LM3560 Datasheet, PDF(21/42 Page) Texas Instruments – LM3560 Synchronous Boost Flash Driver w/ Dual 1A High-Side Current Sources (2A Total Flash Current)

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LM3560 Datasheet, PDF (21/42 Pages) Texas Instruments – LM3560 Synchronous Boost Flash Driver w/ Dual 1A High-Side Current Sources (2A Total Flash Current)

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FIGURE 13. VIN Monitor/VIN Flash Monitor Test Circuit

30113899

INPUT VOLTAGE FLASH MONITOR (FLASH CURRENT

RISING)

A second comparator at the IN terminal is available to monitor

the input voltage during the flash current turn-on (Input Volt-

age Flash Monitor). Bit [3] of the VIN Monitor Register en-

ables/disables this feature. With this bit set to â1â, the VIN

Flash Monitor is active, and bits [5:4] of the VIN Monitor Reg-

ister program the 4 selectable thresholds of (2.9V, 3.0V, 3.1V,

and 3.2V). The VIN Flash Monitor operates as follows: during

flash current turn-on the current sources will transition

through each of the lower flash current levels until the target

flash current is reached. With the Input Voltage Flash Monitor

active, if during the flash current turn-on the input voltage falls

below the VIN Flash Monitor threshold, the flash current is

stopped at the level that the current ramp had risen to, at the

time of the VIN Flash Monitor event. The VIN Flash Monitor

only operates during the ramping up of the flash LED current.

The VIN Flash Monitor ignores the first 2 flash codes during

the flash current turn-on. As a result, if the VIN Flash Monitor

is enabled and VIN were to fall below the VIN Flash Threshold

as the LED current ramps up through either of the first two

levels, then the flash pulse would not be halted until code #3

(168.75mA per current source).

To avoid noise from falsely triggering the VIN Flash Monitor,

this mode incorporates an 8Âµs de-glitch timer as well as an

internal analog filter at the input of the VIN Flash Monitor

Comparator. With the VIN Flash Monitor active, VIN must go

below the VIN Flash Monitor Threshold (VIN_FLASH), and re-

main below it, for 8Âµs before the flash current ramp is halted

and the VIN Flash Monitor Flag is written.

LAST FLASH REGISTER

Once the VIN Flash Monitor is tripped, the flash code that

corresponded to the LED current at which the flash current

ramp was halted is written to the Last Flash Register. The Last

Flash Register is a read only register and has the lower 4 bits

available to latch the code for LED1 and the upper 4 bits to

latch the code for LED2.

For example, suppose that the LM3560 is set-up for a single

LED with a target flash current of 1250 mA and the VIN Flash

Monitor is enabled with the VIN Flash Monitor threshold set

to 3.0V (VIN Monitor Register bits [5:4] = 0, 1). When the

STROBE input is brought high, the LED current begins ramp-

ing up through the torch and flash current codes at 32 Âµs/

code. As the input current increases, the input voltage at the

LM3560âs IN pin begins to fall due to the source impedance

of the battery. By the time the LED current has reached 1000

mA (code 0x77 or 500 mA per current source), VIN falls below

3.0V. The VIN Flash Monitor will then stop the flash current

ramp and the LM3560 will continue to proceed with the flash

pulse, but at 1000 mA instead of 1250 mA. details this se-

quence.

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