5V/3.3V/3V/Adjustable, High-Efficiency,

_______________________________________________________________________________________

7

NAME

FUNCTION

1

VOUT

Sense Input for regulated-output operation. Internally connected to an on-chip voltage divider and to

the variable duty-cycle, on-demand oscillator. It must be connected to the external regulated output.

2

LBO

Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at LBI drops

below 1.28V.

3

LBI

Low-Battery Input. When the voltage at LBI drops below 1.28V, LBO sinks current.

PIN

4

GND

Ground

______________________________________________________________Pin Description

5

LX

Drain of a PMOS power switch that has its source connected to V+. LX drives the external inductor,

which provides current to the load.

6

V+

Positive Supply-Voltage Input. Should not exceed 11.5V

7

VFB

8

SHDN

Dual-Mode Feedback Pin. When VFB is grounded, the internal voltage divider sets the output to 5V

(MAX639), 3.3V (MAX640) or 3V (MAX653). For adjustable operation, connect VFB to an external volt-

age divider.

Shutdown Input — active low. When pulled below 0.8V, the LX power switch stays off, shutting down

the regulator. When the shutdown input is above 2V, the regulator stays on. Tie SHDN to V+ if shut-

down mode is not used.

____________________Getting Started

Designing power supplies with the MAX639/MAX640/

MAX653 is easy. The few required external components

are readily available. The most general applications use

the following components:

(1) Capacitors: For the input and output filter capaci-

tors, try using electrolytics in the 100µF range, or

use low-ESR capacitors to minimize output ripple.

Capacitor values are not critical.

(2) Diode: Use the popular 1N5817 or equivalent

Schottky diode.

(3) Inductor: For the highest output current, choose a

100µH inductor with an incremental saturation cur-

rent rating of at least 600mA. To obtain the highest

efficiencies and smallest size, refer to the

Inductor

Selection section.

_______________Detailed Description

Figure 1 shows a simplified, step-down DC-DC con-

verter. When the switch is closed, a voltage equal to

through the inductor ramps up, storing energy in the

inductor’s magnetic field. This same current also flows

into the output filter capacitor and load. When the switch

opens, the current continues to flow through the inductor

in the same direction, but must also flow through the

diode. The inductor alone supplies current to the load

when the switch is open. This current decays to zero as

the energy stored in the inductor’s magnetic field is

transferred to the output filter capacitor and the load.

V+

L

Figure 1. Simplified Step-Down Converter

SWITCH OFF

0V

SWITCH ON

0A

SWITCH OFF

SWITCH ON

Figure 2. Simplified Step-Down Converter Operation