ACT8810 datasheet(46/52 Pages) ACTIVE-SEMI

46 / 52 page

ActivePathTM CHARGER

ACT8810

Rev 4, 01-Oct-09

Active-Semi

Innovative PowerTM

- 46 -

www.active-semi.com

ActivePMUTM and ActivePathTM are trademarks of Active-Semi.

I2CTM is a trademark of Philips Electronics.

ACT8810

–

VTHH

VTHL

100µA

Li+ Battery

Pack

–

NTC

FUNCTIONAL DESCRIPTION CONT’D

where RNOM is the nominal thermistor resistance at

room temperature, and kHOT and kCOLD are the ratios

of the thermistor's resistance at the desired hot and

cold thresholds, respectively.

Figure 11:

Simple Configuration

Design Procedure

When designing with thermistors it is important to

keep in mind that their nonlinear behavior typically

allows one to directly control no more than one

threshold at a time. As a result, the design

procedure can change depending on which

threshold is most critical for a given application.

Most application requirements can be solved using

one of three cases,

1) Simple solution

2) Fix VTHH, accept the resulting VTHL

3) Fix VTHL, accept the resulting VTHH

The ACT8810 was designed to achieve an

operating temperature range that is suitable for

most applications with very little design effort. The

simple solution is often found to provide reasonable

results and should always be used first, then the

design procedure may proceed to one of the other

solutions if necessary.

In each design example, we refer to the Vishay

NTHS series of NTCs, and more specifically those

which follow a "curve 2" characteristic. For more

information on these NTCs, as well as access to the

resistance/temperature characteristic tables referred

to in the example, please refer to the Vishay

website at http://www.vishay.com/thermistors.

Simple Solution

The ACT8810 was designed to accommodate most

requirements with very little design effort, but also

provides flexibility when additional control over a

design is required. Initial thermistor selection is

accomplished by choosing one that best meets the

following requirements:

RNOM = 5kΩ/kHOT, and

RNOM = 25kΩ/kCOLD

where kHOT and kCOLD for a given thermistor can be

found on its characteristic tables.

Taking a 0°C to 40°C application using a "curve 2"

NTC for this example, from the characteristic tables

one finds that kHOT and kCOLD are 0.5758 and 2.816,

respectively, and the RNOM that most closely

satisfies these requirements is therefore around

8.8kΩ. Selecting 10kΩ as the nearest standard

value, calculate kCOLD and kHOT as:

kCOLD = VTHL/(ITH × RNOM) = 2.5V/(100µA × 10kΩ) = 2.5

kHOT = VTHH/(ITH × RNOM) = 0.5V/(100µA × 10kΩ) = 0.5

Identifying

these

values

the

curve

characteristic tables indicates that the resulting

operating temperature range is 2°C to 44°C, vs. the

design goal of 0°C to 40°C. This example

demonstrates that one can satisfy common

operating temperature ranges with very little design

effort.

Fix VTHH

For demonstration purposes, supposing that we

had selected the next closest standard thermistor

value of 6.8kΩ in the example above, we would

have obtained the following results:

kCOLD = VTHL/(ITH × RNOM) = 2.5V/(100µA × 6.8kΩ) = 3.67

kHOT = VTHH/(ITH × RNOM) = 0.5V/(100µA × 6.8kΩ) = 0.74

which, according to the characteristic tables would

have resulted in an operating temperature range of

-6°C to 33°C vs. the design goal of 0°C to 40°C.

In this case, one can add resistance in series with

the thermistor to shift the range upwards, using the

following equation:

(VTHH/ITH) = kHOT(@40°C) × RNOM + R

R = (VTHH/ITH) - kHOT(@40°C) × RNOM

R = (2.5V/100µA) - 0.5758 × 6.8kΩ

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