ACT8828 datasheet(44/50 Pages) ACTIVE-SEMI

44 / 50 page

ActivePath

TM CHARGER

ACT8828

Rev 7, 15-Nov-12

Innovative Power

- 44 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of NXP.

FUNCTIONAL DESCRIPTION CONT’D

following equation:

(VTHH/ITH) = kHOT(@40°C) × RNOM + R

R = (VTHH/ITH) - kHOT(@40°C) × RNOM

R = (2.5V/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 VTHL

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 VTHL over VTHH,

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:

Figure 8:

Fix VTHH Configuration

Figure 9:

Fix VTHL Configuration

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