Datasheet

19
LTC1325
APPLICATIONS INFORMATION
WUU
U
where,
I
F
= forward diode current,
I
F
′ = forward diode current just prior to turn off,
V
F
= forward drop,
V
R
= reverse diode voltage (approximately equal to V
DD
),
f = PWM frequency (111kHz), and
t
RR
= reverse recovery time
The power and maximum reverse voltage ratings of the
diode should be greater than P
dD1
and V
DD
respectively.
The catch diode should also have fast turn-on times to
reduce the voltage glitch at its cathode when turning on.
Schottky diodes have fast switching times and low
forward drops and are recommended for D1.
5. Trickle Resistor R
TRK
: R
TRK
sets the desired trickle
current in the battery to compensate for self-dis-
charge which is in the order 1% and 2% of capacity per
day for NiCd and NiMH batteries respectively. Trickle
charge rates are typically in the C/30 to C/50 range,
where C is battery capacity.
I
TRK
= (V
DD
– V
BAT
)/R
TRK
where V
BAT
is the voltage of a full charged battery.
Note that I
TRK
varies as the battery is being charged.
6. Thermistor R
T
and Load R
L
: The total resistance of the
thermistor network should be greater than 30k at the
high temperature extreme to minimize effects of load
regulation (see REG pin loading).
7. Fault Setting Resistors R1, R2, R3 and R4: The voltage
levels at the LTF, HTF and MCV pins are tapped from
a resistor divider powered by the REG pin. The voltage
levels are selected taking into account:
a. Manufacturer Recommended Temperature and
Voltage limits,
b. Loading on the REG Pin (< 2mA)
c. Input Voltage Ranges of the LTF, HTF and MCV
Comparators:
1.6V < V
LTF
, V
MCV
< 2.8V and 0.5V < V
HTF
< 1.3V
d. Thermistor Divider Temperature Curve
Typical temperature limits for both NiCd and NiMH
batteries are shown below.
BATTERY
TYPE
MIN MAX MIN MAX
Standard –20 45 to 50 0 45 to 50
Quick –20 45 to 50 10 45 to 50
Fast or Rapid –20 45 to 50 15 45 to 50
Trickle –20 45 to 50 0 45 to 50
Note that the discharge limits are wider than the
charge limits. To prolong battery life, manufacturers
generally recommend discharge temperatures that
are similar to the charge limits. For this reason, the
LTC1325 recognizes the same LTF and HTF limits in
both charge and discharge modes. MCV should be set
just above the charging voltage per cell given in
battery specifications. The voltage at the LTF and HTF
pins should be set to correspond to narrowest tem-
perature range. These are typically 15°C and 45°C.
The corresponding voltages may be read from the
thermistor divider temperature curve such as that
shown in Figure 5. For this thermistor, it works out to
be about for 2.12V for LTF and for 1.13V for HTF. The
MCV may be conveniently tied to LTF since MCV is
typically 2V. If desired, external analog switches under
microprocessor control may be used to vary the LTF,
HTF and MCV voltages between modes or for different
charge rates. The values of R1, R2, R3 and R4 in Figure
3 can be calculated from the following equations:
R4 = V
HTF
(RE/V
REG
)
R3 = V
MCV
(RE – R4)
R2 = V
LTF
(RE) – (R3 + R4)
R1 = RE – (R2 + R3 + R4)
where RE = R1 + R2 + R3 + R4 is chosen to minimize
loading on the REG pin. A minimum value of 30k is
recommended. Note that V
LTF
is assumed to be greater
than V
MCV
. If this is not the case, V
LTF
and V
MCV
in the
above equations should be swapped. If the MCV and
LTF pins are shorted to the same point, R2 should be
set to 0.
CHARGE TEMP
RANGE (°C)
DISCHARGE TEMP
RANGE (°C)