MAX1921 Просмотр технического описания (PDF) - Maxim Integrated
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MAX1921 Datasheet PDF : 12 Pages
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MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Capacitor Selection
For nearly all applications, the input capacitor, CIN,
may be as small as 2.2μF ceramic with X5R or X7R
dielectric. The input capacitor filters peak currents and
noise at the voltage source and, therefore, must meet the
input ripple requirements and voltage rating. Calculate the
maximum RMS input current as:
= IIN ( RMS )
IOUT ( MAX ) ×
VOUT ( VIN − VOUT )
VIN
The output capacitor, COUT, may be either ceramic or
tantalum depending upon the chosen application circuit
(see Figures 1 through 4). Table 3 lists some suggested
capacitor suppliers.
Ceramic Output Capacitor
For ceramic COUT, use the application circuit of Figure 1
or Figure 2. Calculate the minimum capacitor value as:
COUT(MIN) = 2.5 x 10-6 × VCRITICAL
Select the next standard value larger than COUT(MIN).
The COUT(MIN) calculation already includes a margin for
capacitor tolerance. Values much larger than COUT(MIN)
always improve transient performance and stability, but
capacitor size and cost increase.
Tantalum Output Capacitor
For tantalum COUT, use the application circuit of Figure
3 or Figure 4. With tantalum COUT, the equivalent series
resistance (ESR) of COUT must be large enough for sta-
bility. Generally, 25mV of ESR-ripple at the feedback node
is sufficient. The simplified calculation is:
ESRCOUT(MIN) = 8.0 x 10-2 × VOUT
Because tantalum capacitors rarely specify minimum
ESR, choose a capacitor with typical ESR that is about
twice as much as ESRCOUT(MIN). Although ESRs great-
er than this work, output ripple becomes larger.
For tantalum COUT, calculate the minimum output
capacitance as:
COUT ( M= IN )
1.25 ×
L × IOUT ( MAX )
ESR COUT ( MIN ) × VCRITICAL
The 1.25 multiplier is for capacitor tolerance. Select any
standard value larger than COUT(MIN).
Feedback and Compensation
The MAX1921 has factory preset output voltages of 1.5V,
1.8V, 2.5V, 3V, and 3.3V, while the MAX1920 is externally
adjusted by connecting FB to a resistive voltage-divider.
When using a ceramic output capacitor, the feedback
network must include a compensation feed-forward
capacitor, CFF.
INPUT
2V TO 5.5V
CIN
1
IN
6
LX
MAX1920
2 AGND
PGND 5
ON
OFF
3
SHDN
4
FB
L
R1
CFF
OUTPUT
UP TO 400mA
COUT
R2
INPUT
2V TO 5.5V
1
IN
6
L
LX
CIN
MAX1921
2 AGND
PGND 5
ON
OFF
3
SHDN
4
OUT
OUTPUT
UP TO 400mA
COUT
Figure 2. Typical Application Circuit (MAX1920)
Figure 3. MAX1921 Application Circuit Using Tantalum Output
Capacitor
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