GATE connected

to ANODE

GATE to ANODE Voltage Regulated

0 mV

20 mV

50 mV

-5 mV

Forward Regulation Mode

Reverse Current

Protection Mode

Full Conduction Mode

GATE connected

to VCAP

8

LM74700-Q1

SNOSD17 – OCTOBER 2017

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Feature Description (continued)

7.3.3 Gate Driver

The gate driver is used to control the external N-Channel MOSFET by setting the GATE to ANODE voltage to

the corresponding mode of operation. There are three defined modes of operation that the gate driver operates

under; forward regulation, full conduction mode and reverse current protection, according to the ANODE to

CATHODE voltage. Forward regulation mode, full conduction mode and reverse current protection mode are

described in more detail in Regulated Conduction Mode, Full Conduction Mode and Reverse Current Protection

Mode respectively. Figure 2 depicts how the modes of operation vary according to the ANODE to CATHODE

voltage of the LM74700-Q1. The threshold between forward regulation mode and conduction mode is typically

when the ANODE to CATHODE voltage is 50 mV. The threshold between forward regulation mode and reverse

current protection mode is typically when the ANODE to CATHODE voltage is –5 mV.

Figure 2. Gate Driver Mode Transitions

Before the gate driver is enabled the three conditions must be achieved:

•

The EN pin voltage must be greater than the specified input high voltage.

•

The VCAP to ANODE voltage must be greater than the undervoltage lockout voltage.

•

The ANODE voltage must be greater than the specified minimum operating voltage.

If all three conditions are not achieved, the GATE pin is internally connected to the ANODE pin, assuring that the

external MOSFET is disabled. Once these conditions are achieved the gate driver operates in the correct mode

depending on the ANODE to CATHODE voltage.

7.3.4 Enable

The LM74700-Q1 has an available enable pin, EN. The enable pin allows for the gate driver to be either enabled

or disabled by an external signal. If the EN pin voltage is greater than the rising threshold, the gate driver and

charge pump operates as described in Gate Driver and Charge Pump respectively. If the enable pin voltage is

less than the input low threshold, the charge pump and gate driver are disabled placing the LM74700-Q1 in

shutdown mode. The EN pin can withstand a voltage as large as 65 V and as little at –65 V. This allows for the

EN pin to be connected directly to the ANODE pin if enable functionality is not needed. It is not recommended to

leave the EN pin floating.

7.4 Device Functional Modes

7.4.1 Shutdown Mode

The LM74700-Q1 enters shutdown mode when the EN pin voltage is below the specified input low threshold or

the ANODE to GND voltage is below the minimum operating voltage. Both the gate driver and the charge pump

ANODE pin only sinking typically 3 µA and the CATHODE pin sinking typically less than 1 µA. When the

LM74700-Q1 is in shutdown mode, forward current flow through the external MOSFET is not interrupted but is

conducted through the MOSFET's body diode.