RT7277

®
RT7277
3A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter
General Description
Features
The RT7277 is a synchronous step-down DC/DC converter
with Advanced Constant On-Time (ACOTTM) mode control.
z
It achieves high power density to deliver up to 3A output
current from a 4.5V to 18V input supply. The proprietary
ACOTTM mode offers an optimal transient response over a
wide range of loads and all kinds of ceramic capacitors,
which allows the device to adopt very low ESR output
capacitors for ensuring performance stabilization. In
addition, RT7277 keeps an excellent constant switching
frequency under line and load variation and the integrated
synchronous power switches with the ACOTTM mode
operation provides high efficiency in whole output current
load range. Cycle-by-cycle current limit provides an
accurate protection by a valley detection of low side
MOSFET and external soft-start setting eliminates input
current surge during startup. Protection functions also
include output under voltage protection, output over voltage
protection, and thermal shutdown.
z
z
z
z
z
z
z
z
z
z
z
z
z
4.5V to 18V Input Voltage Range
3A Output Current
60mΩ
Ω Internal Low Site N-MOSFET
Advanced Constant On-Time Control
Support All Ceramic Capacitors
Up to 95% Efficiency
700kHz Switching Frequency
Adjustable Output Voltage from 0.765V to 8V
Adjustable Soft-Start
Cycle-by-Cycle Current Limit
Input Under Voltage Lockout
Thermal Shutdown
RoHS Compliant and Halogen Free
Applications
z
z
z
Marking Information
z
z
RT7277GSP : Product Number
RT7277
GSPYMDNN
ACOTTM Mode Enables Fast Transient Response
YMDNN : Date Code
Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
Simplified Application Circuit
RT7277
VIN
SW
VIN
C1
C2
C6
BOOT
Chip Enable
EN
FB
SS
PVCC
C5
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7277-01 March 2013
L1
GND
C4
VOUT
C7
C3
R1
VPVCC
R2
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT7277
Ordering Information
Pin Configurations
(TOP VIEW)
RT7277
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
8
FB
2
PVCC
SS
3
4
GND
9
VIN
7
BOOT
6
SW
5
GND
SOP-8 (Exposed Pad)
Richtek products are :
`
EN
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
Suitable for use in SnPb or Pb-free soldering processes.
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
EN
Enable Input. A logic-high enables the converter; a logic-low forces the IC into
shutdown mode reducing the supply current to less than 10μA.
2
FB
Feedback Input. It is used to regulate the output of the converter to a set value via
an external resistive voltage divider. The feedback threshold voltage is 0.765V
typically.
3
PVCC
Internal Regulator Output. Connect a 1μF capacitor to GND to stabilize output
voltage.
4
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 3.9nF capacitor sets the soft-start period of
VOUT to 2.6ms.
GND
Ground. The Exposed pad should be soldered to a large PCB and connected to
GND for maximum thermal dissipation.
6
SW
Switch Node. Connect this pin to an external L-C filter.
7
BOOT
Bootstrap Supply for High Side Gate Driver. Connect a 0.1μF or greater ceramic
capacitor from BOOT to SW pins.
8
VIN
Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a
suitably large ( ≥10μF x 2) ceramic capacitor.
5, 9
(Exposed Pad)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
RT7277
Function Block Diagram
BOOT
PVCC
Internal
Regulator
VIN
PVCC
VIBIAS
Over Current PVCC
Protection
VREF
UGATE
Switch
Controller
PVCC
SW
Driver
Ripple
Gen.
LGATE
GND
SW
2µA
+
- -
SS
FB
FB
Comparator
On-Time
EN
EN
Operation
In normal operation, the high side N-MOSFET is turned
on when the FB Comparator sets the Switch Controller,
and it is turned off when On-Time Controller resets the
Switch Controller. While the high side N-MOSFET is turned
off, the low side N-MOSFET is turned on and waits for the
FB Comparator to set the beginning of next cycle.
Enable
The FB Comparator sets the Switch Controller by
comparing the feedback signal (FB) from output voltage
with the internal 0.765V reference. When load transient
induces VOUT drop, the FB voltage will be less than its
threshold voltage. This means that the high side N-MOSFET
will turn on again immediately after minimum off-time
expired. The switching frequency will vary during the
transient period thus can provide a very fast transient
response. After the load transient finished, the RT7277
will be back to steady state with a constant switching
frequency.
Provide internal power for logic control and switch gate
drivers.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7277-01 March 2013
Activate internal regulator once EN input level is higher
than the target level. Force IC to enter shutdown mode
when the EN input level is lower than 0.4V
Internal Regulator
On-Time Controller
Control on-time according to VIN and SW to obtain
constant switching frequency.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT7277
Absolute Maximum Ratings
z
z
z
z
z
z
z
z
z
z
(Note 1)
Supply Voltage, VIN ----------------------------------------------------------------------------------------------Switch Voltage, SW ----------------------------------------------------------------------------------------------<10ns ----------------------------------------------------------------------------------------------------------------BOOT to SW, PVCC ---------------------------------------------------------------------------------------------Other Pins Voltage ------------------------------------------------------------------------------------------------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 -------------------------------------------------------------------------------------Junction Temperature Range ------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Storage Temperature Range ------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------
Recommended Operating Conditions
z
z
z
−0.3V to 21V
−0.8V to (VIN + 0.3V)
−5V to 25V
−0.3V to 6V
−0.3V to 21V
2.041W
49°C/W
15°C/W
150°C
260°C
−65°C to 150°C
2kV
(Note 4)
Supply 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
Supply Current
Shutdown Current
ISHDN
VEN = 0V
--
1.5
10
μA
Quiescent Current
IQ
VEN = 3V, VFB = 1V
--
0.7
--
mA
Logic-High
2
--
18
Logic-Low
--
--
0.4
Logic Threshold
EN Voltage
V
VFB Voltage and Discharge Resistance
Feedback Threshold Voltage
VFB
4.5V ≤ VIN ≤ 18V
0.757
0.765
0.773
V
Feedback Input Current
IFB
VFB = 0.8V
−0.1
0
0.1
μA
VPVCC
6V ≤ VIN ≤ 18V, 0 < IPVCC < 5mA
4.7
5.1
5.5
V
VPVCC Output
VPVCC Output Voltage
Line Regulation
6V ≤ VIN ≤ 18V, IPVCC = 5mA
--
--
20
mV
Load Regulation
0 < IPVCC < 5mA
--
--
100
mV
VIN = 6V, VPVCC = 4V
--
110
--
mA
Output Current
IPVCC
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
4
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
RT7277
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
R DS(ON)
Switch On
Resistance
High Side
RDS(ON)_H
--
90
--
Low Side
RDS(ON)_L
--
60
--
ILIM
3.5
4.1
5.7
A
TSD
--
150
--
°C
--
20
--
°C
--
145
--
ns
mΩ
Current Limit
Current Limit
Thermal Shutdown
Thermal Shutdown Threshold
Thermal Shutdown Hysteresis ΔTSD
On-Time Timer Control
On-Time
tON
VIN = 12V, VOUT = 1.05V
Minimum On-Time
tON(MIN)
--
60
--
ns
Minimum Off-Time
tOFF(MIN)
--
230
--
ns
Soft-Start
SS Charge Current
VSS = 0V
1.4
2
2.6
μA
SS Discharge Current
VSS = 0.5V
0.05
0.1
--
mA
VIN Rising to Wake up VPVCC
3.55
3.85
4.15
--
0.3
--
UVLO
UVLO Threshold
Hysteresis
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. The PCB copper area of 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.
DS7277-01 March 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT7277
Typical Application Circuit
VIN
C1
10µF x 2
C2
0.1µF
RT7277
6
8
SW
VIN
1
Chip Enable
5, 9 (Exposed Pad)
C5
3.9nF
EN
GND
4 SS
BOOT
FB
PVCC
7
L1
1.4µH
C6
0.1µF
C3
R1
8.25k
C7
22µF x 2
VOUT
1.05V/3A
2
3
VPVCC
R2
22.1k
C4
1µF
Table 1. Suggested Component Values
VOUT (V)
R1 (kΩ)
R2 (kΩ)
C3 (pF)
L1 (μH)
C7 (μF)
1
6.81
22.1
--
1.4
22 to 68
1.05
8.25
22.1
--
1.4
22 to 68
1.2
12.7
22.1
--
1.4
22 to 68
1.8
30.1
22.1
5 to 22
2
22 to 68
2.5
49.9
22.1
5 to 22
2
22 to 68
3.3
73.2
22.1
5 to 22
2
22 to 68
5
124
22.1
5 to 22
3.3
22 to 68
7
180
22.1
5 to 22
3.3
22 to 68
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
6
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
RT7277
Typical Operating Characteristics
Output Voltage vs. Input Voltage
Efficiency vs. Output Current
100
1.065
90
1.060
Output Voltage (V)
Efficiency (%)
80
70
60
50
VIN = 5V
VIN = 12V
VIN = 17V
40
30
20
1.055
1.050
1.045
1.040
1.035
1.030
10
VOUT = 1.05V
0
0.001
0.01
0.1
1
VOUT = 1.05V
1.025
10
4
6
8
Output Current (A)
1.065
1.060
1.060
1.055
1.055
Output Voltage (V)
Output Voltage (V)
14
16
18
Output Voltage vs. Output Current
Output Voltage vs. Temperature
1.050
1.045
1.040
1.035
1.050
VIN = 17V
VIN = 12V
VIN = 5V
1.045
1.040
1.035
1.030
VIN = 12V, VOUT = 1.05V, IOUT = 0A
1.025
VOUT = 1.05V
1.025
-50
-25
0
25
50
75
100
125
0
0.5
1
Temperature (°C)
1.5
2
2.5
3
Output Current (A)
Frequency vs. Input Voltage
Reference Voltage vs. Temperature
750
0.785
740
0.780
Reference Voltage (V)
730
Frequency (kHz)1
12
Input Voltage (V)
1.065
1.030
10
720
710
700
690
680
670
0.775
0.770
0.765
0.760
0.755
0.750
660
650
0.745
4
6
8
10
12
14
16
Input Voltage (V)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7277-01 March 2013
18
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT7277
Current Limit vs. Input Voltage
Current Limit vs. Temperature
6.0
6.0
5.5
Current Limit (A)
Current Limit (A)
5.6
5.2
4.8
4.4
5.0
4.5
4.0
3.5
VIN = 12V, VOUT = 1.05V
VIN = 12V, VOUT = 1.05V
3.0
4.0
-50
-25
0
25
50
75
100
4
125
8
10
12
Temperature (°C)
Input Voltage (V)
Load Transient Response
Switching
14
16
18
VSW
(10V/Div)
IOUT
(2A/Div)
VOUT
(10mV/Div)
VOUT
(20mV/Div)
VIN = 12V, VOUT = 1.05V, IOUT = 0A to 3A
IL
(2A/Div)
VIN = 12V, VOUT = 1.05V, IOUT = 3A
Time (100μs/Div)
Time (1μs/Div)
Power On from VIN
Power Off from VIN
VIN
(5V/Div)
VOUT
(1V/Div)
VIN
(5V/Div)
VOUT
(1V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 1.05V, IOUT = 3A
Time (5ms/Div)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
8
6
VIN = 12V, VOUT = 1.05V, IOUT = 3A
Time (10ms/Div)
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
RT7277
Power On from EN
Power Off from EN
VEN
(2V/Div)
VEN
(2V/Div)
VOUT
(1V/Div)
VOUT
(1V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 1.05V, IOUT = 3A
Time (1ms/Div)
VIN = 12V, VOUT = 1.05V, IOUT = 3A
Time (10μs/Div)
EN Current vs. EN Voltage
10
9
EN Current (μA)
8
7
6
5
4
3
2
1
VIN = 17V
0
0
2
4
6
8
10
12
14
16
18
EN Voltage (V)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7277-01 March 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
9
RT7277
Application Information
The RT7277 is a synchronous high voltage Buck converter
that can support the input voltage range from 4.5V to 18V
and the output current up to 3A. It adopts ACOTTM mode
control to provide a very fast transient response with few
external compensation components.
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 1).
EN
VIN
PWM Operation
It is suitable for low external component count
configuration with appropriate amount of Equivalent Series
Resistance (ESR) capacitors at the output. The output
ripple valley voltage is monitored at a feedback point
voltage. The synchronous high side MOSFET is turned
on at the beginning of each cycle. After the internal ontime timer expires, the MOSFET is turned off. The pulse
width of this on-time is determined by the converter's input
and output voltages to keep the frequency fairly constant
over the entire input voltage range.
Advanced Constant On-Time Control
The RT7277 has a unique circuit which sets the on-time
by monitoring the input voltage and SW signal. The circuit
ensures the switching frequency operating at 700kHz over
input voltage range and loading range.
Soft-Start
The RT7277 contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timing
can be programmed by the external capacitor between
SS pin and GND. The chip provides a 2μA charge current
for the external capacitor. If a 3.9nF capacitor is used,
the soft-start will be 2.6ms (typ.). The available capacitance
range is from 2.7nF to 220nF.
t SS
C5 (nF) × 1.365
(ms) =
ISS (μ A)
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT7277 quiescent current drops to lower than
10μA. Driving the EN pin high (>2V, <18V) will turn on the
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
REN
EN
RT7277
CEN
GND
Figure 1. External Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2V
is available, as shown in Figure 2. 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.
VIN
REN
100k
EN
Q1
EN
RT7277
GND
Figure 2. Digital Enable Control Circuit
To prevent enabling circuit when VIN is smaller than the
VOUT target value, a resistive voltage divider can be placed
between the input voltage and ground and connected to
the EN pin to adjust IC lockout threshold, as shown in
Figure 3. For example, if an 8V output voltage is regulated
from a 12V input voltage, the resistor REN2 can be selected
to set input lockout threshold larger than 8V.
VIN
REN1
REN2
EN
RT7277
GND
Figure 3. Resistor Divider for Lockout Threshold Setting
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
RT7277
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 4.
VOUT
R1
FB
RT7277
R2
GND
Figure 4. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation. It is recommended to
use 1% tolerance or better divider resistors.
VOUT = 0.765 × (1+
R1
)
R2
inductor to achieve this goal. For the ripple current
selection, the value of ΔIL = 0.2(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)
⎣
⎦ ⎣
⎦
Input and Output Capacitors Selection
The input capacitance, C IN, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
A low ESR input capacitor with larger ripple current rating
should be used for the maximum RMS current. The RMS
current is given by :
Under Voltage Lockout Protection
V
IRMS = IOUT(MAX) OUT
VIN
The RT7277 has Under Voltage Lockout Protection (UVLO)
that monitors the voltage of PVCC pin. When the VPVCC
voltage is lower than UVLO threshold voltage, the RT7277
will be turned off in this state. This is non-latch protection.
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.
Over Temperature Protection
The RT7277 equips an Over Temperature Protection (OTP)
circuitry to prevent 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 main converter will resume operation. To keep operating
at maximum, the junction temperature should be prevented
from rising above 150°C.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and an output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
V
V
ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥
f
×
L
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
highest efficiency operation. However, it requires a large
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7277-01 March 2013
VIN
−1
VOUT
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 and 0.1μF low ESR ceramic
capacitors are recommended.
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.
The output ripple, ΔVOUT, is determined by :
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 need to meet the ESR
and RMS current handling requirements.
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
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
RT7277
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. A sudden inrush of current through the long
wires can potentially cause a voltage spike at VIN large
enough to damage the part.
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 the 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 1N4148
or BAT54. The external 5V can be a 5V fixed input from
system or a 5V output of the RT7277. Note that the external
boot voltage must be lower than 5.5V
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
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) package, the thermal resistance,
θJA, is 49°C/W on a standard JEDEC 51-7 four-layer
PD(MAX) = (125°C − 25°C) / (49°C/W) = 2.041W for
SOP-8 (Exposed Pad) package
0.1µF
SW
Figure 5. External Bootstrap Diode
PVCC Capacitor Selection
Decouple with a 1μF ceramic capacitor. X7R or X5R grade
dielectric ceramic capacitors are recommended for their
stable temperature characteristics.
Over Current Protection
When the output shorts to ground, the inductor current
decays very slowly during a single switching cycle. An
over current detector is used to monitor inductor current
to prevent current runaway. The over current detector
monitors the voltage between SW and GND during the
low side MOS turn-on state. This is cycle-by-cycle
protection. The over current detector also supports
temperature compensated.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 6 allow the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
2.5
Maximum Power Dissipation (W)1
BOOT
www.richtek.com
12
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 :
thermal test board. The maximum power dissipation at
TA = 25°C can be calculated by the following formulas :
5V
RT7277
Thermal Considerations
Four-Layer PCB
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum Power Dissipation
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
RT7277
Layout Consideration
Follow the PCB layout guidelines for optimal performance
of the RT7277
`
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).
The resistor divider must be connected
as close to the device as possible.
R2
GND
C4
C5
SW node is with high frequency voltage swing and
should be kept at small area. Keep sensitive
components away from the SW node to prevent stray
capacitive noise pickup.
`
Connect feedback network behind the output capacitors.
Keep the loop area small. Place the feedback
components near the RT7277 feedback pin.
`
The GND and Exposed Pad should be connected to a
strong ground plane for heat sinking and noise protection.
Input capacitor must be placed
C1 as close to the IC as possible.
VOUT
R1
`
C2
EN
8
FB
2
PVCC
SS
3
GND
4
9
SW should be connected to inductor by
wide and short trace. Keep sensitive
components away from this trace.
VIN
7
BOOT
6
SW
5
GND
C6
C7
L1
VOUT
Figure 7. PCB Layout Guide
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7277-01 March 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT7277
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
DS7277-01 March 2013