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LTC4095EDC

Standalone USB Li-Ion/Polymer Battery Charger in 2mm × 2mm DFN

Linear Technology 

LTC4095

APPLICATIONS INFORMATION

Alternate NTC Thermistors and Biasing

The LTC4095 provides temperature qualified charging if a

grounded thermistor and a bias resistor are connected to

the NTC pin. By using a bias resistor whose value is equal

to the room temperature resistance of the thermistor (R25)

the upper and lower temperatures are pre-programmed

to approximately 40°C and 0°C, respectively (assuming

a Vishay “Curve 1” thermistor).

The upper and lower temperature thresholds can be ad-

justed by either a modification of the bias resistor value

or by adding a second adjustment resistor to the circuit.

If only the bias resistor is adjusted, then either the upper

or the lower threshold can be modified but not both. The

other trip point will be determined by the characteristics

of the thermistor. Using the bias resistor in addition to an

adjustment resistor, both the upper and the lower tempera-

ture trip points can be independently programmed with

the constraint that the difference between the upper and

lower temperature thresholds cannot decrease. Examples

of each technique are given below.

NTC thermistors have temperature characteristics which

are indicated on resistance-temperature conversion tables.

The Vishay-Dale thermistor NTHS0603N011-N1003F, used

in the following examples, has a nominal value of 100k

and follows the Vishay “Curve 1” resistance-temperature

characteristic.

IN

8

RNOM

100k

LTC4095 NTC BLOCK

0.765 • IN

(NTC RISING) –

NTC

+

6

RNTC

100k

–

0.349 • IN

(NTC FALLING) +

TOO_COLD

TOO_HOT

In the explanation below, the following notation is used.

R25 = Value of the thermistor at 25°C

RNTC|COLD = Value of thermistor at the cold trip point

RNTC|HOT = Value of the thermistor at the hot trip point

rCOLD = Ratio of RNTC|COLD to R25

rHOT = Ratio of RNTC|HOT to R25

RNOM = Primary thermistor bias resistor (see Figure 3)

R1 = Optional temperature range adjustment resistor (see

Figure 4)

The trip points for the LTC4095’s temperature qualifica-

tion are internally programmed at 0.349 • IN for the hot

threshold and 0.765 • IN for the cold threshold.

Therefore, the hot trip point is set when:

RNTC|HOT

• IN = 0.349 • IN

RNOM + RNTC|HOT

and the cold trip point is set when:

RNTC|COLD • IN = 0.765 • IN

RNOM + RNTC|COLD

IN

8

RNOM

105k

0.765 • IN

(NTC RISING) –

NTC

+

6

R1

12.7k

RNTC

100k

–

0.349 • IN

(NTC FALLING) +

TOO_COLD

TOO_HOT

+

0.017 • IN –

(NTC FALLING)

NTC_ENABLE

4095 F03

Figure 3. Typical NTC Thermistor Circuit

+

0.017 • IN –

(NTC FALLING)

NTC_ENABLE

4095 F04

Figure 4. NTC Thermistor Circuit with Additional Bias Resistor

4095fa

11

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