MAX1921 datasheet(7/9 Pages) MAXIM

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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:

The output capacitor, COUT, may be either ceramic or

tantalum depending upon the chosen application cir-

cuit (see Figures 1 through 4). Table 3 lists some sug-

gested capacitor suppliers.

Ceramic Output Capacitor

For ceramic COUT, use the application circuit of Figure 1

or Figure 2. Calculate the minimum capacitor value as:

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:

Because tantalum capacitors rarely specify minimum

ESR, choose a capacitor with typical ESR that is about

twice as much as ESRCOUT(MIN). Although ESRs

greater than this work, output ripple becomes larger.

For tantalum COUT, calculate the minimum output

capacitance as:

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 volt-

age-divider. When using a ceramic output capacitor,

the feedback network must include a compensation

feed-forward capacitor, CFF.

MIN

L I

MAX

ESR

MIN

OUT

COUT

CRITICAL

()

=×

125

ESR

MIN

COUT

OUT

()

=×

−

80 10

MIN

OUT

CRITICAL

()

=×

−

25 10

RMS

MAX

OUT

()

=×

−

Low-Voltage, 400mA Step-Down

DC-DC Converters in SOT23

_______________________________________________________________________________________

MAX1920

AGND

SHDN

PGND

OFF

COUT

CFF

OUTPUT

UP TO 400mA

INPUT

2V TO 5.5V

CIN

Figure 2. Typical Application Circuit (MAX1920)

MAX1921

AGND

SHDN

PGND

OUT

OFF

COUT

OUTPUT

UP TO 400mA

INPUT

2V TO 5.5V

CIN

Figure 3. MAX1921 Application Circuit Using Tantalum Output

Capacitor

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