SP6641A Datasheet, PDF (11/13 Pages) Sipex Corporation – 500mA Alkaline DC/DC Boost Regulator in SOT-23

APPLICATION INFORMATION: continued
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
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
130µA 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.
V_BATT
C1
22µF
Diode
Schottky
C2
22µF
L1
22µH
®
1
LX
V_BATT ⁵
U1
2 ^SP6641
GND
3
V_OUT
SHDN ⁴
SHDN
J1
1
2
3
V_OUT
+3.3V
2.7V, 0.9Ω PMOS
SOT23-3 IRLML6302
3 ^Q1 2
3.3V to
Nonvolatile Function
R1
1M
1
C3
.22F Supercap
3
1 ^{Q2 SOT23-3 IRLML2402}
2.7V NMOS
2
Figure 16. SP6641A 3.3V with SuperCap Switch
V_BATT
C1
22µF
L1
22µH
D1
Diode
Schottky
C2
22µF
1 _LX
V_BATT ⁵
®
2
GND
U1
SP6641
3 _VOUT
SHDN ⁴
1.8V THRES. VTHRES = 1.24V(1 + R1/R2)
R1
44k
R2
100k
U2
1
SPX432M
REF
A3
2 _K SOT23-3
BATT GOOD
R4 D2
D3
100k 1N4148 1N4148
R3
20k
2V = BATT GOOD
Q2
0V = LOW BATT
2N3904
R5
100k
V_OUT 3.3V
Figure 17. SP6641A 3.3V with Low Battery Detection
Rev. 3/5/02, *Patent Pending
SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23
11
© Copyright 2002 Sipex Corporation