LTC4090(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4090
(Rev.:RevA)

USB Power Manager with 2A High Voltage Bat-Track Buck Regulator

Linear Technology 

LTC4090/LTC4090-5

APPLICATIONS INFORMATION

In the explanation below, the following notation is used.

R25C = 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 R25C

rHOT= Ratio of RNTC|HOT to R25C

RNOM = Primary Thermistor Bias Resistor (see Figure 8)

R1 = Optional Temperature Range Adjustment resistor

(see Figure 9)

The trip points for the LTC4090/LTC4090-5’s tempera-

ture qualification are internally programmed at 0.29 •

VNTC for the hot threshold and 0.74 • VNTC for the cold

threshold.

Therefore, the hot trip point is set when:

RNTC|HOT • VNTC = 0.29 • VNTC

RNOM + RNTC|HOT

and the cold trip point is set when:

RNTC|COLD • VNTC = 0.74 • VNTC

RNOM + RNTC|COLD

Solving these equations for RNTC|COLD and RNTC|HOT results

in the following:

RNTC|HOT = 0.409 • RNOM

and

RNTC|COLD = 2.815 • RNOM

By setting RNOM equal to R25C, the above equations result

in rHOT = 0.409 and rCOLD = 2.815. Referencing these ratios

to the Vishay Resistance-Temperature Curve 2 chart gives

a hot trip point of about 50°C and a cold trip point of about

0°C. The difference between the hot and cold trip points

is approximately 50°C.

By using a bias resistor, RNOM, different in value from

R25C, the hot and cold trip points can be moved in either

direction. The temperature span will change somewhat due

to the non-linear behavior of the thermistor. The following

equations can be used to easily calculate a new value for

the bias resistor:

RNOM

=

rHOT

0.409

•

R25C

RNOM

=

rCOLD

2.815

•

R25C

VNTC

6

RNOM

10k

NTC

5

RNTC

10k

NTC BLCOK

0.738 • VNTC

–

+

–

0.29 • VNTC

+

TOO_COLD

TOO_HOT

+

0.1V

–

NTC_ENABLE

4090 F08

Figure 8. Typical NTC Thermistor Circuit

VNTC

6

RNOM

13.2k

NTC

5

NTC BLCOK

0.738 • VNTC –

+

TOO_COLD

R1

1.97k

RNTC

10k

–

0.29 • VNTC

+

+

0.1V

–

TOO_HOT

NTC_ENABLE

4090 F09

Figure 9. NTC Thermistor Circuit with Additional Bias Resistor

4090fa

23

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