High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

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11

__________________Design Procedure

Setting the Output Voltage

The MAX746’s dual-mode output voltage can be set

to 5V by grounding FB, or it can be adjusted from

2V to 14V using external resistors R4 and R5 config-

ured as shown in Figure 6. Select feedback resistor

R4 in the 10k

Ω to 60kΩ range. R5 is given by:

R5 = (R4)

2V

The MAX746 is designed to use either internal or exter-

nal feedback mode, but should not be toggled between

the two modes while operating. If two different output

voltages are required, use external feedback mode

with a resistor network similar to the 3.3V/5V adjustable

output circuit shown in Figure 7.

Selecting RSENSE

rent. Once all component values have been deter-

mined, the actual peak current is given by:

V+

CP

HIGH

D2

1N914

C8

0.1

µF

14

13

D3

1N914

C9

1

µF

C10

0.1

µF

D5

1N914

D6

1N914

15

16

D4

1N5817

C11

1

µF

CP

HIGH

14

13

16

V+

15

C8

0.1

µF

D2

1N914

D3

1N914

C9

1

µF

100kHz

OSCILLATOR

T FLIP-

FLOP

5V

Q

AV+

3b. CHARGE-PUMP

VOLTAGE TRIPLER

GND

GND

T

CLK

MAX746

MAX746

3a. CHARGE-PUMP

VOLTAGE DOUBLER

D4

1N5817

Figure 3. Charge-Pump Configurations

PEAK CURRENT LIMIT

vs. SOFT-START VOLTAGE

0

1

2

3

01

2

3

4

SOFT-START VOLTAGE (V)

Figure 4. Peak Current Limit vs. Soft-Start Voltage

-1

GND

V+

LBO

15

1

16

R3

100k

(

)

2

R2

R1

LOW-BATTERY

OUTPUT

LBI

MAX746

Figure 5. Input Voltage Monitor Circuit