LM7480-Q1: 200-V Unsuppressed Load Dump Protection Application in Figure 10-25

Hi Hirokaz-san,

Please see my comments below,

Q1. What does the 85V rectifier diode on D2 do?

Ans: The purpose of D2 is to supply voltage to the VS pin from A pin. The 10 kohms in series with VS pin (which is placed to limit the current into Zener) limits the current required by the charge pump during startup conditions. So, a diode from A to VS provides uninterrupted current into the VS pin.   Also, D2 cannot be removed and directly short A pin to VS pin. This is because this connection will create a parallel path across the Q1 FET which negates the whole purpose of having a Load disconnect FET. 

Q2. Voltage is detected by a voltage divider resistor directly from the input side to the OV terminal without using VSNS and SW.

       Why not use SW? Is the VSNS connected to the Cvs via 10k Ω to avoid voltage delay? Is there any other reason?

Ans: The VSNS, SW pins are rated only upto 70V. You cannot connect them directly to the input where the maximum voltage can be 200V in unsuppressed load dump scenario. To protect these pins, they will have to be connected to VS pin which is protected by the Zener clamp. connecting OV pin resistor ladder to VSNS, VS will cause delay in OV protection due to the RC network on the VS pin.  

Q3. What is the role of CCAP?

Ans: Ccap is the charge pump capacitor. Please refer to '9.3.1 Charge Pump' section of the datasheet for more understanding. 

Q4. What is the reason for the D3 and D4 combination? If you put in D3, does it not have to be bidirectional with D4?

       What are the benefits of this combination?

Ans: Please refer to '11.2 TVS Selection for 12-V Battery Systems' section of the datasheet. 

Hi Hirokaz-san,

The capacitance on the A pin acts as a decoupling capacitor. Without a cap on the A pin, the noise on A pin can cause the voltage across A and C to be less than V(AC_REV)  resulting in turn OFF of DGATE.