Design Procedure

The MAX1920/MAX1921 are optimized for small external

components and fast transient response. There are

several application circuits (Figures 1 through 4) to

allow the choice between ceramic or tantalum output

capacitor and internally or externally set output volt-

ages. The use of a small ceramic output capacitor is

preferred for higher reliability, improved voltage-posi-

tioning transient response, reduced output ripple, and

the smaller size and greater availability of ceramic versus

tantalum capacitors.

Voltage Positioning

Figures 1 and 2 are the application circuits that utilize

small ceramic output capacitors. For stability, the circuit

obtains feedback from the LX node through R1, while

there is no D.C. feedback from the output, the output volt-

age exhibits load regulation that is equal to the output

load current multiplied by the inductor’s series resistance.

This small amount of load regulation is similar to voltage

positioning as used by high-powered microprocessor

supplies intended for personal computers. For the

MAX1920/MAX1921, voltage positioning eliminates or

greatly reduces undershoot and overshoot during load

transients (see the Typical Operating Characteristics),

which effectively halves the peak-to-peak output voltage

excursions compared to traditional step-down converters.

For convenience, Table 1 lists the recommended external

component values for use with the MAX1921 application

circuit of Figure 1 with various input and output voltages.

Inductor Selection

In order to calculate the smallest inductor, several cal-

culations are needed. First, calculate the maximum

duty cycle of the application as:

Second, calculate the critical voltage across the inductor as:

if DutyCycle(MAX) < 50%,

Last, calculate the minimum inductor value as:

Select the next standard value larger than L(MIN). The

L(MIN) calculation already includes a margin for induc-

tance tolerance. Although values much larger than

L(MIN) work, transient performance, efficiency, and

inductor size suffer.

A 550mA rated inductor is enough to prevent saturation

for output currents up to 400mA. Saturation occurs

when the inductor’s magnetic flux density reaches the

maximum level the core can support and inductance

falls. Choose a low DC-resistance inductor to improve

efficiency. Tables 2 and 3 list some suggested inductors

and suppliers.

Capacitor Selection

may be as small as 2.2µF ceramic with X5R or X7R

L MIN

()

.

=×

×

−

25 10

6

DutyCycle MAX

V

V

MIN

OUT

IN

()

()

%

=×100

Low-Voltage, 400mA Step-Down

DC-DC Converters in SOT23

6

_______________________________________________________________________________________

Table 1. MAX1921 Suggested

Components for Figure 1

INPUT SOURCE

OUTPUT

5V

3.3V, 1 Li+,

3 x AA

2.5V, 2 x AA

3.3V

3.0V

R1 = 8.25k

2.5V

R1 = 5.62k

N/A

1.8V

1.5V

L = 10µH,

R1 = 8.25k

Ω,

R1 = 4.75k

Table 2. Suggested Inductors

PART

NUMBER

L

(µH)

(ohms max)

Isat (A)

SIZE

4.7

0.200

1.10

6.8

0.320

0.90

Coilcraft

LPO1704

10

0.410

0.80

6.6 x 5.5 x 1.0

4.7

0.080

0.90

6.8

0.095

0.73

Sumida

CDRH3D16

10

0.160

0.55

3.8 x 3.8 x 1.8

4.7

0.081

0.63

Sumida

CDRH2D18

6.8

0.108

0.57

3.2 x 3.2 x 2.0

4.7

0.38

0.74

Toko

D312F

10

0.79

0.50

3.6 x 3.6 x 1.2

4.7

0.230

0.84

Toko

D412F

10

0.490

0.55

4.6 x 4.6 x 1.2

4.7

0.087

1.14

6.8

0.105

0.95

Toko

D52LC

10

0.150

0.76

5.0 x 5.0 x 2.0