LTC3305_15 Datasheet, PDF (22/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Applications Information
The t_OFF parameter is programmed by a capacitor from
the CTOFF pin to GND. t_OFF is given by:
t_OFF
=
0.48hrs•
C_TOFF
10nF
C0G type capacitors are recommended due to their superior
temperature characteristics.
dissipation in the internal switch connected from V4 to
V_REG, which causes die temperature to increase. In Figure 8,
an external switching regulator generates a 3.3V rail that
backdrives the V_REG pin and provides power to the external
microprocessor and other low voltage circuits. There is
no on-chip loading on the V_REG pin and thus no on chip
power dissipation in the low voltage regulator.
In the continuous operation mode (MODE = 1), the CTON
and CTOFF pins are unused and should be connected to
GND.
Selecting Charge Pump Components
Referring to Figure 2, recommended values for R1, R2
and C_FLY are 249Ω, 1.33k & 10µF respectively for all ap-
plications. For applications in which V4 is no lower than
32V a 10µF capacitor is recommended for C_BOOST. For
applications which may have lower voltages at V4, the
recommended value for C_BOOST is 22µF. Schottky diodes
with a breakdown voltage larger than the maximum V4
voltage are recommended for diodes D1 and D2.
Selecting Decoupling Capacitors
Decoupling capacitors of at least 10µF must be placed
across each battery, from the BOOST pin to V4 and from
the AUXP pin to the AUXN pin. These capacitors must be
placed as close as possible to the LTC3305. The capacitors
must be capable of withstanding the maximum voltage
across each battery. Capacitors with an X5R or X7R type
dielectric should be used.
Thermal Considerations and Limiting On-Chip Power
Dissipation
Excessive on-chip power dissipation will cause the
LTC3305 to enter thermal shutdown. It is important to
understand the source of the power dissipation and how
power dissipation can be reduced. The two contributions
of on-chip power dissipation on the LTC3305 that may be
controlled by the user are the loading on the low voltage
regulator and the power dissipated through the current
sources that provide the NGATE pin currents.
The low voltage linear regulator provides a 2.5V output.
Any current provided by the regulator will cause power
In Figure 8, external resistors R1, R2, R3, R4, and R5 are in
series with the on-chip current sources that provide NGATE
pin current. These resistors reduce the voltage across the
on-chip current sources and thus reduce on-chip power
dissipation. As an example, the current source at NGATE1
delivers current from the V4 pin. When this current source
is turned on, the voltage across it is V4-V_NGATE1. For a
typical application with V4 = 52.8V, programmed I_NGATE =
506µA and V_NGATE1 = 6.12V, the on-chip power dissipated
in the current source is 23mW. In Figure 8, resistor R1
operates with approximately 30.5V across it. The on-chip
power dissipated in the current source is reduced to ap-
proximately 8mW. In similar fashion resistors R2, R3,
R4, and R5 reduce the on-chip power dissipation on the
respective current sources. When choosing these resistors
it is recommended to have the internal current sources
biased with at least 6V across them under all operating
conditions. Power dissipation through the on-chip current
sources may be further reduced by programming a lower
gate current through the NGATE pins.
Balancing Battery Stacks with more than Four Batteries
To balance battery stacks that have more than four bat-
teries, multiple LTC3305 devices may be stacked together.
In this scenario, it is recommended that each LTC3305 be
run in continuous mode and at least one battery in each
sub-stack of four is common to two LTC3305s. Each
LTC3305 needs an auxiliary cell for the balancing opera-
tion. Figure 9 shows an eight battery stack being balanced
using three LTC3305 devices connected together. Figure
10 shows a stack of six batteries being balanced using
two LTC3305 devices. To balance a battery stack with
n batteries, the minimum number of LTC3305 devices
required is [(n-1)/3] rounded up to the nearest integer.
In this calculation, each LTC3305 is assumed to be used
in a four-battery configuration and at least one battery
interleaves two LTC3305 devices.
3305f
22
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