LM74700MDDFREP datasheet(16/32 Pages) TI

16 / 32 page

9.2.2.2 MOSFET Selection

The important MOSFET electrical parameters are the maximum continuous drain current, ID, the maximum

drain-to-source voltage, VDS(MAX), and the maximum source current through body diode and the drain-to-source

On resistance RDSON.

The maximum continuous drain current, ID, rating must exceed the maximum continuous load current. The

maximum drain-to-source voltage, VDS(MAX), must be high enough to withstand the highest differential voltage

seen in the application. This would include any anticipated fault conditions. TI recommends to use MOSFETs

with voltage rating up to 60-V maximum with the LM74700-EP because anode-cathode maximum voltage is 65

V. The maximum VGS LM74700-EP can drive is 13 V, so a MOSFET with 15-V minimum VGS must be selected.

If a MOSFET with < 15-V VGS rating is selected, a zener diode can be used to clamp VGS to safe level. During

startup, inrush current flows through the body diode to charge the bulk hold-up capacitors at the output. The

maximum source current through the body diode must be higher than the inrush current that can be seen in the

application.

To reduce the MOSFET conduction losses, lowest possible RDS(ON) is preferred, but selecting a MOSFET

based on low RDS(ON) can not always be beneficial. Higher RDS(ON) will provide increased voltage information to

LM74700-EP's reverse comparator at a lower reverse current. Reverse current detection is better with increased

RDS(ON). TI recommends to operate the MOSFET in regulated conduction mode during nominal load conditions

and select RDS(ON), such that at nominal operating current, forward voltage drop VDS is close to 20-mV regulation

point and not more than 50 mV.

As a guideline, TI suggests to choose (20 mV / ILoad(Nominal)) ≤ RDS(ON) ≤ ( 50 mV / ILoad(Nominal)).

MOSFET manufacturers usually specify RDS(ON) at 4.5-V VGS and 10-V VGS. RDS(ON) increases drastically

below 4.5-V VGS and RDS(ON) is highest when VGS is close to MOSFET Vth. For stable regulation at light load

conditions, TI recommends to operate the MOSFET close to 4.5-V VGS, that is, much higher than MOSFET gate

threshold voltage. TI recommends to choose MOSFET gate threshold voltage Vth of 2-V to 2.5-V maximum.

Choosing a lower Vth MOSFET also reduces the turn ON time.

Based on the design requirements, preferred MOSFET ratings are:

• 60-V VDS(MAX) and ±20-V VGS(MAX)

• RDS(ON) at 3-A nominal current: (20 mV / 3A ) ≤ RDS(ON) ≤ ( 50 mV / 3A ) = 6.67 mΩ ≤ RDS(ON) ≤ 16.67 mΩ.

• MOSFET gate threshold voltage Vth: 2-V maximum

DMT6007LFG MOSFET from Diodes Inc. is selected to meet this 12-V reverse battery protection design

requirements and it is rated at:

• 60-V VDS(MAX) and ±20-V VGS(MAX)

• RDS(ON) 6.5-mΩ typical and 8.5-mΩ maximum rated at 4.5-V VGS

• MOSFET Vth: 2-V maximum

Thermal resistance of the MOSFET must be considered against the expected maximum power dissipation in the

MOSFET to ensure that the junction temperature (TJ) is well controlled.

9.2.2.3 Charge Pump VCAP, input and output capacitance

Minimum required capacitance for charge pump VCAP and input and output capacitance are:

• VCAP: Minimum 0.1 µF is required; recommended value of VCAP (µF) ≥ 10 × CISS(MOSFET)(µF)

• CIN: minimum 22 nF of input capacitance

• COUT: minimum 100 nF of output capacitance

9.2.3 Selection of TVS Diodes for 12-V Battery Protection Applications

TVS diodes are used in automotive systems for protection against transients. In the 12-V battery protection

application circuit shown in Figure 9-2, a bi-directional TVS diode is used to protect from positive and negative

transient voltages that occur during normal operation of the car and these transient voltage levels, and pulses

are specified in ISO 7637-2 and ISO 16750-2 standards.

LM74700-EP

SNOSDD6 – SEPTEMBER 2021

www.ti.com

Submit Document Feedback

Product Folder Links: LM74700-EP

LM74700MDDFREP Datasheet(PDF) 16 Page - Texas Instruments

LM74700MDDFREP Datasheet(HTML) 16 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL