3.3V/5V or Adjustable-Output,

Step-Up DC-DC Converters

8

_______________________________________________________________________________________

Logic Inputs and Outputs

OUT, and should not be connected to signals outside

this range. The SHDN input and LBO output (open-

drain) are not clamped to V+ and can be pulled as high

control inputs (3/5, LBI, or SHDN) floating.

__________________Design Procedure

Output Voltage Selection

For the MAX856/MAX858,you can select a 3.3V or 5V

output voltage under logic control, or by tying 3/

–

5to

GND or OUT. Efficiency is typically better than 80%

over a 2mA to 100mA (MAX856/MAX857) load range.

The device is internally bootstrapped, with power

derived from the output voltage (via OUT). When the

output is in 5V mode, the higher internal supply voltage

results in lower switch-transistor on-resistance, slightly

greater output power, and higher efficiency.

Bootstrapping allows the battery voltage to sag to 0.8V

once the system is started. Therefore, the battery volt-

forward drop of the Schottky rectifier). If the battery volt-

age exceeds the programmed output voltage, the out-

put will follow the battery voltage. This is acceptable in

many systems; however, the input or output voltage

must not be forced above 7V.

The MAX857/MAX859’s output voltage is set by two

resistors, R1 and R2 (Figure 2b), which form a voltage

divider between the output and FB. Use the following

equation to determine the output voltage:

R1 + R2

R2

To simplify resistor selection:

R1 = R2 ( _______ - 1)

Since the input bias current at FB has a maximum value

of 100nA, large values (10k

Ω to 300kΩ) can be used

for R1 and R2 with no significant accuracy loss. For 1%

error, the current through R1 should be at least 100

times FB’s bias current.

C1*

L1

D1

1N5817

C2*

R1

R2

LX

OUT

FB

LBO

LBI

REF

SHDN

GND

R3

R4

C3

0.1

µF

5

1

3

8

6

2

4

7

MAX857/MAX859

*C1 = C2 = 68

µF FOR MAX857

47

µH

C1 = C2 = 22

µF FOR MAX859

C1*

L1

D1

1N5817

C2*

R1

LX

OUT

3/5

LBO

LBI

REF

SHDN

GND

R3

R4

C3

0.1

µF

5

1

3

8

6

2

4

7

MAX856/MAX858

*C1 = C2 = 68

µF FOR MAX856

OUTPUT

SELECT

47

µH

C1 = C2 = 22

µF FOR MAX858

Figure 2b. Standard Application Circuit—Adjustable Output

Voltage

Figure 2a. Standard Application Circuit—Preset Output

Voltage