Applications Information

Figure 3 shows the standard application circuit for the

MAX685. The values shown in Table 1 will work well for

output currents up to 10mA. However, this circuit can

be optimized to a particular application by using differ-

ent capacitors and a different inductor.

Higher Output Voltages

If the application requires output voltages greater than

-7.5V or +24V, use the circuit of Figure 4. This circuit

uses a charge pump to increase the output voltage

without increasing the voltage stress on the LX_ pin.

The maximum output voltages of the circuit in Figure 4

are -15V and +48V.

The voltage rating on D2, D5, and D6 must be 30V or

greater. For a larger negative output voltage without a

larger positive output (or vice versa), use one-half of

the Figure 4 circuit with one-half of the Figure 3 circuit.

Inductor Selection

A 22µH inductor is suitable for most applications.

Larger inductances will reduce inductor ripple current

and output voltage ripple, but they also typically require

larger physical size if increased resistance and losses

are not also allowed.

Small inductors are typically preferred because of

compact design and low cost. Murata LHQ and

TDK NLC types are examples of small surface-mount

inductors that work for most applications. Because these

small-size inductors use thinner wire, they exhibit higher

resistance and have greater losses than larger ones. If

the application demands higher efficiency, use larger,

lower resistance coils such as the Sumida CD43 or CD54,

Coilcraft DT1608 or DO1608, or Coiltronics UP1V series.

Filter Capacitor Selection

The output ripple voltage is a function of the peak in-

ductor current, frequency, and type and value of the

output capacitors. Capacitors with low equivalent-

series resistance (ESR) and large capacitance reduce

output ripple. Typically, tantalum or ceramic capacitors

are optimal. Tantalum capacitors have higher ESR and

higher capacitance than ceramic capacitors. Therefore

the ESR of tantalum capacitors determines the output

ripple, because at the frequencies used the ESR domi-

nates the impedance of the capacitor. If ceramic

capacitors are used, the capacitance determines the

output ripple.

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

8

_______________________________________________________________________________________

MAX685

REF VP

LXP

1

11

3

8

9

13, 14 16

FBN

10

POK

4

6

7

12

POK

SHDN

SYNC

C5

47pF

R1

1.0M

R5

100k

D2

NBR0520

L1

22

µH

D1

NBR0520

R2

90.9k

+15V

C4

2.2

µF

C3

2.2

µF

C2

0.22

µF

C1

10

µF

R3

750k

R4

124k

-7.5V

SHDN

PGND LXN

GND

SYNC

FBP

Figure 3. Standard Application Circuit

MAX685

REF VP

LXP

C6

1

µF

1

11

3

8

9

13, 14 16 C8

1

µF

FBN

10

POK

4

6

7

12

POK

SHDN

SYNC

C5

47pF

R1

R5

100k

D2

D5

D6

L1

22

µH

D3

D4

D1

R2

C9

2.2

µF

C7

2.2

µF

C3

2.2

µF

C2

0.22

µF

C1

10

µF

R3

R4

SHDN

PGND LXN

GND

SYNC

FBP

C4

2.2

µF

Figure 4. Circuit for Output Voltages < -9V and > +24V