Datasheet

LTC1998
7
1998fb
QUICK DESIGN GUIDE
How to Calculate the External Resistor Values
The LTC1998 is a low battery warning indicator and is es-
pecially designed for monitoring the voltage of single-cell
Lithium-Ion batteries. The LTC1998 compares its supply
pin (BATT) to an accurate internal reference; if the battery
voltage falls below the programmed low battery threshold
voltage of the LTC1998, the battery low pin (BATTLO ) will
change state, from high to low, to indicate a low battery
condition. The low battery threshold voltage is programmed
via the voltage threshold adjust pin (V
TH.A
). A hysteresis
adjust pin (V
H.A
) will add hysteresis to the programmed
value of the low battery threshold voltage.
Typical Application
Table 1: Design Equations for R1, R2, R3, Figure 1
Choose desired values for:
• V
BATT.Th
: Low Battery Threshold Voltage
• V
HYST
: Hysteresis Voltage
• I
R
: Max Allowable Resistor Current
Solve:
RRRR
V
I
RR
V
VV
RR
V
V
R
RR RR
TOTAL
R
TOTAL
BATT Th HYST
TOTAL
BATT Th
TOTAL
=++ =
=
+
⎛
⎝
⎜
⎞
⎠
⎟
=
⎛
⎝
⎜
⎞
⎠
⎟
=
12 3
42
1
5
1
2
5
11
312
.
•–
•––
––
.
.
Figure 1. Low Battery Threshold Detector with Hysteresis
Example 1: A system using a 4.2V (fully charged) Lithium-
Ion battery requires a low battery threshold of 2.7V, 100mV
hysteresis and can allow up to 4.2μA maximum resistor
current.
R
TOTAL
= 1MΩ, R1 = 786k, R2 = 66k and R3 = 148k
Choose standard 1% values.
R1 = 787k, R2 = 66.5k, R3 = 147k
1998 F01
LTC1998
R1
1%
R2
1%
R3
1%
V
H.A
V
TH.A
BATT
V
LOGIC
BATTLO
4
3
2
1
5
6
REGULATOR
μP
Li-Ion
GND
0.1μF
1.5V TO 4.2V
V
LOGIC
+