7.3.1 Charge Pump and Gain Transitions

The input to the 1×/1.5× charge pump is connected to the V_IN pin, and the loosely regulated output of the charge pump is connected to the CP_OUT pin. In 1× mode, as long as the input voltage is less than 4.7 V, the output voltage is approximately equal to the input voltage. When input voltage is over 4.7 V the output voltage is regulated to 4.7 V. In 1.5× mode, the output voltage is always less than or equal to 4.7 V over entire input voltage range.

The gain of the charge pump is selected depending on the headroom voltage across the current sink of LM2758. When headroom voltage V_LED– (at the LED pin) drops below 300 mV (typical) the charge-pump gain transition happens from 1× to 1.5× to maintain current regulation across the LED. Once the charge pump transition to a higher gain, it remains at that gain for as long as the device remains enabled. Shutting down and then re-enabling the device resets the gain mode to the minimum gain required to maintain the load.

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 V_OUT 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 CP_OUT 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 R_SET between the I_SET pin of the LM2758 and GND.

The LED current in torch mode is approximately 1639 times greater than the current of I_SET, while the LED current in flash mode is approximately 7650 times of the same I_SET current. The feedback loop of an internal amplifier sets the voltage of the I_SET pin to 1.3 V (typical). The statements above are simplified in Equation 1:

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

7.3.6 Analog Brightness Control

Figure 7. Analog Brightness Control

The current through the LED can be varied dynamically by changing the I_SET current. Figure 7 shows the circuit. The current though the LED can be calculated with Equation 2:

Equation 2.

7.3.7 Thermal Protection

Internal thermal protection circuitry disables the LM2758 when the junction temperature exceeds 150°C (typical). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device recovers and operates normally when the junction temperature falls below 140°C (typical). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings.

7.4.1 Modes

There are four modes of operation for LM2758: the flash mode, torch mode, indicator mode and shutdown mode (see Table 1). Torch and flash modes sink a constant DC current while indicator mode operates in pulsating DC at 1/32 positive duty cycle with same current magnitude as torch mode.

7.4.2 Logic Control Pins

The LM2758 has two logic pins, EN1 and EN2. There is a 500-kΩ (typical) pulldown resistor connected from EN1 to GND and from EN2 to GND. The operating modes of the part function according to Table 1: