dV

T

C

dI

'

T0

Body Diode Voltage Drop

1.1 V

VOUT

LM74610-Q1

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SNOSCZ1A – JULY 2015 – REVISED OCTOBER 2015

Feature Description (continued)

very small period of time (2% typical) which rules out the chances of overheating the MOSFET, regardless of the

output current. Once the capacitor voltage reaches its high threshold, the MOSFET is turned off and charge

pump circuity is deactivated until the Vcap reaches its low voltage threshold (T1). The voltage difference between

Vcap high and low threshold is typically 1.15V. The LM74610-Q1 charge pump has 46µA charging capability with

5-8MHz frequency.

Figure 9. Vcap Charging and Discarding by the Charge Pump

The Vcap current consumption is 1 µA (typical) to drive the gate. The MOSFET OFF time (T0) and ON time (T1)

can be calculated using the following expression

(1)

Where:

•

C = Vcap Capacitance

•

dV = 1.15V

•

dI = 46 µA for charging

•

dI = 0.95 µA for discharging

Note: Temperature dependence of these parameters – The duty cycle is dependent on temperature since the

capacitance variation over temperature has a direct correlation to the MOSFET OFF and ON periods and the

frequency. If the capacitor varies 20% the periods and the frequency will also vary by 20% so it is recommended

to use a quality X7R/COG cap and not to place the cap in close proximity to high temperature devices. The

variation of the capacitor does not have a thermal impact in the application as the duty cycle does not change.

7.3.3.3 Gate Drive Pin

When the charge pump capacitor is charged to the high voltage level of 6.3V (typ), the Gate Drive pin provides a

6.8µA (typ) of drive current. When the charge pump capacitor reaches its lower voltage threshold of 5.15V (typ),

Gate is pulled down to the Anode voltage (Vin). When Anode voltage goes negative, the Gate voltage is pulled

down to Anode voltage with 160mA pull down current.

7.3.3.4 Gate Pull Down Pin

The Gate Pull Down pin is connected to the Gate Drive pin in a typical application circuit. When the controller

detects negative polarity, possibly due to failure of the input supply or voltage ripple, the Pull-Down quickly

discharges the MOSFET gate through a discharge transistor. This fast pull down react/s regardless of the Vcap

charge level. If the input supply abruptly fails, as would happen if the supply gets shorted to ground, a reverse

current will temporarily flow through the MOSFET. This reverse current can be due to parallel connected supplies

Anode and Cathode pins due to reverse current reaches -20mV (typical), the LM74610-Q1 immediately reacts

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