MIC2755 Datasheet, PDF (6/12 Pages) Micrel Semiconductor – Battery System Supervisor Preliminary Information

MIC2755
Applications Information
Outputs
Since the MIC2755 outputs are open-drain MOSFETs, most
applications will require pull-up resistors. The value of the
resistors should not be too large or leakage effects may
dominate.
Programming Thresholds
There are separate resistive-divider configurations for cir-
cuits that require or do not require manual reset capability.
Configuration Without Manual Reset
See Figure 1. The battery-OK threshold is calculated using:
V_BAT(OK)
=
V_REF

R1+
R2 + R3
R4
+
R4 
The low-battery threshold is calculated using:
V_BAT(low)
=
V_REF
 R1+
R2 + R3 +
R3 + R4
R4 
The dead-battery threshold is calculated using:
V_BAT(dead)
=
V_REF

R1+ R2 + R3 + R4
R2 + R3 + R4

where, for all equations:
V_REF = 1.24V
In order to provide the additional criteria needed to solve for
the resistor values, the resistors can be selected such that
they have a given total value, that is, R1 + R2 + R3 + R4 =
R_total. A value such as 1MΩ for R_total is a reasonable value
because it draws minimum battery current per resistor ladder
but has no significant effect on system accuracy.
When working with large resistors, a small amount of leakage
current can cause voltage offsets that degrade system accu-
racy. The maximum recommended total resistance from
V_BAT to ground is 3MΩ.
V_BAT
V_BAT
R1
572k
R2
28k
R3
55.6k
R4
344k
100k 100k 100k
MIC2755
VDD
/POF
PTH
/NMI
NTH
/RST
RTH(/MR) GND
POF
NMI
RST
Figure 1. Example Circuit without Manual Reset
Once the desired trip points are determined, set the V_BAT(OK)
threshold first.
For a typical single-cell lithium ion battery, 3.6V is a reason-
able “OK threshold” because at 3.6V the battery is moder-
ately charged. Solving for R4:
V_BAT(OK)
=
3.6V
=
1.24V
 1MΩ
 R4 
R4 = 344kΩ
Micrel
To determine the resistor values for V_BAT(low) threshold, set
R4 = 344kΩ and solve for R3.
V_BAT(low)
=
3.1V
=
1.24V


1MΩ 
R3 + R4
R3 = 56k
Once R3 and R4 are determined, the equation for V_BAT(dead)
can be used to determine R2. A single lithium-ion cell should
not be discharged below 2.5V. Many applications limit the
drain to 2.9V. Using 2.9V for the V_BAT(dead) threshold allows
calculating the following resistor values.
V_BAT(dead)
=
2.9V
=
1.24V


R2
+
1MΩ
55.6k +
344k


R2 = 27.4k
R1 = 1MΩ – R2 – R3 – R4 = 572k
Configuration With Manual Reset
See Figure 2. To use manual reset, the MIC2755 requires a
separate resistor ladder for the switch and fresh-battery
threshold. The remaining two thresholds are set by the three-
resistor ladder.
V_BAT
V_BAT
R6
656k
R7
344k
R8
573k
R9
26.7k
SW
R10
400k
100k 100k 100k
MIC2755
VDD
/POF
PTH
/NMI
NTH
/RST
RTH(/MR) GND
POF
NMI
RST
Figure 2. Example Circuit with Manual Reset
V_BAT(OK)
=
V_REF

R6 + R7
R7

V_BAT(low)
=
V_REF
 R8
+
R9 +
R10
R10 
V_BAT(dead)
=
V_REF

R8 + R9 + R10
R9 + R10

where, for all equations:
V_REF = 1.24V
Once the desired trip points are determined, set R6 + R7 =
1MΩ and solve for R7.
V_BAT(fresh)
=
3.6V
=
1.24V


1MΩ
R7


R7 = 344k
R6 = 1MΩ – 344k = 656k
The remaining resistor values are solved in a similar manner
as the above.
1MΩ = R8 + R9 + R10
MIC2755
6
February 2000