MAX1665S Datasheet, PDF (7/12 Pages) Maxim Integrated Products – Lithium-Ion Battery Pack Protector

Lithium-Ion Battery Pack Protector
across each cell to determine if the voltage levels are
within operating range.
As depicted in the typical operating circuit, when CHG
and DSG are high, the MOSFETs are on, allowing the
cells to charge or discharge. However, when the
charge or discharge current becomes excessive, the
device turns off the FETs, enters a standby mode, and
periodically samples the current to determine if the fault
condition is removed. The MAX1665 does not sample
the current directly, but rather measures the differential
voltage across BN and PKN produced by the charge or
discharge current flowing through the drain-to-source
resistance of the MOSFETs. A preset voltage threshold
is used to prevent excess current flow (see Electrical
Characteristics).
In operating mode, all of the MAX1665 versions con-
sume less than 25µA of quiescent current, allowing
long-term battery storage without significantly affecting
battery life. In standby mode, these devices consume
less than 1µA of quiescent current.
Overvoltage Protection
When any individual cell voltage rises above V_OV (over-
voltage limit), the charge MOSFET control pin (CHG) is
driven to PKN, thereby disconnecting the charger from
the cells. The MAX1665 measures each cell of the pack
differentially to prevent overcharging on a cell-by-cell
basis. The charging process resumes when the highest
cell voltage drops below V_OV - 100mV (see Typical
Operating Circuit).
Undervoltage/Overdischarge Protection
Discharge can occur whenever the voltage of each cell
is above the undervoltage threshold voltage (V_UV, typi-
cally 2.50V). If the voltage on any of the cells falls
below V_UV, CHG latches to PKN and DSG latches to
BN. Quiescent current falls to under 1µA as the device
enters the standby mode. The latch resets when BN
exceeds PKN by 18mV.
During charge mode, when BN is greater than PKN, the
latch is held in reset, which disables the undervoltage
comparator feature and allows charging on the cells.
During the initialization process, as cells are connected
to the MAX1665, the device considers this a low-volt-
age condition and disables CHG and DSG until a
charging source is applied to create at least an 18mV
differential between BN and PKN.
During long-term storage, the battery will self-dis-
charge until it reaches the undervoltage threshold.
When this happens, the MAX1665 enters standby
mode. Normal operating mode resumes when a charg-
er is connected, causing BN to rise 18mV above PKN.
Overcurrent Protection
When the MAX1665 detects overcurrent in the system,
it disables the charge or discharge process by con-
necting CHG to PKN and DSG to BN, turning off the
external MOSFETs (see Typical Operating Circuit). In
charge mode, the MAX1665 detects overcharge when
the voltage from BN to PKN exceeds +250mV. In dis-
charge mode, overdischarge occurs when the differen-
tial voltage is less than -250mV. During any overcurrent
condition, CHG and DSG are gated at 12Hz until the
overcurrent is removed.
If both an overvoltage and overcharge condition exist,
the overvoltage condition takes priority. Likewise, if
undervoltage and overdischarge conditions exist, the
overdischarge condition takes priority. For more
details, see Tables 1 and 2.
Cell Current Balancing
When the battery cells are matched, the MAX1665
draws zero current from the intermediate nodes. The
MAX1665 draws current from the top terminal only.
Figure 2 shows a simplified diagram of the voltage
sampling scheme. The following equations show that,
for balanced cells, the differential discharge currents
are zero:
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.
Due to process variations, the MAX1665 does draw a
minute current (70nA ~ 150nA) from the intermediate
node even when the cells are matched. This current
difference exists in the sampling mode, which is 1/32 of
the whole time period, making the average of this cur-
rent 2nA to 5nA.
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