LM3550SPXSLASHNOPB datasheet(22/43 Pages) TI2

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VFB

200 mV/DIV

IFlash = 3A

TIME

(10 ms/DIV)

VCAP = 4.94V

VOH = 600 mV

VCAP

1V/DIV

VLED-

1V/DIV

2V/DIV

VLED+BAL (ave.) = 4V

VLED+BAL (peak) = 4.4V

VFB

200 mV/DIV

IFlash = 3A

TIME

(10 ms/DIV)

VCAP = 5.36V

VOH = 900 mV

VCAP

1V/DIV

VLED-

1V/DIV

2V/DIV

VLED+BAL (ave.) = 4V

VLED+BAL (peak) = 4.4V

LM3550

SNVS569C – MAY 2009 – REVISED OCTOBER 2016

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Device Functional Modes (continued)

Example:

If VLED (peak)= 4.1 V (at 1.5 A), IFLASH (total) = 3 A, CSC = 0.5 F, RESR = 50 mΩ, RDSON = 40 mΩ, VFB = 100 mV,

RBAL = 75 mΩ, and tFLASH = 64 ms, then VOH = 0.604 V and VCAP = 4.97 V.

NOTE

VLED (peak) is equal to the LED voltage before self-heating occurs. Once current flows

through the LED, the LED heats up, and the forward voltage decreases until it reaches a

steady-state level. This voltage drop is dependent on the LED and the PCB layout.

Based on this calculation, setting the overhead voltage to 600 mV in the Current Control Register should ensure

a regulated 3-A flash pulse over the entire flash duration.

Unlike fixed voltage mode, optimal charge mode adjusts the super-capacitor voltage upon changes in LED

forward voltage and variation in super-capacitor ESR, ensuring that the super-capacitor does not charge to a

voltage higher than needed. By charging optimally, the LM3550 can potentially charge the super-capacitor to its

EOC state faster due to the target voltage being lower, and it helps ease the thermal loading on the current

source FET during the flash.

7.4.1.9.1

Optimal Charge Mode vs Fixed Voltage Mode

Figure 42. Optimal Charge Mode

Figure 43. 5.3-V Fixed Voltage Charge Mode

Peak power dissipation across current source FET

PNFET (maximum) = IFLASH × (VOH − VFB)

where

•

Optimal mode = 1.5 W, fixed voltage mode (5.3 V) = 2.4 W

(8)

Average power dissipation across current source FET (64 ms pulse)

PNFET (average) = IFLASH × [VOH − (VDROOP ÷ 2) − VFB]

where

•

Optimal mode = 936 mW, fixed voltage mode (5.3 V) = 1.824 W

(9)

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