The charge pump supplies the voltage necessary to drive the external N-channel MOSFET. An external charge pump capacitor is placed between VCAP+ and VCAP– pins to provide energy to turn on the external MOSFET. For the charge pump to supply current to the external capacitor, the EN pin voltage must be above the specified input high threshold, V_{(EN_IH)}. When enabled, the charge pump sources a charging current of 300-µA typical. If EN pins is pulled low, then the charge pump remains disabled. To make sure that the external MOSFET can be driven above the specified threshold voltage of the MOSFET, the VCAP+ to VCAP–voltage must be above the undervoltage lockout threshold, which is typically 6.6 V, before the internal gate driver is enabled. Use Equation 1 to calculate the initial gate driver enable delay.

where

• C_(VCAP) is the charge pump capacitance connected across ANODE and VCAP pins
• V_{(VCAP_UVLOR)} = 6.6 V (typical)

To remove any chatter on the gate drive, approximately 900 mV of hysteresis is added to the VCAP undervoltage lockout. The charge pump remains enabled until the VCAP+ to VCAP–voltage reaches the typical 13 V, typically, at which point the charge pump is disabled, decreasing the current drawn on the ANODE pin. The charge pump remains disabled until the VCAP+ to VCAP– voltage is below the typical 12.1 V, enabling the charge pump. The voltage between VCAP+ to VCAP– continue to charge and discharge between 12.1 V and 13 V as shown in Figure 8-1. By enabling and disabling the charge pump, the operating quiescent current of the LM74703-Q1 is reduced. When the charge pump is disabled, the charge pump typically sinks 5 µA.

Figure 8-1 Charge Pump Operation