LTC1325CSW datasheet(19/24 Pages) LINER

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LTC1325

APPLICATIONS INFORMATION

where,

IF = forward diode current,

IF′ = forward diode current just prior to turn off,

VF = forward drop,

VR=reversediodevoltage(approximatelyequaltoVDD),

f = PWM frequency (111kHz), and

tRR = reverse recovery time

The power and maximum reverse voltage ratings of the

diode should be greater than PdD1 and VDD 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 RTRK: RTRK 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.

ITRK = (VDD – VBAT)/RTRK

where VBAT is the voltage of a full charged battery.

Note that ITRK varies as the battery is being charged.

6. Thermistor RT and Load RL: 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 < VLTF, VMCV < 2.8V and 0.5V < VHTF < 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

Quick

– 20

45 to 50

Fast or Rapid

– 20

45 to 50

Trickle

– 20

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 = VHTF(RE/VREG)

R3 = VMCV(RE – R4)

R2 = VLTF (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 VLTF is assumed to be greater

than VMCV. If this is not the case, VLTF and VMCV 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)

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