AP74700Q

Document number: DS45004 Rev. 3 - 2

13 of 21

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July 2022

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AP74700Q

Application Information (continued)

DIODES™ DMT6007LFGQ MOSFET is selected to meet this 12V reverse battery protection design requirements and it is rated at:



60V VDS(MAX) and ±20V VGS(MAX)



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



MOSFET VTH: 2V maximum

Thermal resistance of the MOSFET should be considered against the expected maximum power dissipation in the MOSFET to ensure that the

junction temperature (TJ) is well controlled.

Charge Pump C_CP and CIN/COUT

Minimum required capacitance for charge pump VCP and input/output capacitance are:



C_CP: Minimum 0.1μF is required; recommended value of VCP (μF) ≥ 10 x CISS (MOSFET) (μF)



CIN: Minimum 22nF of input capacitance



COUT: Minimum 1

μF of output capacitance.

TVS Selection for 12V Battery Protection Application

TVS diodes are used in automotive systems for protection against transients. In the 12V battery protection application circuit shown below, a

bidirectional TVS diode is used to protect the system 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.

There are two important specifications, breakdown voltage and clamping voltage of the TVS. Breakdown voltage is the voltage at which the TVS

diode goes into avalanche similar to a Zener diode and is specified at a low current value typical 1mA, and the breakdown voltage should be

higher than worst case steady state voltages seen in the system. The breakdown voltage of the TVS+ should be higher than 24V jump start

voltage and 35V suppressed load dump voltage and less than the maximum ratings of AP74700Q (65V). The breakdown voltage of TVS- should

be beyond maximum reverse battery voltage -16V, so that the TVS- is not damaged due to long time exposure to reverse connected battery.

Clamping voltage is the voltage the TVS diode clamps in high current pulse situations and this voltage is much higher than the breakdown voltage.

TVS diodes are meant to clamp transient pulses and should not interfere with steady state operation. In the case of ISO 7637-2 Pulse 1, the input

voltage goes up to -150V with a genera

tor impedance of 10Ω. This translates to 15A flowing through the TVS- and the voltage across the TVS

would be close to its clamping voltage.

AP74700Q

ANODE

GATE

CATHODE

VCP

EN

GND

Voltage

Regulator

(DC/DC, LDO)

TVS

DIODES

SMBJ33CA

Q1,

DMT6007LFG Q

CIN,

22 nF

COUT,

47 uF

C_CP,

0.1 uF

12 V

BATTERY

The next criterion is that the absolute maximum rating of ANODE to CATHODE reverse voltage of the AP74700Q (-75V) and the maximum VDS

rating MOSFET are not exceeded. In the design example, 60V rated MOSFET is chosen and maximum limit on the CATHODE to ANODE voltage

is 60V. In case of ISO 7637-2 Pulse 1, the ANODE of AP74700Q is pulled down by the ISO pulse and clamped by TVS-. The MOSFET is turned

off quickly to prevent reverse current from discharging the bulk output capacitors. When the MOSFET turns off, the CATHODE to ANODE voltage

seen is equal to (TVS clamping voltage + output capacitor voltage). If the maximum voltage on output capacitor is 16V (maximum battery voltage),

then the clamping voltage of the TVS- should not exceed, (60V

– 16V) = -44V.

The SMBJ33CAQ TVS diode can be used for 12V battery protection application. The breakdown voltage of 36.7V meets the jump start, load dump

requirements on the positive side and 16V reverse battery connection on the negative side. During ISO 7637-2 Pulse 1 test, the SMBJ33CAQ

clamps at -44V with 15A of peak surge current as shown below and it meets the clamping voltage

≤ 44V. SMBJ series of TVS' are rated up to

600W peak pulse power levels. This is sufficient for ISO 7637-2 Pulses and suppressed load dump (ISO-16750-2 Pulse B).

TVS

DIODES™

SMBJ33CAQ