MAX11068EVKIT+ Maxim Integrated Products, MAX11068EVKIT+ Datasheet - Page 25

• Manufacturer Part Number: MAX11068EVKIT+
• Description: KIT SMART BATT MEASUREMENT 12CH
• Manufacturer: Maxim Integrated Products

Specifications of MAX11068EVKIT+

• Main Purpose: Power Management, Battery Monitor, Car
• Utilized Ic / Part: MAX11068
• Primary Attributes: Monitors Current, Voltage, Temperature
• Secondary Attributes: 1 ~ 12 Cell- Li-Ion, 1 ~ 12 Cell- NiMH
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Embedded: -

There are two methods of protecting the MAX11068 from
damage due to an open circuit occurring in a series bat-
tery stack:
U An external fuse placed in series with the internal or
U Detection of any cell dropping below ground or vio-
The MAX11068 allows external cell balancing to be
implemented by using the internal switch to control
the bias network of an external transistor. When the
internal switch is closed, the external resistors R
and R
external bipolar or MOSFET transistor. The discharge
current of the battery is set with resistor R
ing sections describe different external cell-balancing
circuits in more detail. Figure 15 is a simplified external
cell-balancing circuit.
When using an external bipolar transistor, it is recom-
mended to select one with current gain (h
than 100 and a V
pack voltage to avoid damage should an open circuit
occur in the cell stack. If a bipolar transistor with a lower
voltage rating is chosen, then series fuses are recom-
Figure 15. Simplified External Cell-Balancing Circuit
Protection from Open-Cell Faults
external cell-balancing circuit protects against high-
voltage damage. If an external MOSFET is used, as in
the circuits described below, the high-value resistors
protect the MAX11068 inputs from damage during an
open-cell condition.
lating the undervoltage condition indicates an open-
cell condition and that the electric motor is supplying
voltage to the battery pack. To prevent damage, the
switches connecting the battery stack to the load
should be opened immediately after the undervolt-
age flag asserts.
BIAS2
External Cell-Balancing Circuit
form the bias network used to turn on the
CE
12-Channel, High-Voltage Sensor, Smart
External Cell Balancing with
voltage that is rated to the overall
R
EQ
a Bipolar Transistor
EQ
R
R
R
BIAS1
SW
BIAS2
. The follow-
FE
) greater
BIAS1
Data-Acquisition Interface
Figure 16. External Cell-Balancing Circuit with a Bipolar
Transistor
mended to protect the circuit. Typical component values
for a 500mA cell discharge current are (see Figure 16):
R
R
Bipolar transistor = MJD50 (for high-voltage tolerance) or
When using an external MOSFET, it is recommended to
select one with low V
V
avoid damage should an open circuit occur in the cell
stack. If a MOSFET with lower voltage rating is chosen,
series fuses are recommended to protect the circuit. See
Figure 17. Typical component values for a 500mA cell
discharge current are:
R
R
R
MOSFET = NTK3134N (for low cost, low voltage)
External cell balancing with a MOSFET switch results in
little to no cell-to-cell interaction. The R
combined with the input bias current requirements does
add a small measurement error of less than 1mV worst
case for a 10kω R
The recommended NTK3134N FETs have built-in gate-
protection diodes. During hot-plug conditions, inrush
current flows through R
charge the HV to DCIN capacitor. This current creates a
negative V
tion diodes and possibly damage the transistor devices.
A series resistor of no less than 470ω should be placed
in series with the transistor gate to make the circuit
robust under cell hot-plug conditions. For other transis-
tors, the negative V
that it is tolerated by the devices.
DS
BIAS
EQ
BIAS1
GATE = 470ω
EQ
= 8I
voltage that is rated to the overall pack voltage to
= 8I
External Cell Balancing with a MOSFET
= 80I
= 10kI
F1
F2
GS
voltage that can turn on the gate-protec-
R
EQ
BIAS value.
MMBTA05 (for low cost and low volt-
age)
GS
D
CLAMP
GS
condition must be controlled so
BIAS
R
BIAS1
(typically around 1.2V) and a
and internal ESD diodes to
R
R
BIAS2
GATE
C
F
BIAS
resistor value
C
C
N+1
N
25