 will
rise to the regulation voltage. This phenomenon often
happens in high frequency operation and with slow rising
input voltage. It can easily be solved by adding a voltage
divider on the EN pin. The RT8280 will be enabled when
the input voltage rises close to the nominal input voltage.
VIN = 12V
3.2
3.1
3.0
⎡
⎤ ⎡
VOUT
VOUT ⎤
L =⎢
x ⎢1 −
⎥
⎥
⎣ fSW x ΔIL(MAX) ⎦ ⎣ VIN(MAX) ⎦
2.9
VIN = 5V
2.8
2.7
2.6
2.5
0.0
0.5
1.0
1.5
2.0
2.5
Load Current (A)
Figure 5. Limit of Maximum Duty
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Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Higher frequency combined with smaller ripple
current is necessary to achieve high efficiency operation.
However, it requires a large inductor to achieve this goal.
For the ripple current selection, setting the maximum value
of the ripple current ΔIL = 0.24(IMAX) is 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 :
3.0
The inductor's current rating (defined by that which causes
a temperature rise from 25°C ambient to 40°C) should be
greater than the maximum load current and its saturation
current should be greater than the short circuit peak current
limit. Refer to Table 2 for the suggested inductor selection.
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RT8280
Table2. Suggested Inductors for Typical
Application Circuit
Component
Supplier
TDK
TDK
TAIYO
YUDEN
Series
Dimensions
(mm)
VLC6045
SLF12565
6 x 6 x 4.5
12.5 x 12.5 x 6.5
NR8040
8x8x4
When the power switch turns off, the path for the current
is through the diode connected between the switch output
and ground. This forward biased diode must have a minimal
voltage drop and recovery time. Schottky diodes are
recommended and should be able to handle those current.
The reverse voltage rating of the diode should be greater
than the maximum input voltage, and the current rating
should be greater than the maximum load current. For
details, please refer to Table 3.
Table 3. Suggested Diode
Component
Series VRRM (V) IOUT (A)
Supplier
Package
DIODES
B330A
30
3
SMA
DIODES
B340A
40
3
SMA
PANJIT
SK33
30
3
DO-214AB
PANJIT
SK34
40
3
DO-214AB
CIN and COUT Selection
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 :
VIN
−1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = I OUT / 2. This simple worst-case condition is
commonly used for design.
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, one 10μF low ESR ceramic
capacitors is recommended. For the recommended
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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 :
Diode Selection
V
IRMS = IOUT(MAX) OUT
VIN
capacitor, please refer to Table 4 below for more details.
1
⎤
ΔVOUT ≤ ΔIL ⎡⎢ESR +
8fCOUT ⎥⎦
⎣
The output ripple will be 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. Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors offer very low ESR value.
However, it provides lower capacitance density than other
types. Although Tantalum capacitors have the highest
capacitance density, it is important to only use types that
pass the surge test for use in switching power supplies.
Aluminum electrolytic capacitors have significantly higher
ESR. However, it can be used in cost sensitive applications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low
ESR characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing.
Nevertheless, high value low 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 the
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.
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RT8280
Checking Transient Response
Choose a capacitor that is greater than the above
calculation result. The frequency of the zero, which
consists of RC and CC, should be lower than one fourth of
fC to get a sufficient phase margin. If the zero moves close
to fC, the phase margin decreases.
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR) and COUT also begins to be charged
or discharged generating a feedback error signal for the
regulator to return VOUT to its steady state value. During
this recovery time, VOUT can be monitored for overshoot
or ringing to indicate any stability problem.
In some applications, the output capacitor will be an
electrolytic capacitor, not a ceramic capacitor. A zero will
be produced by the electrolytic capacitor and its ESR. CP
can be used to produce a pole with RC to cancel the zero.
To calculate CP, follow the equation below :
Compensation Parameters
CP =
The switching frequency of the RT8280 can be programmed
from free running frequency to 3MHz. Table 1 only lists
the recommended compensation parameters for 2.2MHz
switching frequency. Optimized compensation parameters
for other switching frequency can also be determined
through below procedures. The first step is to decide the
crossover frequency, fc. In general, the crossover
frequency is one tenth of the switching frequency. Then,
Rc can be obtained through the following equation :
2π × COUT × fC × VOUT
RC =
gCS × gEA × VFB
EMI Consideration
Since parasitic inductance and capacitance effects in PCB
circuitry would cause a spike voltage on the SW pin when
the high side MOSFET is turned-on/off, this spike voltage
on SW may impact EMI performance in the system. In
order to enhance EMI performance, there are two methods
to suppress the spike voltage. One is to place an R-C
snubber between SW and GND and place them as close
as possible to the SW pin (see Figure 6). Another method
is to add a resistor in series with the bootstrap
capacitor, CBOOT. But this method will decrease the driving
capability to the high side MOSFET. It is strongly
recommended to reserve the R-C snubber during PCB
layout for EMI improvement. Moreover, reducing the SW
trace area and keeping the main power in a small loop will
be helpful for EMI performance. For detailed PCB layout
guide, please refer to the section on Layout Consideration.
where
gCS is Current SenseTransconductance = 1.8 (A/V)
gEA is Error Amplifier Tansconductance = 920 (μA/V)
Once the value of Rc has been determined, the value of
Cc can be obtained by the following equation :
1
CC =
f
2π × RC × C
4
2
VIN
4.5V to 24V
CIN
10µF
REN*
Chip Enable
VIN
BOOT
RT8280
7 EN
1
8 RT
4,
9 (Exposed Pad)
GND
* : Optional
RBOOT*
CBOOT L
10nF 2.2µH
SW 3
RS*
CEN*
RRT
24k
COUT × ESR
RC
B330A
R1
31.6k
CS*
FB 5
COMP
6
VOUT
3.3V/3A
CC
1.8nF
RC
24k
COUT
22µF
R2
10k
NC
CP
Figure 6. Reference Circuit with Snubber and Enable Timing Control
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8280-02 March 2012
RT8280
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
(a) Copper Area = (2.3 x 2.3) mm2, θJA = 75°C/W
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-8 (Exposed Pad) packages, the thermal resistance,
θJA, is 75°C/W on a standard JEDEC 51-7 four-layer
thermal test board. The maximum power dissipation at
TA=25°C can be calculated by the following formulas :
(b) Copper Area = 10mm2, θJA = 64°C/W
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
(70mm2 copper area PCB layout)
The thermal resistance, θJA, of SOP-8 (Exposed Pad) is
determined by the package architectural design and the
PCB layout design. The package architectural design is
fixed. However, it's possible to increase thermal
performance via better PCB layout copper design. The
thermal resistance, θJA, can be decreased by adding
copper area under the exposed pad of the SOP-8
(Exposed Pad) package.
As shown in Figure 7, the amount of copper area to which
the SOP-8 (Exposed Pad) is mounted on affects thermal
performance. When mounted to the standard SOP-8
(Exposed Pad) (Figure 7a), θJA is 75°C/W. Adding copper
area under the SOP-8 (Exposed Pad) (Figure 7b) reduces
θJA to 64°C/W. Further increasing the copper area to
70mm2 (Figure 7e) will reduce θJA to 49°C/W.
The maximum power dissipation depends on operating
ambient temperature for fixed T J (MAX) and thermal
resistance, θJA. The derating curves in Figure 8 allow the
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8280-02 March 2012
(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. Themal Resistance vs. Copper Area Layout
Design
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RT8280
2.2
Four Layer PCB
Power Dissipation (W)
2.0
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 8. Derating Curves for RT8280 Package
Layout Consideration
`
Follow the PCB layout guidelines for optimum performance
of the RT8280.
`
`
Keep the traces of the main current paths as short and
wide as possible.
Place the input capacitor as close as possible to the
device pins (VIN and GND).
` SW node experiences high frequency voltage swing and
Connect the feedback network behind the output
capacitors. Keep the loop area small. Place the feedback
components near the RT8280.
` Connect all analog grounds to a common node and then
connect the common node to ground behind the output
capacitors.
` An
example of the PCB layout guide is shown in Figure
9 for reference.
should be kept in a small area. Keep analog components
away from the SW node to prevent stray capacitive noise
pick up.
VIN
GND
Input capacitor must
be placed as close
to the IC as possible.
RRT
CIN
BOOT
D
CS
COUT
The feedback components
must be connected as close
to the device as possible.
SW GND
RS
8
VIN
2
SW
GND
3
4
GND
9
CC
RT
7
EN
6
COMP
5
FB
CP
R1
R2
L
VOUT
RC
VOUT
GND
SW should be connected to inductor by
wide and short trace. Keep sensitive
components away from this trace.
Figure 9. PCB Layout Guide
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8280-02 March 2012
RT8280
Table 4. 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
C3225X5R1C226M
22
1210
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8280-02 March 2012
is a registered trademark of Richtek Technology Corporation.
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15
RT8280
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
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.
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DS8280-02 March 2012