LTC1325 LINER [Linear Technology], LTC1325 Datasheet - Page 20

• Manufacturer Part Number: LTC1325
• Description: Microprocessor-Controlled Battery Management System
• Manufacturer: LINER [Linear Technology]

LTC1325
APPLICATIONS
8. REG Pin Loading: The 3.072V regulator has a load
9. R
10. PFET(P1) and NFET(N1): For operation of the charge
Charging from Supplies Above 16V
In many applications, the charging supply is greater than
the 16V maximum V
can easily be adapted to charge the batteries from a
charging supply V
external sub-circuits:
1. A regulator to drop V
2. A level shifter between the PGATE and the gate of the
3. A voltage clamp on the V
The Wide Voltage Battery Charger circuit in the Typical
Application section shows low cost implementations of all
three sub-circuits. C1, R11 and D4 generate a 15V V
the LTC1325. D3, R12 and C2 form a level shifter. The
zener D3 is chosen to clamp the source gate voltage of the
20
regulation specification of – 5mV/mA. Since the ADC
uses the same regulator as reference, it is desirable to
reduce loading effects on the REG pin especially over
temperature. Thermistors with R
10k at 25 C are recommended. At 50 C, the ther-
mistor resistance could drop by a factor of 3 from its
value at 25 C. R
on Temperature Sensing. The temperature coefficient
of R
dominates the sensing circuit.
a value within the battery discharge specifications and
must have a power rating above I
I
and discharge loops, V
and DIS pins swing between 0 and V
to minimize power dissipation. The power ratings of
P1 and N1 should be above I
I
above V
range of the LTC1325.
PFET, P1, to ensure that P1 can be completely turned
off when PGATE rises to V
pulling V
DIS
DIS
DIS
2
= V
: R
L
[R
is not critical since the thermistor tempco
DIS
DS(ON)
BAT
DD
BAT
is selected to limit the discharge current to
.
/[R
above V
(N1)] respectively. V
DIS
DC
DD
U
L
is chosen as explained in the section
+ R
that is above 16V by adding three
rating of the LTC1325. The LTC1325
INFORMATION
DD
DS(ON)
DC
U
.
down to within the supply
BAT
GS
DD
(N1)]
pin to prevent R
CHRG
.
< V
W
DD
TO
2
DIS
DS(MAX)
[R
DD
values of at least
since the PGATE
2
DS(ON)
(R
. V
DIS
GS
should be
U
(P1)] and
) where:
TRK
<< V
DD
from
for
DD
PFET to within the maximum gate source voltage rating of
the latter. Finally, D2 clamps V
Charging Batteries with Voltages Above 16V
To charge a battery with a maximum (fully charged) voltage
of above 16V, the charging supply V
Thus the charger will need the regulator, level shifter and
clamp mentioned in the previous section. In addition, an
external battery divider must be added to limit the voltage at
the V
application circuit, Wide Voltage Battery Charger. The resis-
tors R9 and R10 are selected to divide the battery voltage by
the number of cells in the battery and the battery divider
internal to the LTC1325 is set to divide-by-1. The external
divider prevents V
the BATP (Battery Present Flag) to be high regardless of
whether the battery is physically present or not. This does not
affect the other operations of the LTC1325.
SOFTWARE DESIGN
A general charging algorithm consists of the following
stages:
Under some operating and storage conditions, NiCd and
NiMH batteries may not provide full capacity. In particular,
repeated shallow charge and discharge cycles cause the
“memory effect” in NiCd batteries. In order to restore full
capacity (battery conditioning), these batteries have to be
subjected to several deep discharge/charge cycles which
will be provided by repetitions of the above algorithm.
Figure 6 shows a simplified flowchart of a charging algo-
rithm. In practice, this flowchart has to be augmented to
take into account the occurrence of fail-safes at any point
in the algorithm. For example, the battery temperature
could rise above HTF during discharging or charging.
General programming notes are as follows:
1. The start bit is always high.
2. The SGL/DIFF bit is generally set to low so that the ADC
Discharge Before Charge
Fast Charge
Top Off Charge
Trickle Charge
makes conversions with respect to ground.
BAT
pin to less than V
BAT
from ever rising to V
DD
. This is shown in the typical
BAT
to 15V.
DC
must be above 16V.
DD
and this causes