LTC1733 查看數據表（PDF） - Linear Technology

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LTC1733

Monolithic Linear Lithium-Ion Battery Charger with Thermal Regulation

Linear Technology 

LTC1733

APPLICATIO S I FOR ATIO

resistance of the NTC thermistor rises. The LTC1733 is

designed to go into hold mode when the value of the NTC

thermistor increases to seven times the value of RHOT. For

a Vishay NTHS0603N02N1002J thermistor, this value is

28.2k which corresponds to approximately 0°C. The hot

and cold comparators each have approximately 2°C of

hysteresis to prevent oscillation about the trip point. The

NTC function can be disabled by grounding the NTC pin.

VCC

RHOT

1%

7/8 VCC

–

+

NTC

RNTC

10k

1/2 VCC

+

–

TOO COLD

TOO HOT

3/160 VCC

+

–

LTC1733

Figure 3.

DISABLE NTC

1733 F03

Thermistors

The LTC1733 NTC trip points were designed to work with

thermistors whose resistance-temperature characteris-

tics follow Vishay Dale’s “R-T Curve 2”. The Vishay

NTHS0603N02N1002J is an example of such a ther-

mistor. However, Vishay Dale has many thermistor prod-

ucts that follow the “R-T Curve 2” characteristic in a variety

of sizes. Futhermore, any thermistor whose ratio of RCOLD

to RHOT is about 7.0 will also work (Vishay Dale R-T Curve

2 shows a ratio of RCOLD to RHOT of 2.816/0.4086 = 6.9).

NTC Layout Considerations

It is important that the NTC thermistor not be in close

thermal contact with the LTC1733. Because the LTC1733

package can reach temperatures in excess of the 50°C trip

point, the NTC function can cause a hysteretic oscillation

which turns the charge current on and off according to the

package temperature rather than the battery temperature.

This problem can be eliminated by thermally coupling the

NTC thermistor to the battery and not to the LTC1733.

Furthermore, it is essential that the VCC connection to

RHOT is made according to standard Kelvin sense tech-

niques. Since VCC is a high current path into the LTC1733,

it is essential to minimize voltage drops between the VCC

input pin and the top of RHOT.

NTC Trip Point Errors

When a 1% resistor is used for RHOT, the major error in

the 50°C trip point is determined by the tolerance of the

NTC thermistor. A typical 10k NTC thermistor has a ±10%

tolerance. By looking up the temperature coefficient of the

thermistor at 50°C, the tolerance error can be calculated

in degrees centigrade. Consider the Vishay

NTHS0603N02N1002J thermistor which has a tempera-

ture coefficient of –3.3%/°C at 50°C. Dividing the toler-

ance by the temperature coefficient, ±10%/(–3.3%/°C) =

±3°C, gives the temperature error of the hot trip point.

The cold trip point is a little more complicated because its

error depends on the tolerance of the NTC thermistor and

the degree to which the ratio of its value at 0°C and its value

at 50°C varies from 7 to 1. Therefore, the cold trip point

error can be calculated using the tolerance, TOL, the

temperature coefficient of the thermistor at 0°C, TC

(in %/°C), the value of the thermistor at 0°C, RCOLD, and

the value of the thermistor at 50°C, RHOT. The formula is:

Temperature Error (°C) =

 1+ TOL

 7

• RCOLD

RHOT



– 1

• 100

TC

For example, the Vishay NTHS0603N02N1002J thermistor

with a tolerance of ±10%, TC of –4.5%/°C, and RCOLD/

RHOT of 6.89, has a cold trip point error of:

Temperature

Error

(°C)

=





1±

0.10

7

•

6.89

–



1

• 100

–4.5

= –1.8°C, +2.5°C

sn1733 1733fs

11

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