LM3550SPXSLASHNOPB datasheet(30/43 Pages) TI2

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TA (°C)

1.0e1

1.0

1.0e-1

1e5

1e4

1e3

105

VALD/TEMP

VBIAS = 1.8V

RTHERMISTOR = 100k

@ 25°C

=4500, R1 = 6.367k

R1 is then:

= 7.959 k

)

100

298

273

0.8V x 7.959 k

1.8V ± 0.8V

6.367 k

(

)

TRIP

- V

BIAS

)

TRIP

(

TRIP

( )

298

273

LM3550

SNVS569C – MAY 2009 – REVISED OCTOBER 2016

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8.2.2.1.4

ALD/TEMP Components

8.2.2.1.4.1

NTC Selection

NTC thermistors have a temperature-to-resistance relationship of:

where

•

β is given in the thermistor data sheet

•

R25°C is the thermistor's value at 25°C

(11)

R1 in is chosen so that it is equal to:

where

•

RT(TRIP) is the thermistors value at the temperature trip point

•

VBIAS is shown in Figure 50

•

VTRIP = 800 mV (typical)

(12)

Choosing R1 here gives a more linear response around the temperature trip voltage. For example, with VBIAS =

1.8 V, a thermistor whose nominal value at 25°C is 100 k

Ω, and a β = 4500 K, the trip point is chosen to be

85°C. The value of R(T) at 85°C is:

(13)

Setting the ALD/TEMP Sense High Register to N = 50 or hex 0x32 places the upper trip point to approximately

800 mV. Voltages higher than 800 mV prevent the flash LED from turning on. Based on Figure 50, the Sense

Low Register can be set to a lower code to give a second LED current threshold (70% flash). Voltages lower

than the value stored in the Sense Low Register allow a full current flash.

Figure 50. Thermistor Resistive Divider Response vs Temperature

If the temperature changes during a flash event, meaning VALS/TEMP crosses the sense high and/or sense low

values, the current scales to the appropriate zone current.

Place the thermistor as close as possible to the flash LEDs. This provides the best thermal coupling (lowest

thermal resistance).

LM3550SPX/NOPB Datasheet(PDF) 30 Page - Texas Instruments

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