MAX1666A Datasheet, PDF (12/17 Pages) Maxim Integrated Products – Advanced Lithium-Ion Battery-Pack Protector

Advanced Lithium-Ion
Battery-Pack Protector
B4P
I₄
+
V4
-
B3P
I_CB
=
V4
R
I_3P
+
V3
I₃
-
B2P
I_CB
=
V4
R
I_2P
+
V2
I₂
-
B1P
I_CB
=
V4
R
I_1P
+
+
+
+
+
V1
I₁
V1 R V2 R
-
-
V3 R
-
V4 R
-
-
V1
V2
V3
V4
R
R
R
R
Figure 4. Cell Voltage Sampling
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:
±2% ^{4.4V − 4.0V} = ± 0.2%
4.4V
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 = 1MΩ. 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
pulldown 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 = 1MΩ. Calculate
R5 and R6:
R6 = (VCMT / 500mV) RTOTAL
R5 = RTOTAL - R6
Maximum mismatch of a 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: I₄ = 3I_CB + V₄ / R = 4V₄ / R = BAT4 current
B3P: I₃ = I_3P + I₄ = BAT3 current
I_3P + I_CB = V3 / R ⇒ I_3P = V₃ / R - V₄ / R
I₃ = I₄ + (V₃ - V₄) / R = (3V₄ + V₃) / R
B2P: I₂ = I_2P + I₃ = BAT2 current
I_2P + I_CB = V₂ / R ⇒ I_2P = V₂ / R - V₄ / R
I₂ = I₃ + V₂ / R - V₄ / R = I₄ + (V3 - V₄) / R +
(V₂ - V₄) / R = (2V₄ + V₃ + V₂) / R
B1P: I₁ = I_1P + I₂ = BAT1 current
I_1P + I_CB = V₁ / R ⇒ I_1P = V₁ / R - V₄ / R
I₁ = I₂ + V₁ / R - V₄ / R = I₄ + (V₃ - V₄) / R +
(V₂ - V₄) / R + (V₁ - V₄) / R
= (V₄ + V₃ + V₂ + V₁) / R
when V₁ = V₂ = V₃ = V₄, I_1P = I_2P = I_3P = 0, and
I₁ = I₂ = I₃ = I₄ = 4V₄ / R
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