MAX1920/MAX1921

Low-Voltage, 400mA Step-Down

DC-DC Converters in SOT23

Design Procedure

The MAX1920/MAX1921 are optimized for small external

components and fast transient response. There are sev-

eral application circuits (Figures 1 through 4) to allow the

choice between ceramic or tantalum output capacitor and

internally or externally set output voltages. The use of a

small ceramic output capacitor is preferred for higher reli-

ability, improved voltage-positioning 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

load transients are fed-forward through CFF. Because

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

voltage 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 sup-

plies intended for personal computers. For the MAX1920/

MAX1921, voltage positioning eliminates or greatly reduc-

es undershoot and overshoot during load transients (see

the Typical Operating Characteristics), which effectively

halves the peak-to-peak output voltage excursions com-

pared to traditional step-down converters.

Table 1. MAX1921 Suggested

Components for Figure 1

OUTPUT

3.3V

3.0V

2.5V

1.8V

1.5V

INPUT SOURCE

5V

3.3V, 1 Li+,

3 x AA

2.5V, 2 x AA

L = 10µH, COUT = 10µF,

R1 = 8.25kΩ, CFF = 3300pF

L = 6.8µH, COUT = 6.8µF,

R1 = 5.62kΩ, CFF = 4700pF

N/A

L = 10µH,

COUT = 10µF,

R1 = 8.25kΩ,

CFF = 3300pF

L = 4.7µH, COUT = 4.7µF,

R1 = 4.75kΩ, CFF = 5600pF

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.

Induction Selection

In order to calculate the smallest inductor, several calcula-

tions are needed. First, calculate the maximum duty cycle

of the application as:

DutyCyc=le( MAX ) VOUT × 100%

VIN ( MIN )

Second, calculate the critical voltage across the inductor as:

if DutyCycle(MAX) < 50%,

then VCRITICAL = (VIN(MIN) - VOUT),

else VCRITICAL = VOUT

Last, calculate the minimum inductor value as:

L(MIN) = 2.5 ×10-6 × VCRITICAL

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

L(MIN) calculation already includes a margin for inductance

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.

Table 2. Suggested Inductors

PART

NUMBER

Coilcraft

LPO1704

Sumida

CDRH3D16

Sumida

CDRH2D18

Toko

D312F

Toko

D412F

Toko

D52LC

L

(μH)

4.7

6.8

10

4.7

6.8

10

4.7

6.8

4.7

10

4.7

10

4.7

6.8

10

RL

(ohms max)

0.200

0.320

0.410

0.080

0.095

0.160

0.081

0.108

0.38

0.79

0.230

0.490

0.087

0.105

0.150

Isat

(A)

1.10

0.90

0.80

0.90

0.73

0.55

0.63

0.57

0.74

0.50

0.84

0.55

1.14

0.95

0.76

SIZE

6.6 x 5.5 x 1.0

= 36.3mm3

3.8 x 3.8 x 1.8

= 26.0mm3

3.2 x 3.2 x 2.0

= 20.5mm3

3.6 x 3.6 x 1.2

= 15.6mm3

4.6 x 4.6 x 1.2

= 25.4mm3

5.0 x 5.0 x 2.0

= 50.0mm3

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