The charge pump supplies the voltage necessary to drive the external N-channel MOSFET. An external charge pump capacitor is placed between VCAP and SOURCE pin to provide energy to turn on the external MOSFET. In order 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 typically. If EN pins is pulled low, then the charge pump remains disabled. To ensure that the external MOSFET can be driven above its specified threshold voltage, the VCAP to SOURCE voltage must be above the undervoltage lockout threshold, typically 6.5 V, before the internal gate driver is enabled. Use Equation 1 to calculate the initial gate driver enable delay.

Equation 1.

where

• C_(VCAP) is the charge pump capacitance connected across SOURCE and VCAP pins
• V_{(VCAP_UVLOR)} = 6.5 V (typical)

To remove any chatter on the gate drive approximately 800 mV of hysteresis is added to the VCAP undervoltage lockout. The charge pump remains enabled until the VCAP to SOURCE voltage reaches 12.4 V, typically, at which point the charge pump is disabled decreasing the current draw on the SOURCE pin. The charge pump remains disabled until the VCAP to SOURCE voltage is below to 11.6 V typically at which point the charge pump is enabled. The voltage between VCAP and SOURCE continue to charge and discharge between 11.6 V and 12.4 V as shown in Figure 8-1. By enabling and disabling the charge pump, the operating quiescent current of the LM74500-Q1 is reduced. When the charge pump is disabled it sinks 5-µA typical.

Figure 8-1 Charge Pump Operation