LTC3305_15 Datasheet, PDF (21/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Applications Information
Selection of External NMOS Switches
The external NMOS switches must be capable of with-
standing a reverse voltage equal to the battery stack volt-
age. They should also be capable of carrying DC current
up to the PTC thermistor trip point. The maximum power
dissipated in the NMOS should not cause it to operate
outside of its Safe Operating Area. Refer to Table 5 for a
list of recommended NMOS switches.
Table 5. Recommended NMOS Switches
PART NUMBER MANUFACTURER I_DS(MAX)
V_DC(MAX)
SiR882DP
Vishay
60A
100V
SiS892DN
Vishay
25A
100V
IPD70N10S3-12
Infineon
70A
100V
IPB35N10S3L-26
Infineon
35A
100V
RJK1051DPB
Renesas
60A
100V
RJK1054DPB
Renesas
92A
100V
the NMOS device. Programming a higher current reduces
the NMOS device turn on time. Programming a large gate
source voltage reduces the on resistance of the NMOS
device. During turn off, the gate capacitor discharges
through the gate source resistor.
Programming Undervoltage and Overvoltage
Thresholds
Referring to the Block Diagram, the voltage at the I_SET pin
is servoed to 1.2V. An external resistor, R_ISET, from this
pin to GND programs a current which is divided down and
mirrored to the V_L and V_H pins. The I_SET pin current has
a programmed range from 50µA to 150µA.
The I_SET pin current is given by:
I_ISET
=
1.2V
R_ISET
Programming NMOS Turn On
The NMOS switches are turned on by developing a voltage
across an external resistor from the gate to the source.
The current through the resistor is delivered from the
NGATE pins and is programmed by the current at the
I_SET pin. The internal current sources that provide the
NGATE pin currents operate from the V4 and BOOST
supplies as shown in the Block Diagram. The BOOST
pin voltage is regulated at 8.45V greater than V4. It is
recommended that the gate turn on voltage be set to no
more than 7.5V. The current flowing through the gate
turn on resistor connected to the NGATE3 pin is given by:
I_NGATE3
=
26.4V
R_ISET
The current flowing through the other NGATE pins is
given by:
I_NGATE
=
13.2V
R_ISET
The NGATE3 current has a programmed range from 1mA
to 3mA. All other NGATE currents have a programmed
range from 500µA to 1.5mA.
The NGATE current initially charges the gate capacitor of
the NMOS device to turn it on. The external gate source
resistor maintains a constant gate to source voltage on
The current out of the V_L and V_H pins is given by:
I_VL
=
I_VH
=
I _ISET
3
External resistors R_VL from the V_L pin to GND and R_VH
from the V_H pin to GND program the undervoltage and
overvoltage thresholds for each battery. The undervolt-
age threshold for a battery is given by:
V_BAT,UV
=
4V
•
R_VL
R_ISET
The overvoltage threshold for a battery is given by:
V_BAT,OV
=
4V •
R_VH
R_ISET
Programming the t_BAT Parameter
The t_BAT parameter is programmed using a capacitor
from the CTBAT pin to GND. t_BAT is given by:
t_BAT
=
5sec•
C_TBAT
10nF
A C0G type capacitor is recommended due to its superior
temperature characteristics.
Programming the t_ON and t_OFF Parameters
The t_ON parameter is programmed by a capacitor from
the CTON pin to GND. t_ON is given by:
t_ON
=
0.48hrs
•
C_TON
10nF
3305f
For more information www.linear.com/LTC3305
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