Two important specifications are 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 1 mA and the breakdown voltage must be higher than worst case steady state voltages seen in the

system. The breakdown voltage of the TVS+ must be higher than 24-V jump start voltage and 35-V suppressed

load dump voltage and less than the maximum ratings of LM74700-EP (65 V). The breakdown voltage of TVS–

must be beyond than maximum reverse battery voltage –16 V, 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 must not interfere

with steady state operation. In the case of an ISO 7637-2 pulse 1, the input voltage goes up to –150 V with a

generator impedance of 10 Ω. This action translates to 15 A flowing through the TVS– and the voltage across

the TVS would be close to its clamping voltage.

LM74700

GATE

CATHODE

EN

ANODE

VCAP

GND

TVS

SMBJ33CA

Voltage

Regulator

22 nF

VCAP

0.1 µF

Q1

47 µF

Figure 9-2. Typical 12-V Battery Protection With Single Bi-directional TVS

The next criterion is that the absolute maximum rating of Anode to Cathode reverse voltage of the LM74700-EP

chosen and maximum limit on the cathode to anode voltage is 60 V.

In case of ISO 7637-2 pulse 1, the anode of LM74700-EP 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 16-V (maximum battery voltage), then the

clamping voltage of the TVS– must not exceed (60 V – 16) V = –44 V.

The SMBJ33CA TVS diode can be used for 12-V battery protection application. The breakdown voltage of 36.7

V meets the jump start, load dump requirements on the positive side and 16-V reverse battery connection on the

negative side. During ISO 7637-2 pulse 1 test, the SMBJ33CA clamps at –44 V with 15 A of peak surge current

as shown in Figure 9-5 and it meets the clamping voltage ≤ 44 V.

SMBJ series of TVS are rated up to 600-W peak pulse power levels. This rating is sufficient for ISO 7637-2

pulses and suppressed load dump (ISO-16750-2 pulse B).

9.2.4 Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications

Typical 24-V battery protection application circuit shown in Figure 9-3 uses two uni-directional TVS diodes to

protect from positive and negative transient voltages.

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LM74700-EP

SNOSDD6 – SEPTEMBER 2021

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