11

Rev. 3/5/02,

*Patent Pending

SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23

© Copyright 2002 Sipex Corporation

Low Battery Circuit for SP6641

Application

The circuit in figure 17 uses the Sipex SPX432

shunt regulator as a reference and comparator

circuit to detect a low battery condition and give

a high level, typically 1.7V output. When the

battery is good, the SPX432 output is low, but

not at ground but at 0.8V or about one Vbe

below the 1.24V reference. To translate that

level to a CMOS Low of less than 0.4V, an NPN

and 2 signal diodes can be added to the SPX432

Figure 17. SP6641A 3.3V with Low Battery Detection

APPLICATION INFORMATION: continued

output, as shown.

The small SOT23-3pin

SPX432 and 2N3904 bipolar transistor and di-

odes are small and inexpensive to add to the

SP6641 circuit and work well to add a Battery

Low detection circuit, with the addition of about

130

µA current from 3.3V out. As a bonus, the

output of this circuit can be used to drive the

SP6641 SHDN_N pin 3 to GND when the bat-

tery is removed, which would reset the SP6641

and eliminate the need for the SuperCap Switch

shown in figure 16.

GND

LX

L1

22

µH

100k

U1

SP6641

SHDN

®

®

C2

22

µF

R3

20k

Diode

Schottky

1

2

3

C1

22

µF

5

4

R5

100k

BATT GOOD

D2

D3

1N4148

R2

100k

R1

44k

1N4148

Q2

2N3904

2V = BATT GOOD

0V = LOW BATT

R4

1

2

3

REF

K

A

1.8V THRES.

VTHRES = 1.24V(1 + R1/R2)

U2

SPX432M

SOT23-3

D1

GND

LX

SHDN

J1

L1

22

µH

2.7V, 0.9

Ω PMOS

SOT23-3 IRLML6302

Q1

U1

SP6641

SHDN

®

®

C2

22

µF

+3.3V

Diode

Schottky

1

2

3

C1

22

µF

5

4

1

2

3

C3

.22F Supercap

3.3V to

Nonvolatile Function

32

1

3

1

2

Q2 SOT23-3 IRLML2402

2.7V NMOS

R1

1M

Figure 16. SP6641A 3.3V with SuperCap Switch