_______________Detailed Description

Operating Principle

The MAX866/MAX867 combine a switch-mode regula-

tor, N-channel power MOSFET, precision voltage refer-

ence, and power-fail detector in a single monolithic

device. The MOSFET is a “sense-FET” type for best effi-

ciency, and has a very low gate threshold voltage to

ensure start-up with low battery voltages (0.8V typ).

PFM Control Scheme

The MAX866/MAX867 control scheme (Figure 1) com-

bines low-voltage efficiency (80% typ) with low battery

drain (100µA typ). There is no oscillator; switching is

accomplished by a pair of one shots that set a maxi-

mum LX on-time (4.5µs typ) and a minimum LX off-time

(1µs). LX on-time will be terminated early if the inductor

current reaches 0.5A before 4.5µs elapses. With the

standard application circuit (Figure 2a), LX current is

typically less than 50mA, so LX on-time is normally not

terminated by the 0.5A limit and lasts the complete

4.5µs. The LX on-resistance is typically 1

Ω to minimize

switch losses. The MAX866/MAX867 switching frequen-

cy depends on load, input voltage, and inductor value,

and it can range up to 250kHz with typical component

values.

3.3V/5V or Adjustable-Output,

Single-Cell DC-DC Converters

_______________________________________________________________________________________

7

MAX866/MAX867

SHDN

3/5*

LBO

LBI

N

LBI COMPARATOR

ERROR COMPARATOR

CURRENT-LIMIT

COMPARATOR

ONE-SHOT

TRIG

Q

Q

ONE-SHOT

TRIG

S

Q

R

F/F

MINIMUM

OFF-TIME

ONE-SHOT

LX

N

GND

OUT

**

FB**

**

*

*

REF

REFERENCE

MAXIMUM

ON-TIME

ONE-SHOT

*MAX866 ONLY

**MAX867 ONLY

Figure 1. Block Diagram