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ACT3780

ActivePath™ Battery Charger

Active-Semi, Inc 

ACT3780

Rev 8, 09-Jul-13

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 ACT3780 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 on the curve 2

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 VTHL

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 = (0.5V/100µA) - 0.5758 × 6.8kΩ

Finally,

R = 5kΩ - 3.9kΩ = 1.1kΩ

This result shows that adding 1.1kΩ in series with

the thermistor sets the net resistance from TH to G

to be 0.5V at 40°C, satisfying VTHH at the correct

temperature. Adding this resistance, however, also

impacts the lower temperature limit as follows:

VTHL/ITH = kCOLD(@TC) × RNOM + R

kCOLD(@TC) = (VTHL/ITH - R)/RNOM

Finally,

kCOLD(@TC) = (25kΩ - 1.1kΩ)/6.8kΩ = 3.51

Reviewing the characteristic curves, the lower

threshold is found to move to -5°C, a change of only

1°C. As a result, the system satisfies the upper

threshold of 40°C with an operating temperature

range of -5°C to 40°C, vs. our design target of 0°C

to 40°C. It is informative to highlight that due to the

NTC behavior of the thermistor, the relative impact

on the lower threshold is significantly smaller than

the impact on the upper threshold.

Fix VTHH

Following the same example as above, the

"unadjusted" results yield an operating temperature

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

40°C. In applications that favor VTHH over VTHL,

however, one can control the voltage present at TH

at low temperatures by connecting a resistor in

parallel with ITH. The desired resistance can be

found using the following equation:

(ITH + (VCHG_IN - VTHL)/R) × kCOLD(@0°C) × RNOM = VTHL

Rearranging yields

R = (VCHG_IN - VTHL)/(VTHL/(kCOLD(@0°C) × RNOM) - ITH)

R = (5V - 2.5V)/(2.5V/(2.816 × 6.8kΩ) - 100µA)

R = 82kΩ

Adding 82kΩ in parallel with the current source

increases the net current flowing into the

thermistor, thus increasing the voltage at TH.

Adding this resistance, however, also impacts the

upper temperature limit:

VTHH = (ITH + (VCHG_IN - VTHH)/R) × kHOT(@40°C) × RNOM

Rearranging yields,

Innovative PowerTM

- 10 -

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