The charge pump supplies the voltage
necessary to drive the external N-channel MOSFET. An external charge pump capacitor is
placed between VCAP and VS pin to provide energy to turn on the external MOSFET. For the
charge pump to supply current to the external capacitor the EN/UVLO 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/UVLO 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 VS voltage must be above the undervoltage
lockout threshold, typically 6.5 V, before the internal gate driver is enabled. Use
Equation 3 to calculate the
initial gate driver enable delay.
Equation 1.
where
C(VCAP) is the charge
pump capacitance connected across VS 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 VS voltage reaches 12.4 V,
typically, at which point the charge pump is disabled decreasing the current draw on
the VS pin. The charge pump remains disabled until the VCAP to VS voltage is below
to 11.6 V typically at which point the charge pump is enabled. The voltage between
VCAP and VS 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
LM74502 is reduced. When the charge pump is disabled it sinks 5-µA typical.