2.7A, 1MHz, Low-Voltage, Step-Down Regulator with

Internal Synchronous Rectification in QFN Package

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9

Design Procedure

For typical applications, use the recommended compo-

nent values in Table 1. For other applications, take the

following steps:

1) Select the desired PWM-mode switching frequency.

See Figure 3 for maximum operating frequency.

2) Select the constant off-time as a function of input

voltage, output voltage, and switching frequency.

4) Select the inductor as a function of output voltage,

off-time, and peak-to-peak inductor current.

Setting the Output Voltage

The output of the MAX1843 is selectable between one

of three preset output voltages: +2.5V, +1.8V, and

+1.5V. For a preset output voltage, connect FB to the

output voltage, and connect FBSEL as indicated in

Table 2. For an adjustable output voltage, connect

FBSEL to GND, and connect FB to a resistive divider

between the output voltage and ground (Figure 4).

Regulation is maintained for adjustable output voltages

Ω to 50kΩ

range for R1. R2 is given by the equation:

R2

R1

V

V

1

OUT

REF

=









−

LX

R2

R1

FB

MAX1843

Figure 4. Adjustable Output Voltage

0

400

200

800

600

1200

1000

1400

2.6

3.6

4.1

3.1

4.6

5.1

5.6

MAXIMUM RECOMMENDED

OPERATING FREQUENCY vs. INPUT VOLTAGE

Figure 3. Maximum Recommended Operating Frequency vs.

Input Voltage

(V)

(k

Ω)

2.2

39

L

(µH)

5

3.3

2.2

(V)

47

2.2

75

5

1.8

1.5

39

3.3

2.5

1.5

43

3.3

1.8

1.5

56

3.3

1.5

5

2.5

2.2

100

5

1.5

(kHz)

800

850

570

985

940

715

1180

Table 1. Recommended Component

Output voltage

Unconnected

FBSEL

OUTPUT

VOLTAGE

(V)

FB

Resistive

divider

GND

Adjustable

Output voltage

REF

1.8

1.5

Output voltage

2.5

PIN

Table 2. Output Voltage Programming