__________Applications Information

Figure 4 shows a MAX877 step-up application circuit.

This circuit starts up and operates with inputs ranging

from 1.0V to 6.2V. Start-up time is a function of the load,

typically less than 5ms. Output current capability is a

function of the input voltage (see

Typical Operating

Characteristics).

The converters will regulate down to the output voltage

and seamlessly switch into boost mode as the input

drops below the output voltage. This is especially use-

ful in battery-powered applications, where the battery

voltage may initially exceed the output voltage. To gen-

erate 5V from four alkaline cells in series, the input

ranges from 6.2V to 3.6V. When the battery pack is

fresh, the MAX877 will step down with the active rectifi-

er acting as the switch. As the batteries approach 5V,

or the desired output voltage, the converter’s control

circuitry will ensure a smooth transition into step-up

mode. The converter operates until the batteries are

less than 3V; efficiency is typically 80% with fresh bat-

Inductor Selection

The 22µH inductor shown in the

Typical Operating

Circuit is sufficient for most MAX877/MAX878/MAX879

designs. Other inductor values ranging from 10µH to

47µH are also suitable. The inductor should have a satu-

ration rating equal to or greater than the peak switch-

current limit, which is 1A without an external current limit

(ILIM connected to IN). It is acceptable to operate the

inductor at 120% of its saturation rating; however, this

may slightly reduce efficiency. For highest efficiency,

use an inductor with a low DC resistance, preferably

under 0.2

Ω. Table 1 lists suggested inductor suppliers.

Capacitor Selection

The 100µF, 10V surface-mount tantalum (SMT) output

capacitor shown in the

Typical Operating Circuit will

provide a 25mV output ripple or less, stepping up from

3V to 5V at 200mA. Smaller capacitors, down to 10µF,

are acceptable for light loads or in applications that

can tolerate higher output ripple. The input capacitor

may be omitted if the supply has low output impedance

and the input lead length is less than 2 inches (5cm) or

the loads are small.

The primary factor in selecting both the output and input

filter capacitor is low ESR. The ESR of both bypass and

filter capacitors affects efficiency. Optimize performance

by increasing filter capacitors or using specialized low-

ESR capacitors. The smallest low-ESR SMT tantalum

capacitors currently available are Sprague 595D or 695D

series. Sanyo OS-CON organic-semiconductor through-

hole capacitors also exhibit very low ESR, are rated for

the wide temperature range, and are especially suitable

for operation at cold temperatures (below 0°C).

Table 1 lists suggested capacitor suppliers.

5V/3.3V/3V/Adjustable-Output,

Step-Up/Step-Down DC-DC Converters

_______________________________________________________________________________________

9

INPUT

1.0V TO 6.2V

IN

ILIM

SHDN

100µF

OUTPUT

5V

AGND

MAX877

LX

OUT

PGND

22µH

ON/OFF

22

µF

1

2

7

4

3

6

5

SHDN

PGND

MAX879

OUT

AGND

7

1

3

4

8

6

L1

22

µH

5

LX

FB

2

IN

ILIM

C1

22µF

R1

C3

C2

100µF

R2

Figure 3. MAX879 Adjustable Voltage

Figure 4. MAX877 Standard Application Circuit