
M
Advanced Lithium-Ion
Battery-Pack Protector
______________________________________________________________________________________
11
Maximum mismatch of 1% external resistor-dividers is
±2%. The external adjusted overvoltage threshold accu-
racy includes the internal overvoltage threshold accuracy
(±0.5%) and the error due to the external resistor-divider
multiplied by maximum adjustment.
The total external adjusted overvoltage threshold (V
OVT
)
accuracy is ±0.7%.
Undervoltage Protection
The MAX1666 provides undervoltage protection to
avoid overdischarging the cells. When any cell is under
voltage, UVO,
DSO
, and
CGO
go high, turning off the
external charging and discharging MOSFETs.
TKO
remains low to keep the trickle charge current on.
Undervoltage threshold is linearly adjustable through
an external resistor-divider (Figure 3) from REF.
Determine the undervoltage threshold (V
UVT
) required.
V
UVT
must be between 2V and 3V. Set R
TOTAL
= R3 +
R4 = 1M
. Calculate R3 and R4 as follows:
R4 = [(V
UVT
- 2V) / (3V - 2V)] R
TOTAL
R3 = R
TOTAL
- R4
Maximum mismatch of 1% external resistor-dividers is
±2.0%, and the error due to the external resistor-divider
multiplied by maximum adjustment is ±2% (3V - 2V) / 3V
= ±0.7%. The total external adjusted undervoltage
threshold (V
UVT
) accuracy including the internal under-
voltage threshold accuracy (±2.5%) is ±3.2%.
Undervoltage Warning
When any cell drops to 100mV above the undervoltage
threshold,
WRN
goes low.
WRN
returns high when all
cells are 300mV above the undervoltage threshold.
Cell-Mismatch Protection
The MAX1666 disables charging or discharging when
mismatch occurs. When any two cells are mismatched,
TKO
,
CGO
, UVO, and
DSO
go high, turning off the
external MOSFETs.
PKF
goes low.
PKF
has a strong
pull-down current (>4mA), and can be used to control
an external thermal fuse. The cell-mismatch threshold is
linearly adjustable through an external resistor-divider
(Figure 3) from REF. Determine the cell-mismatch
threshold (V
CMT
) required. V
CMT
has to be between 0
and 500mV. Set R
TOTAL
= R5 + R6 = 1M
. Calculate
R5 and R6:
R6 = (V
CMT
/ 500mV) R
TOTAL
R5 = R
TOTAL
- R6
Maximum mismatch of 1% external resistor-divider is
±2%. The total external adjusted cell-mismatch thresh-
old accuracy, including the internal cell-mismatch
threshold accuracy (±10%), is ±12%.
Cell Voltage Sampling
The MAX1666 does not introduce cell mismatch. When
the battery cells are matched, the MAX1666 draws
close to zero current from the intermediate cells.
Figure 4 shows a simplified diagram of the voltage
sampling scheme.
B4P: I
4
= 3I
CB
+ V
4
/ R = 4V
4
/ R = BAT4 Current
B3P: I
3
= I
3P
+ I
4
= BAT3 Current
I
3P
+ I
CB
= V3 / R
I
3P
= V
3
/ R - V
4
/ R
I
3
= I
4
+ (V
3
- V
4
) / R = (3V
4
+ V
3
) / R
B2P: I
2
= I
2P
+ I
3
= BAT2 Current
I
2P
+ I
CB
= V
2
/ R
I
2P
= V
2
/ R - V
4
/ R
I
2
= I
3
+ V
2
/ R - V
4
/ R = I
4
+ (V3 - V
4
) / R +
(V
2
- V
4
) / R = (2V
4
+ V
3
+ V
2
) / R
B1P: I
1
= I
1P
+ I
2
= BAT1 Current
I
1P
+ I
CB
= V
1
/ R
I
1P
= V
1
/ R - V
4
/ R
I
1
= I
2
+ V
1
/ R - V
4
/ R = I
4
+ (V
3
- V
4
) / R +
(V
2
- V
4
) / R + (V
1
- V
4
) / R
= (V
4
+ V
3
+ V
2
+ V
1
) / R
When V
1
= V
2
= V
3
= V
4
, I
1P
= I
2P
= I
3P
= 0, and
I
1
= I
2
= I
3
= I
4
= 4V
4
/ R
±
.
= ±
2
4 4
.
V
4 4
0. %
%
4 0
V
V
+
V4
-
B3P
B2P
B1P
B4P
+
V3
-
I
3
I
4
I
3P
I
2P
I
1P
I
2
I
1
+
V2
-
+
V2
-
+
V1
-
R
R
R
R
+
V1
-
I
CB =
V4
R
I
CB =
V4
R
I
CB =
V4
R
V1
R
V2
R
+
V3
-
V3
R
+
V4
-
V4
R
Figure 4. Cell Voltage Sampling