Questions about "AN11967, S12ZVM Family in 48 V Applications"

[maxxu]

Hello everyone：

      According to the description of "AN11967, S12ZVM Family in 48 V Applications" 5.2 chapters。

       The text is as follows：

        5.2 Drive BLDC/PMSM through GDU outputs

       In this configuration, GDU outputs are used to control the external pre-driver. This solution brings advantages and disadvantages. The principal advantage is that PWM outputs functionalities are not sacrificed to control the external predriver, but this implies the use of additional components to reduce the output voltage from the gate drivers to the external pre-driver inputs. Another consideration is the intrinsic delay time introduced by the GDU to the pre-driver inputs (tdelon and tdeloff).

      Most of the external pre-drivers accept an input (logical) voltage from 2.5 V to 10 V. For this reason, it is necessary to add a voltage divider of ¼ from the HG and LG pins to fit with this requirement.

       In the case of the HGx pin where the output voltage is around 20 V, the voltage going into the pre-driver’s inputs will be around 5 V. For LGx pins the output voltage is 12 V so the voltage going into pre-driver inputs would be 3 V.

My questions are as follows

1、Why the voltage of HGx is about 20v, when the bootstrap capacitor is not connected to the circuit, HGx should not be able to output.

2、If the default bootstrap capacitor is connected, I think HGx should be a floating level, ranging from 12V to 20V. Then, even if the voltage divider is 4:1, it can not drive the pre-driver correctly, because the level range is 3V to 5V, which is logical 1 for the pre-driver.

I think there is a mistake in the description of the circuit in the original text. Maybe I misunderstood it. Please ask the experts to correct it.