5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

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7

_______________Detailed Description

The MAX608 is a BiCMOS, step-up, switch-mode pow-

er-supply controller that provides a preset 5V output, in

addition to adjustable-output operation. Its unique con-

trol scheme combines the advantages of pulse-frequen-

cy modulation (low supply current) and pulse-width

modulation (high efficiency with heavy loads), providing

high efficiency over a wide output current range, as well

as increased output current capability over previous

PFM devices. In addition, the external sense resistor

and power transistor allow the user to tailor the output

current capability for each application. Figure 1 shows

the MAX608 functional diagram. The device has a shut-

down mode that reduces the supply current to 5µA

max.

Figure 2 shows the standard application circuits. The

IC is powered from the output, and the input voltage

known as bootstrap operation). The voltage applied to

the gate of the external power transistor is switched

The MAX608’s output voltage can be set to 5V by con-

necting FB to ground; it can also be adjusted from 3V

to 16.5V using external resistors. Use 1% external feed-

back resistors when operating in adjustable-output

mode (Figures 2b, 2c) to achieve an overall output volt-

age accuracy of ±5%.

PFM Control Scheme

The MAX608 uses a proprietary current-limited PFM con-

trol scheme to provide high efficiency over a wide range

of load currents. This control scheme combines the ultra-

low supply current of PFM converters (or pulse skippers)

with the high full-load efficiency of PWM converters.

Figure 2a. 5V Preset Output

Figure 2b. 12V Output

Figure 2c. 3.3V Output

MAX608

REF

SHDN

AGND

GND

N

7

EXT

CS

FB

C1

150

µF

L1

22

µH

D1

1N5817

R1

58k

C4

200

µF

C3

0.1

µF

5

4

6

1

8

3

2

OUT

@ 0.3A

R2

402k

50m

Ω

C2

0.1

µF

R2 = (R1)

(

-1

)

MMFT3055EL

MAX608

REF

SHDN

AGND

GND

N

7

EXT

CS

FB

C1

150

µF

L1

22

µH

D1

1N5817

R1

50k

C4

200

µF

C5

47pF

C3

0.1

µF

5

4

6

1

8

3

2

OUT

@ 0.6A

R2

60k

50m

Ω

C2

0.1

µF

R2 = (R1)

(

-1

)

SI6426

MAX608

REF

SHDN

FB

AGND

GND

N

7

EXT

CS

C2

0.1

µF

C1

150

µF

L1

22

µH

D1

1N5817

MMFT3055EL

50m

Ω

C4

200

µF

C3

0.1

µF

5

4

3

6

1

8

2

OUT

@ 0.5A