LM74500-Q1: TVS, input/output capacitor, gate resistor

Hi Dejan

Thanks for reaching out to us.

 "on LM74500Q1EVM board, 33 V bidirectional TVS diode is used. Is there any reason that 36 V TVS diode is not used? I'm planning to use SMLJ36CA TVS diode, which has 58.1 V clamping voltage, which is lower than 60 V, so this diode should be still acceptable?"

The EVM has as 33V bidirectional TVS diode taking into consideration the ISO 7637-2. During ISO 7637-2 pulse 1, the source of LM74500-Q1 is pulled down by the ISO pulse, clamped by TVS- and the MOSFET Q1 is turned off quickly to prevent reverse current from discharging the bulk output capacitors. When the MOSFET turns off, the VDS of the FET 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 applications. 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 an ISO 7637-2 pulse 1 test, the SMBJ33CA clamps at –42 V with 15 A of peak surge current and meets the clamping voltage ≤ 44 V

Considering SMBCJ36CA, the clamping voltage will cross 44V mark and damage the FET because of VDS violation.

Below are the points that need to be taken into account while selecting a TVS,

• The breakdown voltage of the TVS has to be greater than than the maximum DC voltage that can be applied at the input - both positive and negative side.
• All the input positive transient voltages that can occur at the input have to be clamped to less than +65V and all the negative transients voltages have to be clamped within  -65V. 
• The TVS needs to be properly rated for power handling capacity to withstand power dissipation in it during transients.

In your case, are there any standards or test cases which are to be passed?

Since TVS selection is not based only on positive side of working range.

"is there any reason why 220 uF capacitor is used as output capacitor on EVM board? I'm planning to use 2.2 uF/100 V capacitor on the output and input side. Any concerns regarding this?"

No, there is no specific reason to use a 220uF capacitor. It generally is a place holder for various capacitors values to be tested. Although, a bigger value capacitor is typically considered to protect the immediate output voltage collapse as a result of line disturbance.

 "is there any maximal value for Vcap capacitor that could be used? I'm planning to use 470 nF / 50 V capacitor, because the same capacitor type is already used for other purposes on the same circuit."

You can use a 470nF capacitor. As far as the condition mentioned below is satisfied, you can use a capacitor. Please note that bigger charge pump cap will take more time to charge and hence the startup will take longer. During startup load current will pass through body diode.

"do you suggest to use gate resistor to minimise peak gate current in to MOSFET transistor?"

You can consider using a resistor in the order of 10Ω in series with the gate of the FET to dampen the oscillations caused by the parasitics of the FET if any. 

Thanks and Regards

Shiven Dhir

Hello Shiven,

thanks for your very detailed explanation. In my example there is no specific tests which are to be passed. I just would like to implement a basic reverse polarity protection for input voltage of 36 V max. Bidirectional type is choosen because there could be some transients over 36 V (less then 50 V) and these transients could be "filtered" without problems. However, Enable pin of LM74500 is, in my example, connected directly to the source pin. That means that transistor will not go off till input voltage will drop too low, so any specific voltage spike on the FET is not expected in my example?

Since LM74500 device can survive input voltages up to +65/-65 V and such transients are not expected, can I go maybe even without TVS diode at the input?

Many thanks, Kind regards, Dejan

Hi Dejan

Although the device is rated for +65/-65V, it is limited by the 60V FET. Just in case the output is charged, and input goes negative and is failed to be filtered, your FET will be damaged.

If you don't expect any transients, or your transients are short enough that can be filtered out with the capacitors, you can go TVS less also.

TVS is used so that your FET is within VDS working range at all times or the device abs max is not violated, whichever is lower.

Thanks and Regards

Shiven Dhir.