The LED can be turned off using the MAX684's shutdown control pin or the MAX1916's enable control pin. For white LEDs with a forward voltage of 3.5 to 4.2V (at 20mA), a boost converter is usually required. A charge pump (such as MAX682 to MAX684) can be used together with the MAX1916 to form the driving circuit of the LED. Show. The MAX682~MAX684 convert the 2.7V input voltage to a 5.05V output with output currents of 250mA, 100mA, and 50mA, respectively. In the circuit of Figure 1, the MAX684 is in shutdown mode, the supply current is reduced to 22μA; when RSET=43kΩ, the LED current is 22mA;
Figure 2 shows the LED current control circuit using a charge pump. The typical value of the feedback regulation voltage is 1.235V, Ipk=1.235/R SENSE. The 24Ω current-sense resistor can supply 50mA to the diode. When the charge pump is operating, the output voltage rises to the turn-on voltage of the LED and the LED begins to conduct. The typical forward voltage of a white LED is 3.5 (1 ± 0.1) V. With the feedback regulation voltage, the bias voltage provided at the output of the MAX1759 is 4.735V. The output voltage ripple of the circuit is within 40mV, which will not cause significant changes in the LED output, which is usually not visible to the human eye. In addition, the circuit of Figure 2 has no DC path between the input and output in the off state.
