MAX1920 Maxim, MAX1920 Datasheet - Page 8

• Manufacturer Part Number: MAX1920
• Description: PLASTIC ENCAPSULATED DEVICES
• Manufacturer: Maxim

When using the application circuit of Figure 1, the
inductor’s series resistance causes a small amount of
load regulation, as desired for a voltage-positioning
load transient response. Choose R1 such that V
high at no load by about half of this load regulation. The
simplified calculation is:
where R
inductor. Select a standard resistor value that is within
20% of this calculation.
Next, calculate C
back node. The simplified calculation is:
where R1 is the standard resistor value that is used.
Select a standard capacitor value that is within 20% of
the calculated C
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Table 3. Component Suppliers
Figure 4. MAX1920 Application Circuit Using Tantalum Output
Capacitor
8
Coilcraft
Kemet
Murata
Sumida
Taiyo
Yuden
Toko
2V TO 5.5V
_______________________________________________________________________________________
INPUT
C
SUPPLIER
OFF
IN
ON
L
(MAX) is the maximum series resistance of the
USA
Japan
USA
Japan
USA
Japan
1
2
3
IN
AGND
SHDN
R
1 5
FF
MAX1920
=
FF
C
.
for 25mV ripple at the internal feed-
FF
×
81-3-3607-5111
81-3-3833-5441
81-3-3727-1161
847-639-6400
408-986-0424
814-237-1431
847-956-0666
408-573-4150
847-297-0070
PGND
MAX1921 Using Ceramic C
=
10
LX
FB
PHONE
2 5
4
.
6
5
4
×
×
R MAX
L
10
L
(
−
5
R
www.T-Yuden.com
)
www.coilcraft.com
www.sumida.com
www.tokoam.com
www.murata.com
www.yuden.co.jp
1
www.kemet.com
www.toko.co.jp
R1
R2
WEBSITE
UP TO 400mA
OUTPUT
C
OUT
OUT
OUT
is
When using the application circuit of Figure 2, 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 and R2 such that V
high at no load by about half of this load regulation:
where R2 is chosen in the 50kΩ to 500kΩ range, V
= 1.25V and R
inductor. Use 1% or better resistors.
Next, calculate the equivalent resistance at the FB node as:
Then, calculate C
fied calculation is:
Select a standard capacitor value that is within 20% of
the calculated C
When using the application circuit of Figure 4, choose
R1 and R2 such as to obtain the desired V
where R2 is chosen to be less than 50kΩ and V
1.25V. Use 1% or better resistors.
High switching frequencies make PC board layout a
very important part of design. Good design minimizes
excessive EMI on the feedback paths and voltage gra-
dients 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 configuration.
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.
R
1
=
R
2
×
L
R
Re
FF



C
1
is the typical series resistance of the
FF
V
.
=
FF
q R
OUT
R
=
for 25mV ripple at FB. The simpli-
=
MAX1920 Using Tantalum C
2
MAX1920 Using Ceramic C
2 5
1
×
+
.
||
R



Layout Considerations
R
L
×
V
2
V
OUT
REF
V
×
=
10
REF
R
R
I
−
OUT
1
1
5
−
×
+
1
Re



(
MAX
R
R
q
2
2
OUT
) /
2
:
−
1



OUT
REF
OUT
OUT
REF
is
=