MP28253EL [MPS]
21V, 3A, 500kHz Synchronous Step-down Converter;
| 型号： | MP28253EL |
| 厂家： | 
MONOLITHIC POWER SYSTEMS |
| 描述： | 21V, 3A, 500kHz Synchronous Step-down Converter |
| 文件： | 总15页 (文件大小：373K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
MP28253
21V, 3A, 500kHz
Synchronous Step-down Converter
The Future of Analog IC Technology
DESCRIPTION
FEATURES
The MP28253 is a high frequency synchronous
rectified step-down switch mode converter with
built in internal power MOSFETs. It offers a
very compact solution to achieve 3A continuous
output current over a wide input supply range
with excellent load and line regulation. The
MP28253 operates at high efficiency over a
wide output current load range.
•
•
•
•
Wide 4.5V to 21V Operating Input Range
3A Output Current
Low RDS(ON) Internal Power MOSFETs
Proprietary Switching Loss Reduction
Technique
Fixed 500kHz Switching Frequency
External Soft-Start
Sync from 300kHz to 2MHz External Clock
Internal Compensation
Integrated Bootstrap Diode
Over-Current Protection and Hiccup
Thermal Shutdown
Output Adjustable from 0.8V
Available in 14-pin QFN3x4 Package
•
•
•
•
•
•
•
•
•
Current mode operation provides fast transient
response and eases loop stabilization.
Full protection features include OCP and thermal
shut down.
The MP28253 requires a minimum number of
readily available standard external components
and is available in a space saving 3mm x 4mm
14-pin QFN package.
APPLICATIONS
•
•
•
•
•
•
Notebook Systems and I/O Power
Networking Systems
Digital Set Top Boxes
Personal Video Recorders
Flat Panel Television and Monitors
Distributed Power Systems
All MPS parts are lead-free and adhere to the RoHS directive. For MPS green
status, please visit MPS website under Products, Quality Assurance page.
“MPS” and “The Future of Analog IC Technology” are registered trademarks of
Monolithic Power Systems, Inc.
The information in this datasheet about the product and its associated
technologies are proprietary and intellectual property of Monolithic Power
Systems and are protected by copyright and pending patent applications
TYPICAL APPLICATION （FOR NOTEBOOK）
1
6
VIN
IN
BST
4.5V-21V
C1
MP28253
9
2,3,4,5
PG
PG
VOUT 1.2V/3A
SW
R3
100k
R1
11
VCC
4.99k
Rt
24k
8
FB
SS
C5
47nF
7
R2
10k
10
ON/OFF
EN/SYNC
GND AGND
12,13 14
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
1
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
ORDERING INFORMATION
Part Number*
Package
Top Marking
Free Air Temperature (TA)
MP28253EL
3x4 QFN14
28253
-20°C to +85°C
For Tape & Reel, add suffix –Z (e.g. MP28253EL–Z).
For RoHS compliant packaging, add suffix –LF (e.g. MP28253EL–LF–Z)
PACKAGE REFERENCE
TOP VIEW
PIN 1 ID
IN
SW
1
2
3
4
5
6
7
14 AGND
13 GND
12 GND
11 VCC
SW
SW
SW
SS
PG
FB
10
9
BST
EN/SYNC
8
EXPOSED PAD
ON BACKSIDE
Thermal Resistance (5)
3x4 QFN14 .............................48...... 11... °C/W
θJA
θJC
ABSOLUTE MAXIMUM RATINGS (1)
Supply Voltage VIN ....................................... 22V
V
SW..........................-0.3V (-5V for<10ns) to 23V
Notes:
1) Exceeding these ratings may damage the device.
2) Please refer to Page 9, Enable Control Section for absolute
maximum rating of EN pin.
VBS .......................................................VSW + 6V
(2)
All Other Pins............................. -0.3V to +6V
3) The maximum allowable power dissipation is a function of the
maximum junction temperature TJ(MAX), the junction-to-
ambient thermal resistance θJA, and the ambient temperature
TA. The maximum allowable continuous power dissipation at
any ambient temperature is calculated by PD(MAX)=(TJ(MAX)-
TA)/θJA. Exceeding the maximum allowable power dissipation
will cause excessive die temperature, and the regulator will go
into thermal shutdown. Internal thermal shutdown circuitry
protects the device from permanent damage.
Operating Temperature.............. -20°C to +85°C
Continuous Power Dissipation (TA = +25°C)
(3)
……………………………………………....2.6W
Junction Temperature...............................150°C
Lead Temperature ....................................260°C
Storage Temperature............... -65°C to +150°C
Recommended Operating Conditions (4)
Supply Voltage VIN ...........................4.5V to 21V
Maximum Junction Temp. (TJ) ...............+125°C
4) The device is not guaranteed to function outside of its
operating conditions.
5) Measured on JESD51-7, 4-layer PCB.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
2
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
ELECTRICAL CHARACTERISTICS
VIN = 12V, TA = +25°C, unless otherwise noted.
Parameters
Symbol
IIN
Condition
Min
Typ
0
Max
Units
μA
Supply Current (Shutdown)
Supply Current (Quiescent)
HS Switch On Resistance
LS Switch On Resistance
VEN = 0V
Iq
VEN = 2V, VFB = 1V
0.7
150
50
mA
HSRDS-ON
LSRDS-ON
mΩ
mΩ
V
12V
EN = 0V, VSW = 0V or
Switch Leakage
SWLKG
0
10
μA
Current Limit (6)
ILIMIT
FSW
4
5.6
500
0.25
90
A
kHz
fSW
%
Oscillator Frequency
Fold-back Frequency
Maximum Duty Cycle
Sync Frequency Range
Feedback Voltage
VFB = 0.75V
VFB = 0V
425
575
FFB
DMAX
FSYNC
VFB
VFB = 700mV
85
0.3
789
2
MHz
mV
nA
V
805
10
1.3
0.4
2
821
50
Feedback Current
IFB
VFB = 800mV
VEN = 2V
EN Rising Threshold
EN Threshold Hysteresis
VEN_RISING
VEN_HYS
1
1.6
V
μA
EN Input Current
IEN
VEN = 0V
0
EN Turn Off Delay
ENTd-Off
PGVth-Hi
PGVth-Lo
PGTd
5
μs
VFB
VFB
μs
Power Good Rising Threshold
Power Good Falling Threshold
Power Good Delay
0.9
0.7
20
Power Good Sink Current
Capability
VPG
Sink 4mA
0.4
10
V
Power Good Leakage Current
Soft-start current
IPG_LEAK
ISS
VPG = 3.3V
nA
10.5
4.0
μA
VIN Under Voltage Lockout
Threshold Rising
INUVVth
3.8
4.2
V
VIN Under Voltage Lockout
Threshold Hysteresis
INUVHYS
VCC
880
mV
VCC Regulator
5
5
V
%
VCC Load Regulation
Soft-Start Period
Thermal Shutdown
ICC=5mA
CSS=47nF
2
4
6.5
ms
°C
TSD
150
Note:
6) Guaranteed by design.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
3
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
PIN FUNCTIONS
Pin #
Name
Description
Supply Voltage. The MP28253 operates from a +4.5V to +21V input rail. C1 is
needed to decouple the input rail. Use wide PCB traces and multiple vias to make
the connection.
1
IN
2,3,4,5
6
SW
Switch Output. Use wide PCB traces and multiple vias to make the connection.
Bootstrap. A capacitor connected between SW and BS pins is required to form a
floating supply across the high-side switch driver.
BST
EN=1 to enable the chip. External clock can be applied to EN pin for changing
switching frequency. For automatic start-up, connect EN pin to VIN by proper EN
resistor divider. It shows in Page 9, Enable Control Section.
7
8
EN/SYNC
FB
Feedback. An external resistor divider from the output to GND, tapped to the FB
pin, sets the output voltage. To prevent current limit run away during a short circuit
fault condition the frequency fold-back comparator lowers the oscillator frequency
when the FB voltage is below 500mV.
Power Good Output, the output of this pin is open drain. Power good threshold is
90% low to high and 70% high to low of regulation value.
9
10,
PG
SS
Soft-Start control input. SS controls the soft-start period. Connect a capacitor from
SS to Gnd to set the soft-start period.
Bias Supply. Decouple with 0.1μF~0.22μF cap. And the capacitance should be no
more than 0.22μF.
11
VCC
GND
AGND
System Ground. This pin is the reference ground of the regulated output voltage.
For this reason care must be taken in PCB layout.
12,13
14
Signal Ground. AGND is not internally connected to System Ground, make sure
AGND connected to system Ground in PCB layout.
Exposed No Internal Connection. It is recommended to connect exposed pad to GND plane
Pad
for optimal thermal performance.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
4
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 12V, VOUT = 1.2V, L=1.8μH, TA = +25ºC, unless otherwise noted.
Enabled Supply Current vs.
Input Voltage
Disabled Supply Current vs.
Input Voltage
Vcc Regulator Line Regulation
V
=1V
FB
V
=0V
EN
0.2
0.15
0.1
6
1000
950
900
850
800
750
700
650
600
550
500
5.5
5
0.05
0
4.5
4
-0.05
-0.1
-0.15
-0.2
3.5
0
5
10
15
20
25
0
5
10
15
20
25
0
5
10
15
20
25
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Current Limit vs.
Duty Cycle
Operating Range
Load Regulation
0.3
0.2
0.1
0
100
10
8
6
4
2
0
DmaxLimit
V
=4.5V
IN
Minimum on time Limit
V
=21V
IN
-0.1
-0.2
-0.3
V
=12V
IN
1
0.1
0
0.5
1
1.5
2
2.5
3
0
20
40
60
80
100
0
5
10
15
20
25
DUTY CYCLE (%)
INPUT VOLTAGE (V)
OUTPUT CURRENT (A)
Case Temperature Rise vs.
Output Current
Line Regulation
0.3
0.2
0.1
0
15
10
5
I
=0A
OUT
I
=1.5A
OUT
-0.1
-0.2
-0.3
I
=3A
OUT
0
0
5
10
15
20
25
0
1
2
3
4
INPUT CURRENT (V)
OUTPUT CURRENT (A)
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
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© 2013 MPS. All Rights Reserved.
5
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 12V, VOUT = 1.2V, L=1.8μH, TA = +25ºC, unless otherwise noted.
Efficiency
VOUT=1.2V
Efficiency
VOUT=1.8V
Efficiency
VOUT=2.5V
100
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
VIN=4.5V
VIN=4.5V
VIN=4.5V
VIN=12V
VIN=12V
VIN=21V
VIN=12V
VIN=21V
VIN=21V
0
0.5
1
1.5
2
2.5
3
0
0.5
1
1.5
2
2.5
3
0
0.5
1
1.5
2
2.5
3
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
Efficiency
VOUT=3.3V
100
90
80
70
60
50
40
30
20
10
0
VIN=5V
VIN=12V
VIN=21V
0
0.5
1
1.5
2
2.5
3
OUTPUT CURRENT (A)
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
6
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 12V, VOUT = 1.2V, L=1.8μH, TA = +25ºC, unless otherwise noted.
Short Entry
Short Recovery
Power Up without Load
V
V
V
OUT
1V/div
OUT
1V/div
OUT
1V/div
V
SW
5V/div
V
V
SW
SW
5V/div
5V/div
V
IN
10V/div
I
I
INDUCTOR
5A/div
INDUCTOR
5A/div
I
INDUCTOR
5A/div
1ms/div
2ms/div
2ms/div
Power Up with 3A Load
Enable Startup
without Load
Enable Startup
with 3A Load
V
V
V
OUT
OUT
OUT
1V/div
1V/div
1V/div
V
V
SW
V
SW
SW
5V/div
5V/div
5V/div
V
V
EN
V
EN
IN
5V/div
5V/div
10V/div
I
I
I
INDUCTOR
5A/div
INDUCTOR
5A/div
INDUCTOR
5A/div
2ms/div
2ms/div
2ms/div
Output Ripple Voltage
IOUT=3A
Input Ripple Voltage
IOUT=3A
Load Transient Response
IOUT=1.5A-3A
V
OUT/AC
10mV/div
V
OUT/AC
V
IN/AC
50mV/div
100mV/div
V
SW
5V/div
I
V
INDUCTOR
5A/div
SW
I
OUT
2A/div
5V/div
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
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© 2013 MPS. All Rights Reserved.
7
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
BLOCK DIAGRAM
Figure 1—Function Block Diagram
MP28253 Rev. 1.02
12/25/2013
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© 2013 MPS. All Rights Reserved.
8
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
OPERATION
The MP28253 is a high frequency synchronous
rectified step-down switch mode converter with
built in internal power MOSFETs. It offers a very
compact solution to achieve 3A continuous
output current over a wide input supply range
with excellent load and line regulation.
1) Enabled by external logic H/L signal
The chip starts up once the enable signal goes
higher than EN/SYNC input high voltage (2V),
and is shut down when the signal is lower than
EN/SYNC input low voltage (0.4V). To disable
the chip, EN must be pulled low for at least 5µs.
The input is compatible with both CMOS and TTL.
The MP28253 operates in a fixed frequency,
peak current control mode to regulate the output
voltage. A PWM cycle is initiated by the internal
clock. The integrated high-side power MOSFET
is turned on and remains on until its current
reaches the value set by the COMP voltage.
When the power switch is off, it remains off until
the next clock cycle starts. If, in 90% of one PWM
period, the current in the power MOSFET does
not reach the COMP set current value, the power
MOSFET will be forced to turn off
2) Enabled by Vin through voltage divider.
Connect EN with VIN through a resistive voltage
divider for automatic startup as the figure 2
shows.
Power Good Indicator
When the FB is below 0.7VFB, the PG pin will be
internally pulled low. When the FB is above
0.9VFB, the PG becomes an open-drain output.
Internal Regulator
Most of the internal circuitries are powered from
the 5V internal regulator. This regulator takes the
VIN input and operates in the full VIN range.
When VIN is greater than 5.0V, the output of the
regulator is in full regulation. When VIN is lower
than 5.0V, the output decreases, 0.1uF ceramic
capacitor for decoupling purpose is required.
Figure 2—Enable Divider Circuit
Choose the value of the pull-up resistor REN1 and
pull-down resistor REN2 to reset the automatic
start-up voltage:
(REN1 + REN2 ||1MΩ)
V
= VEN_RISING ⋅
IN_START
REN2 ||1MΩ
Error Amplifier
(REN1 + REN2 ||1MΩ)
REN2 ||1MΩ
The error amplifier compares the FB pin voltage
with the internal FB reference (VFB) and outputs a
current proportional to the difference between the
two. This output current is then used to charge or
discharge the internal compensation network to
form the COMP voltage, which is used to control
the power MOSFET current. The optimized
internal compensation network minimizes the
external component counts and simplifies the
control loop design.
V
=
VEN-FALLING ⋅
IN_STOP
As shown in Figure 2, the EN pin is also clamped
internally using a 6.7V series-Zener-diode. The
EN input pin can be connected through a pullup
resistor to any voltage connected to the VIN pin
such that the pullup resistor limits the EN input
current to less than 100µA.
For example, with 12V connected to Vin, RPull-
up ≥ (12V – 6.7V) ÷ 100µA = 53kꢀ.
Enable/Sync Control
EN/Sync is a digital control pin that turns the
regulator on and off. Drive EN high to turn on the
regulator, drive it low to turn it off. There is an
internal 1MEG resistor from EN/Sync to GND
thus EN/Sync can be floated to shut down the
chip.
If the EN pin is directly connected to a voltage
source without any pullup resistor, then the
amplitude of the voltage source should be limited
below 6V to prevent from causing damage to the
Zener diode.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
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© 2013 MPS. All Rights Reserved.
9
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
external decoupled cap. The soft-start time can
1ms Turn On Delay
5us Turn Off Delay
be caculated as below:
VIN_STA
RT
VIN_STOP
Vref(V)×C5(nF)
tss(ms) =
Vin
VEN_Rising
10.5μA
To reduce the susceptibility to noise, do not leave
SS pin open. Use a capacitor with small value if
soft –start function not needed.
VEN_Falling
EN/Sync
VCC_Rising
Vcc
Over-Current-Protection and Hiccup
The MP28253 has cycle-by-cycle over current
limit when the inductor current peak value
exceeds the set current limit threshold.
Meanwhile, output voltage starts to drop until FB
is below the Under-Voltage (UV) threshold,
typically 30% below the reference. Once a UV is
triggered, the MP28253 enters hiccup mode to
periodically restart the part. This protection mode
is especially useful when the output is dead-short
to ground. The average short circuit current is
greatly reduced to alleviate the thermal issue and
to protect the regulator. The MP28253 exits the
hiccup mode once the over current condition is
removed.
Vout
Figure 3—Startup Sequence Using EN Divider
3) Synchronized by External Sync Clock Signal
The chip can be synchronized to external clock
range from 300kHz up to 2MHz through this pin
2ms right after output voltage is set, with the
internal clock rising edge synchronized to the
external clock rising edge.
5us
1ms
2ms
Vin
EN/Sync
VCC_Rising
Vcc
Thermal Shutdown
Vout_set
Thermal shutdown is implemented to prevent the
chip from operating at exceedingly high
temperatures. When the silicon die temperature
is higher than 150°C, it shuts down the whole
chip. When the temperature is lower than its
lower threshold, typically 140°C, the chip is
enabled again.
0.625*Vout_set
Vout
CLK
Foldback
External CLK
500kHz
Figure 4—Startup Sequence Using External
Sync Clock Signal
Floating Driver and Bootstrap Charging
The floating power MOSFET driver is powered by
an external bootstrap capacitor. This floating
driver has its own UVLO protection. This UVLO’s
rising threshold is 2.2V with a hysteresis of
150mV. The bootstrap capacitor voltage is
regulated internally by VIN through D1, M3, C4,
L1 and C2 (Figure 5). If (VIN-VSW) is more than
5V, U2 will regulate M3 to maintain a 5V BST
voltage across C4.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented to
protect the chip from operating at insufficient
supply voltage. The MP28253 UVLO comparator
monitors the output voltage of the internal
regulator, VCC. The UVLO rising threshold is
about 4.0V while its falling threshold is a
consistent 3.2V.
External Soft-Start
The soft-start is implemented to prevent the
converter output voltage from overshooting
during startup. When the chip starts, the internal
circuitry generates a soft-start voltage (SS)
ramping up from 0V to 1.2V. When it is lower
than the internal FB reference (REF), SS
overrides REF so the error amplifier uses SS as
the reference. When SS is higher than REF, REF
regains control. The SS time can be set by
SW
Figure 5—Internal Bootstrap Charging Circuit
MP28253 Rev. 1.02
12/25/2013
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10
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
Startup and Shutdown
If both VIN and EN are higher than their
appropriate thresholds, the chip starts. The
reference block starts first, generating stable
reference voltage and currents, and then the
internal regulator is enabled. The regulator
provides stable supply for the remaining
circuitries.
Three events can shut down the chip: EN low,
VIN low and thermal shutdown. In the shutdown
procedure, the signaling path is first blocked to
avoid any fault triggering. The COMP voltage and
the internal supply rail are then pulled down. The
floating driver is not subject to this shutdown
command.
MP28253 Rev. 1.02
12/25/2013
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© 2013 MPS. All Rights Reserved.
11
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
APPLICATION INFORMATION
VOUT × (VIN − VOUT
VIN × ΔIL × fOSC
)
Setting the Output Voltage
L =
The external resistor divider is used to set the
output voltage (see Typical Application on page
1). The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor (see Typical Application
on page 1). Choose R1 to be around 40.2kꢀ for
optimal transient response. R2 is then given by:
Where ΔIL is the inductor ripple current.
Choose inductor ripple current to be
approximately 30% if the maximum load current,
3A. The maximum inductor peak current is:
ΔIL
IL(MAX) = ILOAD
+
R1
R2 =
VOUT
2
−1
Under light load conditions below 100mA, larger
inductance is recommended for improved
efficiency.
VFB
The T-type network is highly recommended when
Vo is low, as Figure 6 shows.
Selecting the Input Capacitor
R1
Rt
The input current to the step-down converter is
discontinuous, therefore a capacitor is required to
supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low ESR
capacitors for the best performance. Ceramic
capacitors with X5R or X7R dielectrics are highly
recommended because of their low ESR and
small temperature coefficients. For most
applications, a 22µF capacitor is sufficient.
1
FB
VOUT
R2
Figure 6— T-type Network
Table 1 lists the recommended T-type resistors
value for common output voltages.
Table 1—Resistor Selection for Common
Output Voltages
Since the input capacitor (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input capacitor
can be estimated by:
VOUT
(V)
R1
R2
Rt
L
COUT
(kΩ) (kΩ) (kΩ) (μH) (μF, Ceramic)
1.05 4.99 16.5 24.9 1-4.7
47
47
47
47
47
47
47
1.2
1.5
1.8
2.5
3.3
5
4.99 10.2 24.9 1-4.7
4.99 5.76 24.9 1-4.7
4.99 4.02 24.9 1-4.7
⎛
⎞
⎟
VOUT
VIN
VOUT
VIN
⎜
IC1 = ILOAD
×
× 1−
⎜
⎝
⎟
⎠
The worse case condition occurs at VIN = 2VOUT,
40.2 19.1
40.2 13
40.2 7.68
0
0
0
1-4.7
1-4.7
1-4.7
where:
ILOAD
IC1
=
2
Note:
For simplification, choose the input capacitor
whose RMS current rating greater than half of the
maximum load current.
The above feedback resistor table applies to a specific load
capacitor condition as shown in the table 1. Other capacitive loading
conditions will require different values.
Selecting the Inductor
The input capacitor can be electrolytic, tantalum
or ceramic. When electrolytic or tantalum
capacitor is used, a small, high quality ceramic
capacitor, i.e. 0.1μF, should be placed as close
to the IC as possible. When using ceramic
capacitors, make sure that they have enough
capacitance to provide sufficient charge to
prevent excessive voltage ripple at input. The
A 1µH to 10µH inductor with a DC current rating
of at least 25% percent higher than the maximum
load current is recommended for most
applications. For highest efficiency, the inductor
DC resistance should be less than 15mꢀ. For
most designs, the inductance value can be
derived from the following equation.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
12
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
3) Ensure all feedback connections are short
input voltage ripple caused by capacitance can
be estimated by:
and direct. Place the feedback resistors and
compensation components as close to the
chip as possible.
⎛
⎜
⎝
⎞
⎟
⎟
⎠
ILOAD
VOUT
VIN
VOUT
⎜
ΔV
=
×
× 1−
IN
fS × C1
V
IN
4) Route SW away from sensitive analog areas
such as FB.
5) Connect IN, SW, and especially GND
respectively to a large copper area to cool
the chip to improve thermal performance and
long-term reliability.
Selecting the Output Capacitor
The output capacitor (C2) is required to maintain
the DC output voltage. Ceramic, tantalum, or low
ESR electrolytic capacitors are recommended.
Low ESR capacitors are preferred to keep the
output voltage ripple low. The output voltage
ripple can be estimated by:
6) Adding RC snubber circuit from IN pin to SW
pin can reduce SW spikes.
⎛
⎜
⎝
⎞
⎟
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎟
⎠
C1
VOUT
VOUT
VIN
1
⎜
⎜
ΔVOUT
=
× 1−
× RESR
+
GND
VIN
fS × L
8 × fS × C2
AGND
IN
1
2
3
4
5
6
7
14
13
12
11
10
9
Where L is the inductor value and RESR is the
equivalent series resistance (ESR) value of the
output capacitor.
SW
SW
SW
SW
BST
GND
GND
VCC
Css
L1
SS
In the case of ceramic capacitors, the impedance
at the switching frequency is dominated by the
capacitance. The output voltage ripple is mainly
caused by the capacitance. For simplification, the
output voltage ripple can be estimated by:
PG
8
EN
FB
R2
C2
⎛
⎜
⎝
⎞
⎟
⎟
⎠
VOUT
8 × fS2 × L × C2
VOUT
⎜
ΔVOUT
=
× 1−
V
IN
Top Layer
In the case of tantalum or electrolytic capacitors,
the ESR dominates the impedance at the
switching frequency. For simplification, the output
ripple can be approximated to:
VOUT
VOUT
VIN
⎛
⎞
ΔVOUT
=
× ⎜1−
⎟ ×RESR
⎜
⎟
fS ×L
⎝
⎠
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP28253 can be optimized for a wide range of
capacitance and ESR values..
The recommended external BST diode is IN4148,
and the BST cap is 0.1~1µF.
PCB Layout
PCB layout is very important to achieve stable
operation. Please follow these guidelines and
take Figure 7 for references.
Bottom Layer
Figure 7—PCB Layout
1) Keep the connection of input ground and
GND pin as short and wide as possible.
2) Keep the connection of input capacitor and
IN pin as short and wide as possible.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
13
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
External Bootstrap Diode
An external bootstrap diode may enhance the
efficiency of the regulator, the applicable
conditions of external BST diode is:
VOUT
z Duty cycle is high: D=
>65%
VIN
In this case, an external BST diode is
recommended from the VCC pin to BST pin, as
shown in Figure 8
External BST Diode
IN4148
BST
VCC
CBST
MP28253
SW
L
COUT
Figure 8—Add Optional External Bootstrap
Diode to Enhance Efficiency
The recommended external BST diode is IN4148,
and the BST cap is 0.1~1µF.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
14
MP28253 – 3A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
PACKAGE INFORMATION
3mm x 4mm QFN14
1.60
1.80
2.90
3.10
0.30
0.50
PIN 1 ID
SEE DETAIL A
PIN 1 ID
MARKING
1
14
0.18
0.30
3.20
3.40
3.90
4.10
PIN 1 ID
INDEX AREA
0.50
BSC
8
7
TOP VIEW
BOTTOM VIEW
PIN 1 ID OPTION A
0.30x45º TYP.
PIN 1 ID OPTION B
R0.20 TYP.
0.80
1.00
0.20 REF
0.00
0.05
SIDE VIEW
DETAIL A
2.90
1.70
NOTE:
0.70
1) ALL DIMENSIONS ARE IN MILLIMETERS.
2) EXPOSED PADDLE SIZE DOES NOT INCLUDE MOLD FLASH.
3) LEAD COPLANARITY SHALL BE0.10 MILLIMETER MAX.
4) JEDEC REFERENCE IS MO-229, VARIATION VGED-3.
5) DRAWING IS NOT TO SCALE.
0.25
0.50
3.30
RECOMMENDED LAND PATTERN
NOTICE: The information in this document is subject to change without notice. Users should warrant and guarantee that third
party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not
assume any legal responsibility for any said applications.
MP28253 Rev. 1.02
12/25/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
15
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