LTC1325CSW datasheet(17/24 Pages) LINER

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LTC1325

APPLICATIONS INFORMATION

(3). Alternatively,

β may be calculated from the RT/RTO

ratio using equation (4) or from

α, using equation (6).

As a numerical example, consider the Panasonic

ERT-D2FHL103S thermistor which has the following char-

acteristics:

1. RT (25°C) = RTO = 10k

α = –4.6%/°C at TO = 25°C

3. Ratio R25/R50 = 2.9

Using equation (4) and R25/R50 = 2.9, β = (323 × 298)In

(2.9)/(298 – 323) = 4099k. Alternatively, using equation

(6) and

α = –4.6%/°C, β = –(–0.046)(298)2 = 4085k.

Both values of

β are close to each other. Substituting

β = 4085k into equation (3) gives RL = 10k [4085 – (2 ×

298)]/[4085 + (2

× 298)] = 7.45k. The nearest 1% resistor

value is 7.5k. Figure 5 shows a plot of VDIV(T) measured

at various temperatures for this thermistor with a 7.5k RL.

Figure 5. ERT-D2FHL103S Divider

T = [2.605 – VDIV(T)]/0.034. The straight line approxima-

tion is accurate to within 2

°C over a temperature range of

°C to 45°C, assuming 3% β and 10% RTO tolerances.

PTC (Positive Temperature Coefficient) Thermistors

Positive Temperature Coefficient (PTC) thermistors may

be used in battery chargers that do not require accurate

temperature measurements. The resistance vs tempera-

ture characteristics of PTC exhibits a sharp increase at a

selectable switch temperature TS. This sharp change is

exploited in chargers which use TCO (Temperature Cutoff)

∆TCO (Difference between battery and ambient tem-

perature). With TCO termination, a voltage divider consist-

ing of a PTC and a low temperature coefficient load resistor

is connected between REG and GND with the top end of the

PTC at REG. The PTC is mounted on the battery to sense

its temperature. The divider output is tied to TBAT. When

the switch temperature is reached, the PTC resistance

increases sharply causing TBAT to fall below HTF. This

causes an HTF fault and charging is terminated. To imple-

ment

∆TCO termination, the load resistor can, in principle,

be replaced by a matching PTC and the divider now

responds to differences between battery and ambient

temperature. With both TCO and

∆TCO terminations, the

position of the battery temperature PTC can be swapped

with the load resistor or ambient temperature PTC. In both

cases, an LTF fault terminates charge when the trip point

is reached. Note that in practice, matched PTCs are not

readily available and for

∆TCO termination, NTC ther-

mistors are recommended.

HARDWARE DESIGN PROCEDURE

This section discusses the considerations in selecting

each component of a simple battery charger (see Figures

3 and 4). Further applications assistance is provided in

Application Note 64, using the LTC1325 Battery Manage-

ment IC.

1. RSENSE: There are three factors in selecting RSENSE:

a. LTC1325 VREF and Duty Ratio Settings

b. Sense Resistor Dissipation

c. ILOAD(RSENSE) < – 450mV for Gas Gauge Linearity

TEMPERATURE (

°C)

–60

– 0.5

1.0

1.5

2.0

4.5

3.0

–20

LTC1325 • F05

0.5

3.5

4.0

2.5

–40

ACTUAL

IDEAL

There are two methods of calculating battery or ambient

temperature from ADC readings of the TBAT or TAMB

channels. The first method is to store the VDIV(T) vs T

curve as a lookup table. The second method is to use a

straight line approximation. The equation of this line may

be calculated from the slope dVDIV/dT at TO [see equation

(7)] and assuming that the line passes through the point

[TO, VDIV(TO)] on the curve. For the ERT-D2FHL103S, the

slope is minus 34mV/

°C and the equation of the line is

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