MP2108DQ-LF [MPS]

Switching Regulator, Current-mode, 3.5A, 920kHz Switching Freq-Max, PDSO10, 3 X 3 MM, ROHS COMPLIANT, MO-229VEED-5, QFN-10;
MP2108DQ-LF
型号: MP2108DQ-LF
厂家: MONOLITHIC POWER SYSTEMS    MONOLITHIC POWER SYSTEMS
描述:

Switching Regulator, Current-mode, 3.5A, 920kHz Switching Freq-Max, PDSO10, 3 X 3 MM, ROHS COMPLIANT, MO-229VEED-5, QFN-10

开关 光电二极管
文件: 总13页 (文件大小:454K)
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MP2108  
2A, 6V, 740KHz  
Synchronous Buck Converter  
The Future of Analog IC Technology  
DESCRIPTION  
FEATURES  
The MP2108 is a 2A, 740KHz synchronous  
buck converter designed for low voltage  
applications requiring high efficiency. It is  
capable of providing output voltages as low as  
0.9V, and integrates top and bottom switches to  
minimize power loss and component count. The  
740KHz switching frequency allows for small  
filtering components, further reducing the  
solution size.  
2A Output Current  
Synchronous Rectification  
Internal 160mand 190mPower Switches  
Input Range of 2.6V to 6V  
Over 95% Efficiency  
Under Voltage Lockout Protection  
Soft-Start Operation  
Thermal Shutdown  
Internal Current Limit (Source & Sink)  
Tiny 10-Pin MSOP and 3x3 QFN Packages  
The MP2108 includes cycle-by-cycle current  
limiting and under voltage lockout. Internal  
power switches, combined with the tiny 10-pin  
MSOP or 3mm x 3mm QFN packages, provide  
a solution requiring a minimum of board space.  
QFN package is recommended if output current  
is higher than 1.5A.  
APPLICATIONS  
SOHO Routers, PCMCIA Cards, Mini PCI  
Handheld Computers, PDAs  
Cell phones, Digital Still and Video Cameras  
Small LCD Displays  
EVALUATION BOARD REFERENCE  
“MPS” and “The Future of Analog IC Technology” are Registered Trademarks of  
Monolithic Power Systems, Inc.  
Board Number  
Dimensions  
EV2108DQ/DK-00A  
2.5”X x 2.0”Y x 0.5”Z  
TYPICAL APPLICATION  
100  
V
=2.5V  
OUT  
90  
80  
70  
60  
50  
40  
30  
20  
10  
0
2
1
V
=1.2V  
OUT  
VIN  
10  
BST  
3
7
RUN  
LX  
6
SS  
8
COMP  
FB  
VREF SGND PGND  
9
5
4
V
=3.3V  
IN  
0.01  
0.1  
1
10  
LOAD CURRENT (A)  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
1
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
PACKAGE REFERENCE  
TOP VIEW  
TOP VIEW  
BST  
VIN  
1
2
3
4
5
10 RUN  
BST  
VIN  
1
2
3
4
5
10  
9
RUN  
VREF  
COMP  
FB  
9
8
7
6
VREF  
COMP  
FB  
LX  
LX  
8
PGND  
SGND  
7
PGND  
SGND  
6
SS  
SS  
EXPOSED PAD  
ON BACKSIDE  
Part Number**  
Package***  
Temperature  
Part Number*  
Package  
Temperature  
QFN10  
(3mm x 3mm)  
MP2108DQ  
MP2108DK  
MSOP10  
–40°C to +85°C  
–40°C to +85°C  
For Tape & Reel, add suffix –Z (eg. MP2108DQ–Z)  
For RoHS compliant packaging, add suffix –LF (eg.  
MP2108DQ–LF–Z)  
For Tape & Reel, add suffix –Z (eg. MP2108DK–Z)  
For RoHS compliant packaging, add suffix –LF (eg.  
MP2108DK–LF–Z)  
**  
*
*** Recommended for output currents higher than 1.5A  
ABSOLUTE MAXIMUM RATINGS (1)  
Input Supply Voltage VIN ............................. 6.5V  
LX Voltage VLX ..................... 0.3V to VIN + 0.3V  
BST to LX Voltage .........................0.3V to +6V  
Voltage on All Other Pins...............0.3V to +6V  
Storage Temperature...............55°C to +150°C  
Recommended Operating Conditions (2)  
Input Supply Voltage VIN ...................... 2.6V to 6  
Output Voltage VOUT...........................0.9V to 5V  
Operating Temperature..............40°C to +85°C  
Thermal Resistance (3)  
MSOP10................................150..... 65... °C/W  
QFN10 (3mm x 3mm).............50...... 12... °C/W  
θJA  
θJC  
Notes:  
1) Exceeding these ratings may damage the device.  
2) The device is not guaranteed to function outside of its  
operating conditions.  
3) Measured on approximately 1” square of 1 oz copper.  
ELECTRICAL CHARACTERISTICS  
VIN = 5.0V, TA = +25°C, unless otherwise noted.  
Parameter  
Symbol Condition  
Min  
Typ  
Max  
Units  
Input Voltage Range  
Input Undervoltage Lockout  
VIN  
2.6  
6
V
V
2.2  
Input Undervoltage Lockout  
Hysteresis  
100  
mV  
Shutdown Supply Current  
Operating Supply Current  
VREF Voltage  
RUN Input Low Voltage  
RUN Input High Voltage  
RUN Hysteresis  
VRUN 0.3V  
VRUN > 2V, VFB = 1.1V  
VIN = 2.6V to 6V  
0.5  
1.2  
2.4  
1.0  
1.8  
µA  
mA  
V
V
V
VREF  
VIL  
VHL  
0.4  
1
1.5  
100  
mV  
µA  
RUN Input Bias Current  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
2
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
ELECTRICAL CHARACTERISTICS (continued)  
VIN = 5.0V, TA = +25°C, unless otherwise noted.  
Parameter  
Symbol Condition  
Min  
Typ  
740  
200  
Max  
Units  
Oscillator  
Switching Frequency  
Maximum Duty Cycle  
Minimum On Time  
Error Amplifier  
fSW  
620  
85  
920  
KHz  
%
ns  
DMAX VFB = 0.7V  
TON  
Voltage Gain  
AVEA  
GEA  
400  
450  
±40  
895  
–100  
V/V  
µA/V  
µA  
mV  
nA  
Transconductance  
COMP Maximum Output Current  
FB Regulation Voltage  
FB Input Bias Current  
Soft-Start  
VFB  
IFB  
875  
915  
VFB = 0.9V  
Soft-Start Current  
Output Switch On-Resistance  
ISS  
2
µA  
VIN = 5V  
VIN = 3V  
VIN = 5V  
VIN = 3V  
190  
280  
160  
230  
3.5  
mΩ  
mΩ  
mΩ  
mΩ  
A
Switch On Resistance  
Synchronous Rectifier On Resistance  
Switch Current Limit (Source)  
2.5  
Synchronous Rectifier Current Limit  
(Sink)  
350  
160  
mA  
Thermal Shutdown  
°C  
PIN FUNCTIONS  
Pin #  
Name  
Description  
Power Switch Boost. BST powers the gate of the high-side N-Channel power MOSFET  
switch. Connect a 10nF or greater capacitor between BST and LX.  
1
BST  
Internal Power Input. VIN supplies the power to the MP2108 through the internal LDO  
regulator. Bypass VIN to PGND with a 10µF or greater capacitor. Connect VIN to the input  
source voltage.  
Output Switching Node. LX is the source of the high-side N-Channel switch and the drain  
of the low-side N-Channel switch. Connect the output LC filter between LX and the output.  
2
3
VIN  
LX  
Power Ground. PGND is the source of the N-Channel MOSFET synchronous rectifier.  
Connect PGND to SGND as close to the MP2108 as possible.  
SGND Signal Ground.  
4
5
PGND  
Soft-Start Input. Place a capacitor from SS to SGND to set the soft-start period. The  
6
SS  
MP2108 sources 2µA from SS to the soft-start capacitor at start-up. As the voltage at SS  
rises, the feedback threshold voltage increases to limit inrush current at startup.  
Feedback Input. FB is the inverting input of the internal error amplifier. Connect a resistive  
voltage divider from the output voltage to FB to set the output voltage.  
Compensation Node. COMP is the output of the error amplifier. Connect a series RC  
network to compensate the regulation control loop.  
Internal 2.4V Regulator Bypass. Connect a 10nF capacitor between VREF and SGND to  
bypass the internal regulator. Do not apply any load to VREF.  
On/Off Control Input. Drive RUN high to turn on the MP2108, drive RUN low to turn the  
7
8
FB  
COMP  
VREF  
RUN  
9
10  
MP2108 off. For automatic startup, connect RUN to VIN via a 100kpull-up resistor.  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
3
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
TYPICAL PERFORMANCE CHARACTERISTICS  
Circuit of Figure 2, VIN = 5V, VOUT = 2.5V, L1 = 5µH, C1 = 10µF, C2 = 22µF, TA = +25°C, unless  
otherwise noted.  
Efficiency vs  
Load Current  
Peak Current vs  
Duty Cycle  
100  
90  
80  
70  
60  
50  
40  
30  
20  
10  
0
5.0  
4.5  
4.0  
3.5  
3.0  
2.5  
2.0  
V
=3.3V  
OUT  
V
=1.2V  
OUT  
V
=2.5V  
OUT  
V
=5V  
IN  
0.01  
0.1  
1
10  
0
20  
40  
60  
100  
80  
LOAD CURRENT (A)  
DUTY CYCLE (%)  
Feedback Voltage vs  
Die Temperature  
Switching Frequency vs  
Die Temperature  
830  
810  
790  
770  
750  
730  
710  
690  
670  
650  
630  
0.904  
0.902  
0.900  
0.898  
0.896  
0.894  
0.892  
0.890  
0.888  
0.886  
0.884  
-50 -25  
0
25 50 75 100 125  
-50 -25  
0
25 50 75 100 125 150  
DIE TEMPERATURE (OC)  
DIE TEMPERATURE (OC)  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
4
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
TYPICAL PERFORMANCE CHARACTERISTICS (continued)  
Circuit of Figure 2, VIN = 5V, VOUT = 2.5V, L1 = 5µH, C1 = 10µF, C2 = 22µF, TA = +25°C, unless  
otherwise noted.  
Load Transient  
Steady State Operation  
Steady State Operation  
0.5A-1A Step Resistive Load  
Full Load  
No Load  
V
OUT  
AC Coupled  
10mV/div.  
V
OUT  
AC Coupled  
10mV/div.  
V
OUT  
AC Coupled  
200mV/div.  
I
L
1A/div.  
I
L
2A/div.  
I
L
V
1A/div.  
SW  
V
SW  
5V/div.  
5V/div.  
400ns/div.  
400ns/div.  
Start-up through Enable  
Shut-down through Enable  
Start-up through Enable  
Full Load  
No Load  
No Load  
V
V
V
OUT  
OUT  
OUT  
2V/div.  
2V/div.  
1V/div.  
V
V
V
EN  
EN  
EN  
2V/div.  
5V/div.  
2V/div.  
1ms/div.  
100ms/div.  
400ms/div.  
Shut-down through Enable  
Full Load  
V
OUT  
2V/div.  
V
EN  
2V/div.  
200ms/div.  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
5
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
OPERATION  
V
IN  
2.6V to 6V  
V
IN  
2
C1  
CURRENT  
SENSE  
AMPLIFIER  
ENABLE  
CKT & LDO  
REGULATOR  
GATE  
DRIVE  
REGULATOR  
RUN  
Vdr  
OFF ON  
+
--  
10  
BST  
LX  
V
REF  
2.4V  
PWM  
1
3
COMPARATOR  
9
C6  
C7  
+
--  
L1  
V
OUT  
CONTROL  
LOGIC  
Vdr  
C2  
720KHz  
OSCILLATOR  
RAMP  
CURRENT  
+
LIMIT  
--  
COMPARATOR  
V
BP  
R2  
UVLO &  
THERMAL  
SHUTDOWN  
+
--  
PGND  
FB  
SS  
4
7
6
C5  
--  
--  
+
GM  
ERROR  
AMPLIFIER  
V
FB  
0.9V  
CURRENT  
LIMIT  
THRESHOLD  
R1  
5
8
COMP  
R3  
C3  
SGND  
C4  
Figure 1—Functional Block Diagram  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
6
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
The MP2108 measures the output voltage  
The average inductor current is controlled by  
the voltage at COMP, which in turn, is  
controlled by the output voltage. Thus the  
output voltage controls the inductor current to  
satisfy the load.  
through an external resistive voltage divider and  
compares it to the internal 0.9V reference to  
generate the error voltage at COMP. The  
current-mode regulator uses the voltage at  
COMP and compares it to the inductor current  
to regulate the output voltage. The use of  
current-mode regulation improves transient  
response and control loop stability.  
Since the high-side N-Channel MOSFET  
requires voltage above VIN to drive its gate, a  
bootstrap capacitor from LX to BST is required  
to drive the high-side MOSFET gate. When LX  
is driven low (through the low-side MOSFET),  
the BST capacitor is internally charged. The  
voltage at BST is applied to the high-side  
MOSFET gate to turn it on. Voltage is  
maintained until the high-side MOSFET is  
turned off and the low-side MOSFET is turned  
on, and the cycle repeats. Connect a 10nF or  
greater capacitor from BST to SW to drive the  
high-side MOSFET gate.  
At the beginning of each cycle, the high-side  
N-Channel MOSFET is turned on, forcing the  
inductor current to rise. The current at the drain  
of the high-side MOSFET is internally  
measured and converted to a voltage by the  
current sense amplifier.  
That voltage is compared to the error voltage at  
COMP. When the inductor current rises  
sufficiently, the PWM comparator turns off the  
high-side switch and turns on the low-side  
switch; forcing the inductor current to decrease.  
APPLICATION INFORMATION  
2
1
VIN  
BST  
3
10  
RUN  
SS  
LX  
6
8
7
COMP  
FB  
VREF SGND PGND  
9
5
4
Figure 2—Typical Application Circuit  
Internal Low-Dropout Regulator  
Soft-Start  
The internal power to the MP2108 is supplied  
from the input voltage (VIN) through an internal  
2.4V low-dropout linear regulator, whose output  
is VREF. Bypass VREF to SGND with a 10nF  
or greater capacitor for proper operation. The  
internal regulator can not supply more current  
than is required to operate the MP2108.  
Therefore, do not apply any external load to  
VREF.  
The MP2108 includes a soft-start timer that  
slowly ramps the output voltage at startup to  
prevent excessive current at the input.  
When power is applied to the MP2108, and  
RUN is asserted. A 2µA internal current source  
charges the external capacitor at SS. As the  
capacitor charges, the voltage at SS rises. The  
MP2108 internally limits the feedback threshold  
voltage at FB to that of the voltage at SS. This  
forces the output voltage to rise at the same  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
7
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
rate as the voltage at SS, forcing a linear  
output voltage ramp from 0V to the desired  
regulation voltage during soft-start.  
type. All ceramic capacitors should be placed  
close to the IC. For most applications, a 10µF  
ceramic capacitor will work.  
The soft-start period is determined by the  
equation:  
Selecting the Output Capacitor  
The output capacitor (C2) is required to  
maintain the DC output voltage. Low ESR  
capacitors are preferred to keep the output  
voltage ripple to a minimum. The characteristics  
of the output capacitor also affect the stability of  
the regulation control system. Ceramic,  
tantalum, or low ESR electrolytic capacitors are  
recommended.  
tSS = 0.45 × C5  
Where C5 (in nF) is the soft-start capacitor from  
SS to GND, and tSS (in ms) is the soft-start  
period. Determine the capacitor required for a  
given soft-start period by the equation:  
C5 = 2.22× tSS  
The output voltage ripple is:  
Use values for C5 between 10nF and 22nF to  
set the soft-start period between 4ms and 10ms.  
VRIPPLE  
=
VOUT  
VOUT  
VIN  
1
Setting the Output Voltage (see Figure 2)  
Set the output voltage by selecting the resistive  
voltage divider ratio. The voltage divider drops  
the output voltage to the 0.9V feedback voltage.  
Use 10kfor the low-side resistor of the  
voltage divider. Determine the high-side resistor  
by the equation:  
× 1−  
× RESR  
+
fSW × L  
8 × fSW × C2  
Where VRIPPLE is the output voltage ripple, fSW is  
the switching frequency, VIN is the input voltage  
and RESR is the equivalent series resistance of  
the output capacitors.  
Choose an output capacitor to satisfy the output  
ripple requirements of the design. A 22µF  
ceramic capacitor is suitable for most  
applications.  
V
OUT  
R2 = ⎜  
1⎟ × R1  
0.9V  
Where R2 is the high-side resistor, R1 is the  
low-side resistor and VOUT is the output voltage.  
Selecting the Inductor  
The inductor is required to supply constant  
current to the output load while being driven by  
the switched input voltage. A larger value  
inductor results in less ripple current that in turn  
results in lower output ripple voltage. However,  
the larger value inductor is likely to have a  
larger physical size and higher series  
resistance. Choose an inductor that does not  
saturate under the worst-case load conditions.  
A good rule for determining the inductance is to  
allow peak-to-peak ripple current to be  
approximately 30% to 40% of the maximum  
load current. Make sure that the peak inductor  
current (the load current plus half the peak-to-  
peak inductor ripple current) is below 2.5A to  
prevent loss of regulation due to the current  
limit.  
Selecting the Input Capacitor  
The input current to the step-down converter is  
discontinuous, so a capacitor is required to  
supply the AC current to the step-down  
converter while maintaining the DC input  
voltage. A low ESR capacitor is required to  
keep the noise at the IC to a minimum. Ceramic  
capacitors are preferred, but tantalum or low  
ESR electrolytic capacitors are also an option.  
The capacitor can be electrolytic, tantalum or  
ceramic. Because it absorbs the input switching  
current, it must have an adequate ripple current  
rating. Use a capacitor with RMS current rating  
greater than 1/2 of the DC load current.  
For stable operation, place the input capacitor  
as close to the IC as possible. A smaller high  
quality 0.1µF ceramic capacitor may be placed  
closer to the IC with the larger capacitor placed  
further away.  
If using this technique, it is recommended that  
the larger capacitor be a tantalum or electrolytic  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
8
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
Calculate the required inductance value by the  
equation:  
If large value capacitors with relatively high  
equivalent-series-resistance (ESR) are used,  
the zero due to the capacitance and ESR of the  
output capacitor can be compensated by a third  
pole set by R3 and C4. The pole is:  
VOUT  
×
(
VIN VOUT  
)
L =  
VIN × fSW × ∆I  
Where I is the peak-to-peak inductor ripple  
current. It is recommended to choose I to be  
30%~40% of the maximum load current.  
1
fP3  
=
2π ×R3× C4  
The system crossover frequency (the frequency  
where the loop gain drops to 1dB or 0dB) is  
important. Set the crossover frequency below  
one tenth of the switching frequency to insure  
stable operation. Lower crossover frequencies  
result in slower response and worse transient  
load recovery. Higher crossover frequencies  
degrade the phase and/or gain margins and  
can result in instability.  
Compensation  
The system stability is controlled through the  
COMP pin. COMP is the output of the internal  
transconductance error amplifier. A series  
capacitor-resistor combination sets a pole-zero  
combination to control the characteristics of the  
control system.  
The DC loop gain is:  
Table 1—Compensation Values for Typical  
Output Voltage/Capacitor Combinations  
VFB  
AVDC  
=
× AVEA × GCS ×RLOAD  
VOUT  
VOUT  
C2  
22µF  
R3  
C3  
C4  
R2  
R1  
Where VFB is the feedback voltage, 0.9V, AVEA  
is the transconductance error amplifier voltage  
gain, 400 V/V and GCS is the current sense  
transconductance, (roughly the output current  
divided by the voltage at COMP), 4.5A/V.  
1.8V  
6.8k3.3nF None 10k10kꢀ  
9.1k2.2nF None 17.8k10kꢀ  
12k1.8nF None 27k10kꢀ  
Ceramic  
22µF  
Ceramic  
2.5V  
3.3V  
22µF  
Ceramic  
RLOAD is the load resistance:  
VOUT  
47µF  
Tantalum  
(300m)  
RLOAD  
=
IOUT  
1.8V  
13k2nF  
1nF  
10k10kꢀ  
Where IOUT is the output load current.  
47µF  
The system has 2 poles of importance, one is  
due to the compensation capacitor (C3), and  
the other is due to the load resistance and the  
output capacitor (C2), where:  
2.5V Tantalum 18k1.2nF 750pF 17.8k10kꢀ  
(300m)  
47µF  
3.3V Tantalum 24k1nF 560pF 27k10kꢀ  
(300m)  
GEA  
fP1  
=
47µF  
Ceramic  
2π × AVEA × C3  
1V  
6.98k3.3nF None 1.18k10kꢀ  
6.98k3.3nF None 3.4k10kꢀ  
P1 is the first pole, and GEA is the error amplifier  
transconductance (450µA/V) and  
47µF  
Ceramic  
1.2V  
1
fP2  
=
Choosing the Compensation Components  
The values of the compensation components  
listed in Table 1 yields a stable control loop for  
the given output voltage and capacitor. To  
optimize the compensation components for  
conditions not listed in Table 1, use the  
following procedure.  
2π×RLOAD × C2  
The system has one zero of importance, due to  
the compensation capacitor (C3) and the  
compensation resistor (R3). The zero is:  
1
fZ1  
=
2π ×R3× C3  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
9
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
Choose the compensation resistor to set the  
External Bootstrap Diode  
desired crossover frequency. Determine the  
value by the following equation:  
An external bootstrap diode may enhance the  
efficiency of the regulator, the applicable  
conditions of external BST diode are:  
z VOUT is 5V or 3.3V; and  
2π × C2× fC VOUT  
R3 =  
×
GEA × GCS  
VFB  
VOUT  
z Duty cycle is high: D=  
>65%  
VIN  
Where GEA is the EA transconductance  
(450µA/V) and fC is the desired crossover  
frequency (preferably 33KHz).  
In these cases, an external BST diode is  
recommended from the output of the voltage  
regulator to BST pin, as shown in Fig.3  
Choose the compensation capacitor to set the  
zero below one fourth of the crossover  
frequency. Determine the value by the following  
equation:  
External BST Diode  
IN4148  
BST  
CBST  
MP2108  
2
C3 >  
5V or 3.3V  
SW  
π × R3 × fC  
L
COUT  
Determine if the second compensation  
capacitor, C4 is required. It is required if the  
ESR zero of the output capacitor happens at  
less than half of the switching frequency or:  
Figure 3—Add Optional External Bootstrap  
Diode to Enhance Efficiency  
The recommended external BST diode is  
IN4148, and the BST cap is 0.1~1µF.  
π × C2× RESR × fSW > 1  
where RESR is the equivalent series resistance  
of the output capacitor.  
PCB Layout Guide  
PCB layout is very important to achieve stable  
operation. It is highly recommended to duplicate  
EVB layout for optimum performance.  
The second compensation capcacitor is  
determined by the equation:  
C2×RESR(max)  
If change is necessary, please follow these  
guidelines and take Figure 4 for reference.  
C4 =  
R3  
Where RESR(MAX) is the maximum ESR of the  
output capacitor.  
1) Keep the path of switching current short  
and minimize the loop area formed by Input  
cap, high-side MOSFET and low-side  
MOSFET.  
2) Bypass ceramic capacitors are suggested  
to be put close to the Vin Pin.  
3) Ensure all feedback connections are short  
and direct. Place the feedback resistors  
and compensation components as close to  
the chip as possible.  
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.  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
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© 2010 MPS. All Rights Reserved.  
10  
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
L1  
CBST  
VIN  
VOUT  
COUT  
CIN  
GND  
GND  
R4  
Css  
C1  
R1  
R2  
R3  
C2  
Top Layer  
GND  
Bottom Layer  
Figure 4—PCB Layout (Double Layers)  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
11  
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
PACKAGE INFORMATION  
MSOP10  
MP2108 Rev 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
12  
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER  
QFN10 (3mm x 3mm)  
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.  
MP2108 Rev. 1.2  
1/22/2010  
www.MonolithicPower.com  
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.  
© 2010 MPS. All Rights Reserved.  
13  

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