C

°

x 298

E

C

°

273

-

C

T

=

° )

(

+ E

LN

C x

°

298

»

¼

º

«

¬

ª

(

V

V

TRIP

BIAS

x

-

)

R

C

°

25

RBIAS

V

30

Temperature (°C)

60

50

60

70

80

90

100

2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

=

(T

R

)

x

:

=

k

10

e

-

+

298

1

273

93

1

E

»

¼

º

1.215 k

:

=

V

1

(

)

V

- 1

V

x 1.8

1.215 k

:

972

:

LM3561

SNOSB44C – MARCH 2011 – REVISED MAY 2013

www.ti.com

3380K, the trip point is chosen to be +93°C. The value of R(T) at 93°C is:

(6)

Figure 39 shows the linearity of the thermistor resistive divider of the previous example.

Figure 39. Thermistor Resistive Divider Response vs Temperature

and R(T) and solving for temperature. This gives the following relationship.

(7)

NTC THERMISTOR PLACEMENT

The termination of the thermistor must be done directly to the cathode of the Flash LED in order to adequately

couple the heat from the LED into the thermistor. Consequentially, the noisy environment generated from the

switching of the LM3561's boost converter can introduce noise from GND into the thermistor sensing input. To

filter out this noise it is necessary to place a 0.1µF or larger ceramic capacitor close to the LEDI/NTC pin. The

filter capacitor's return must also connect with a low-impedance trace, as close as possible to the GND pin of the

LM3561.

30

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