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LM2758

SNVS551E – APRIL 2008 – REVISED MAY 2016

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Feature Description (continued)

7.3.2 Soft Start

The LM2758 contains internal soft-start circuitry to limit inrush currents when the part is enabled. Soft start is

implemented internally with a controlled turnon of the internal voltage reference.

7.3.3 Current Limit Protection

The LM2758 charge pump contains current limit protection circuitry that protects the device during VOUT fault

conditions where excessive current is drawn. Output current is limited to 1.2 A typically.

7.3.4 Flash Time-out Feature

Flash time-out protection circuitry disables the current sinks when the signal on EN1 and EN2 is held high for

more than 814 msec (typical). This prevents the device from self-heating due to the high power dissipation during

flash conditions. During the time-out condition, voltage is still present on CPOUT but the current sinks are shut off,

resulting in no current through the flash LED. When the device goes into a time-out condition, placing a logic low

signal on EN1 and EN2 resets the timeout; a subsequent logic high signal on EN1 or EN2 returns the device to

normal operation.

7.3.5 Setting LED Currents

The current through the LED can be set by connecting an appropriately sized resistor RSET between the ISET pin

of the LM2758 and GND.

The LED current in torch mode is approximately 1639 times greater than the current of ISET, while the LED

current in flash mode is approximately 7650 times of the same ISET current. The feedback loop of an internal

amplifier sets the voltage of the ISET pin to 1.3 V (typical). The statements above are simplified in Equation 1:

ILED = GAINFLASH/TORCH × (1.3 / RSET)

(1)

The maximum recommended current through LED is 500 mA in torch mode / 700 mA in flash mode.

NOTE

If the ISET for torch mode setting at 500 mA, the flash mode would be over 700 mA

(maximum). See Figure 6. Using the device in conditions where the junction temperature

might rise above the rated maximum requires that the operating ranges and/or conditions

be de-rated. The printed circuit board also must be carefully laid out to account for high

thermal dissipation in the part.