LM3554 datasheet(33/47 Pages) TI1

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IN x

(

)

OUT - V

OUT

(

)

PEAK

- '

LOAD =

OUT

1.05V

TX2

TX1/TORCH

Force Torch or

LED Shutdown (VIN Monitor, TX2 or

NTC only)

Internal to

LM3554

LEDI/

NTC

R(T)

0.1 PF

VIN Monitor

BIAS

LM3554

www.ti.com

SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

Product Folder Links: LM3554

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Figure 43. Thermistor Voltage Divider and Sensing Circuit

8.2.2.5 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. Consequently, the noisy environment generated from the boost

converter switching can introduce noise from GND into the thermistor sensing input. To filter out this noise it is

necessary to place a 0.1-µF 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 to the PGND pin of the device as possible.

8.2.2.6 Maximum Load Current (Voltage Mode)

Assuming the power dissipation in the LM3554 and the ambient temperature are such that the device does not

hit thermal shutdown, the maximum load current as a function of IPEAK is:

where

•

η is efficiency and is found in the efficiency curves in the Typical Characteristics

(8)

and

(9)

Figure 44 shows the theoretical maximum output current vs theoretical efficiency at different input and output

voltages using Equation 8 and Equation 9 for

ΔIL and ILOAD with a peak current of 2.5 A. Figure 44 represents the

theoretical maximum output current (for the LM3554 in voltage-output mode) that the device can deliver just

before hitting current limit.

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