MAX1921EUT18T Просмотр технического описания (PDF) - Maxim Integrated
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MAX1921EUT18T Datasheet PDF : 12 Pages
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MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Table 3. Component Suppliers
SUPPLIER
Coilcraft
Kemet
Murata
USA
Sumida
Japan
Taiyo
Yuden
USA
Japan
Toko
USA
Japan
PHONE
847-639-6400
408-986-0424
814-237-1431
847-956-0666
81-3-3607-5111
408-573-4150
81-3-3833-5441
847-297-0070
81-3-3727-1161
WEBSITE
www.coilcraft.com
www.kemet.com
www.murata.com
www.sumida.com
www.T-Yuden.com
www.yuden.co.jp
www.tokoam.com
www.toko.co.jp
MAX1921 Using Ceramic COUT
When using the application circuit of Figure 1, the induc-
tor’s series resistance causes a small amount of load
regulation, as desired for a voltage-positioning load tran-
sient response. Choose R1 such that VOUT is high at no
load by about half of this load regulation. The simplified
calculation is:
R1 = 5 x 104 x RL(MAX)
where RL(MAX) is the maximum series resistance of the
inductor. Select a standard resistor value that is within
20% of this calculation.
Next, calculate CFF for 25mV ripple at the internal feed-
back node. The simplified calculation is:
CFF = 2.5 × 10-5/R1
where R1 is the standard resistor value that is used.
Select a standard capacitor value that is within 20% of
the calculated CFF.
INPUT
2V TO 5.5V
1
IN
6
L
LX
CIN
MAX1920
2 AGND
PGND 5
ON
OFF
3
SHDN
4
FB
OUTPUT
UP TO 400mA
COUT
R1
R2
Figure 4. MAX1920 Application Circuit Using Tantalum Output
Capacitor
MAX1920 Using Ceramic COUT
When using the application circuit of Figure 2, the induc-
tor’s series resistance causes a small amount of load reg-
ulation, as desired for a voltage-positioning load transient
response. Choose R1 and R2 such that VOUT is high at
no load by about half of this load regulation:
R1
= R 2 × VOUT + RL V×RIOEFUT ( MAX
)
/
2
−
1
where R2 is chosen in the 50kΩ to 500kΩ range, VREF
= 1.25V and RL is the typical series resistance of the
inductor. Use 1% or better resistors.
Next, calculate the equivalent resistance at the FB node as:
= Req R= 1|| R 2 R1× R 2
R1+ R2
Then, calculate CFF for 25mV ripple at FB. The simplified
calculation is:
CFF = 2.5 × 10-5/Req
Select a standard capacitor value that is within 20% of the
calculated CFF.
MAX1920 Using Tantalum COUT
When using the application circuit of Figure 4, choose R1
and R2 such as to obtain the desired VOUT:
R1
= R 2 ×
VOUT
VREF
− 1
where R2 is chosen to be less than 50kΩ and VREF =
1.25V. Use 1% or better resistors.
Layout Considerations
High switching frequencies make PC board layout a very
important part of design. Good design minimizes exces-
sive EMI on the feedback paths and voltage gradients in
the ground plane, both of which can result in instability or
regulation errors. Connect the inductor, input filter capacitor,
and output filter capacitor as close to the device as possible,
and keep their traces short, direct, and wide. Connect their
ground pins at a single common node in a star ground con-
figuration. The external voltage-feedback network should
be very close to the FB pin, within 0.2in (5mm). Keep
noisy traces, such as the LX trace, away from the voltage-
feedback network; also keep them separate, using grounded
copper. The MAX1920/MAX1921 evaluation kit data sheet
includes a proper PC board layout and routing scheme.
www.maximintegrated.com
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