Datasheet

MAX1920/MAX1921
MAX1921 Using Ceramic COUT
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
OUT
is
high at no load by about half of this load regulation. The
simplified calculation is:
where R
L
(MAX) is the maximum series resistance of the
inductor. Select a standard resistor value that is within
20% of this calculation.
Next, calculate C
FF
for 25mV ripple at the internal feed-
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
FF
.
MAX1920 Using Ceramic C
OUT
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
OUT
is
high at no load by about half of this load regulation:
where R2 is chosen in the 50kΩ to 500kΩ range, V
REF
= 1.25V and R
L
is the typical series resistance of the
inductor. Use 1% or better resistors.
Next, calculate the equivalent resistance at the FB node as:
Then, calculate C
FF
for 25mV ripple at FB. The simpli-
fied calculation is:
Select a standard capacitor value that is within 20% of
the calculated C
FF
.
MAX1920 Using Tantalum C
OUT
When using the application circuit of Figure 4, choose
R1 and R2 such as to obtain the desired V
OUT
:
where R2 is chosen to be less than 50kΩ and V
REF
=
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
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.
RR
V
V
OUT
REF
12 1=× −






Cq
FF
=×
−
25 10
5
. Re
Re ||
qRR
RR
RR
==
×
+
12
12
12
RR
VRIMAX
V
OUT L OUT
REF
12
2
1=×
+×
−






()/
CR
FF
=×
−
25 10 1
5
.
RRMAX
L
15 10
4
=× × ()
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
8 _______________________________________________________________________________________
Table 3. Component Suppliers
SUPPLIER PHONE WEBSITE
Coilcraft 847-639-6400
www.coilcraft.com
Kemet 408-986-0424 www.kemet.com
Murata 814-237-1431
www.murata.com
USA
847-956-0666
Sumida
Japan
81-3-3607-5111
www.sumida.com
USA
408-573-4150
www.T-Yuden.com
Taiyo
Yuden
Japan
81-3-3833-5441
www.yuden.co.jp
USA
847-297-0070
www.tokoam.com
Toko
Japan
81-3-3727-1161
www.toko.co.jp
MAX1920
IN
AGND
SHDN
LX
PGND
FB
OFF
ON
R1
C
OUT
L
OUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
C
IN
R2
2
1
3
5
6
4
Figure 4. MAX1920 Application Circuit Using Tantalum Output
Capacitor