MPQ28261
21V, 3A, 500kHz Synchronous
Step-Down Converter
DESCRIPTION
FEATURES
The MPQ28261 is a synchronous, rectified,
step-down, switch-mode converter with built-in
120mΩ high-side MOSFET and 20mΩ low-side
MOSFET. It offers a very compact solution to
achieve a continuous 3A output current over a
wide input supply range, with excellent load and
line regulation. Its synchronous mode operation
increases its efficiency over its output current
load range, and uses a fixed 500kHz switching
frequency.
Current mode operation provides a fast
transient response and improves loop
stabilization. Full protection features include
over-current protection and thermal shut down.
The MPQ28261 has an external SS pin to
program the soft-start time and power-good
signal output.
The MPQ28261 requires a minimal number of
readily-available standard external components
and is available in a space saving 3mmx4mm
14-pin QFN package.
Wide 4.5V-to-21V Operating Input Range
3A Maximum Continuous Output Current
Integrated 120mΩ High-Side and 20mΩ
Low-Side Power MOSFETs
1% Reference Voltage Accuracy
Up to 85% Efficiency (for Drops of 12V to
1.2V)
Fixed 500kHz Switching Frequency
External Soft-Start
PG Output
OCP and Thermal Shutdown
Output Adjustable from 0.603V to 18V
Available in a 3mm x 4mm 14-pin QFN
Package.
APPLICATIONS
DSL Modems
Cable Modems
Set-Top Boxes
Telecom
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.
TYPICAL APPLICATION
4.5V-21V
VIN
1
IN
BST
6
VCC
C1
22uF
R10
100K
MPQ28261
9
11
C4
0.1uF
ON/OFF
SW
PG
2,3,4,5
Rt
24K
VCC
FB
7 EN
SS
GND
12,13
AGND
14
C3
0.1uF L1
2.8uH
8
10
C7
47nF
VOUT
1.2V/3A
R1
10K
C2
47uF
R2
10K
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
1
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
ORDERING INFORMATION
Part Number*
MPQ28261DL
Package
QFN14 (3x4mm)
Top Marking
28261
Operation Junction Temperature (Tj)
-40C to +125C
* For Tape & Reel, add suffix –Z (e.g. MPQ28261DL–Z);
For RoHS, compliant packaging, add suffix –LF (e.g. MPQ28261DL–LF–Z).
PACKAGE REFERENCE
ABSOLUTE MAXIMUM RATINGS (1)
Thermal Resistance
VIN .............................................. -0.3V to +23V
VSW .............. -0.3V(-5V for 5V.
Startup and Shutdown
When VIN and EN exceed their respective
thresholds, the chip starts. The reference block
starts first, generating stable reference voltages
and currents, and then the internal regulator is
enabled. The regulator provides a stable power
supply for the rest of the device.
Three events can shut down the chip: EN low,
VIN low, and thermal shutdown. In shutdown,
the signaling path shuts down first to avoid
triggering any faults. Then the COMP voltage
and the internal supply rail are pulled down.
The floating driver is not subject to this
shutdown command.
Figure 4: Internal Bootstrap Charging Circuit
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
14
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
APPLICATION INFORMATION
COMPONENT SELECTION
Output Voltage Selection
The external resistor divider sets the output
voltage (see Typical Application circuits on
pages 1 and 17). The feedback resistor R1 also
sets the bandwidth of the feedback loop in
conjunction with the internal compensation
capacitor. Choose R1 with a value of ~10kΩ.
Use the following equation to then estimate R2:
R2
Inductor Selection
For most applications, chose an inductor value
between 1µH and 10µH with a DC current
rating that is at least 25% percent higher than
the maximum load current. Select an inductor
with a DC resistance less than 15mΩ for best
efficiency. Use the following equation to
estimate the inductance value for most designs:
L1
R1
VOUT
VOUT (VIN VOUT )
VIN IL fOSC
Where ΔIL is the inductor ripple current.
1
0.6V
Use a T-type network for low values VOUT, as
shown in Figure 5.
Choose the inductor current to equal ~30% of
the maximum load current. Estimate the
maximum inductor peak current as:
IL(MAX ) ILOAD
Figure 5: T-Type Network
Table 2 lists the recommended T-type resistors
value for common output voltages.
Table 2: Resistor Selection for Common
VOUT (V)
R1 (kΩ)
R2 (kΩ)
Rt (kΩ)
1.0
1.2
1.8
2.5
3.3
10(1%)
10(1%)
10(1%)
10(1%)
10(1%)
15(1%)
10(1%)
4.99(1%)
3.16(1%)
2.20(1%)
24 (1%)
24 (1%)
24 (1%)
24 (1%)
24 (1%)
5
10(1%)
1.36(1%)
24 (1%)
IL
2
A larger value inductor provides a higher
maximum load current, and reduces the output
voltage ripple. If the load is lower than the
maximum load current, then a lower-value
inductor will suffice and the chip then operates
with higher ripple current; this allows for the use
of a either a physically smaller inductor, or one
with a lower DCR that can result in higher
efficiency. If the inductance differs from the
conditions described above, then the maximum
load current will depend on the input voltage.
Choose an inductor value that allows for
maximum output current near the switch current
limit. Table 3 lists some of the recommended
inductors.
Table 3: Recommended Inductor
Part Number
Manufacturer
Inductance ( µH )
DCR ( mΩ )
Current Rating ( A )
HC8LP-1R2
Cooper
1.2
7.5
12.4
D104C-919AS-1R8N
TOKO
1.8
7.6
10.4
7443552280
Wurth
2.8
10.5
11
FDA1055-3R3M
TOKO
3.3
7.3
11.7
7447709004
Wurth
4.7
11
13
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
15
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
Selecting the Input Capacitor
The MPQ28261 requires an input capacitor (C1) to
supply the AC current to the step-down
converter while maintaining the DC input
voltage, because the input current to the stepdown converter is discontinuous. Use
capacitors with low equivalent series resistance
(ESR)—such as ceramic capacitors with X5R or
X7R dielectrics with low ESR and small
temperature coefficients. For most applications,
use a 22µF capacitor.
Since C1 absorbs the input switching current, it
must withstand significant ripple current. The
RMS current in the input capacitor can be
estimated by:
IC1 ILOAD
V
V
OUT 1 OUT
VIN
VIN
The worse case condition occurs at VIN = 2VOUT,
where:
IC1
ILOAD
2
For simplification, choose an input capacitor
with an RMS current rating greater than half of
the maximum load current.
The input capacitor can be electrolytic, tantalum
or ceramic. Include a small ceramic capacitor
with a value of ~0.1µF as close to the IC as
possible when using electrolytic or tantalum
capacitors. Use sufficiently-large ceramic
capacitors that can provide sufficient charge to
prevent excessive voltage ripple. The capacitorinduced input voltage ripple can be estimated
by:
VIN
Where L1 is the inductor value and RESR is the
ESR value of the output capacitor.
For
ceramic capacitors,
the capacitor
dominates the impedance at the switching
frequency, and subsequently dominates the
output voltage ripple. For simplicity, the output
voltage ripple can be estimated as:
ΔVOUT
V
VOUT
1 OUT
8 fS L1 C2
VIN
2
For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency. Subsequently, the output ripple can
be approximated as:
ΔVOUT
VOUT
V
1 OUT
fS L1
VIN
RESR
The characteristics of the output capacitor also
affect the system regulation stability. The
MPQ28261 can be optimized for a wide range
of capacitances and ESR values.
The maximum capacitance is 1100μF. (Tested
on Chroma 63030, CCH mode, 3A load current,
soft start capacitor is 100nF, VOUT=5V).
External Bootstrap Diode
An external bootstrap diode may enhance the
efficiency of the regulator under the following
applicable conditions:
VOUT is 5V or 3.3V; and
Duty cycle is high: D=
VOUT
>65%
VIN
In these cases, an external BST diode is
recommended from the VCC pin to BST pin, as
shown in Figure 6.
V
ILOAD
V
OUT 1 OUT
fS C1 VIN
VIN
Selecting the Output Capacitor
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or lowESR electrolytic capacitors to minimize the
output voltage. The output voltage ripple can be
estimated as:
VOUT
V
V
1
OUT 1 OUT RESR
fS L1
VIN
8 fS C2
Figure 6: Add Optional External Bootstrap Diode
to Enhance Efficiency
The recommended external BST diode is
IN4148 where the BST capacitor value is
between 0.1 and 1μF.
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
16
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
TYPICAL APPLICATIONS
Figure 7: VOUT = 1V
Figure 8: VOUT = 1.2V
Figure 9: VOUT = 3.3V
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
17
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
Figure 10: VOUT = 5V
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
18
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
Layout Recommendation
1) Lay the large switching current paths
(GND, IN, and SW) very close to the device
using short, wide, and direct traces.
Minimize the loop length and area formed
by these components. These components,
including the inductor and output capacitor,
should be placed on the same side of the
circuit board, with their connections on that
layer. Place a localized unbroken ground
plane below these components.
2) Place input capacitors as close as possible
to the IN and GND pins with wide PCB
traces to avoid excess inductance and to
prevent large spikes. Add thermal vias to
the bottom side to improve thermal
performance.
3)
4)
5)
6)
Place the decoupling capacitor as close as
possible to the VCC and GND pins. Avoid
routing the VCC trace near the noisy SWto-BST trace.
Place the inductor as close as possible to
the SW pin. Keep the switching node SW
short and away from the sensitive nodes
such as the feedback network.
Place external feedback resistors next to
the FB pin. Make sure that there is no via
on the FB trace and keep the FB trace
short. Route the FB trace away from the
noisy SW and BST node.
Keep the BST voltage path (BST, C3, and
SW) as short as possible. Keep the BST
trace should away from sensitive nodes.
VIN
GND
C1
IN 1
SW
C3
SW 2
13 GND
SW 3
12 GND
SW 4
11 VCC
SW 5
10 SS
BST 6
9 PG
EN
L1
14 AGND
7
8 FB
C4
C7
R10
Rt
R2
R1
U1
C2
VOUT
GND
Top Layer
GND
Bottom Layer
Figure 11: PCB Layout
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
19
MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
PACKAGE INFORMATION
QFN14 (3mmx4mm)
NOTICE: The information in this document is subject to change without notice. Please contact MPS for current specifications.
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.
MPQ28261 Rev. 1.12
www.MonolithicPower.com
11/6/2015
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
20
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