L9945TR Mosfet Pre-driver peak drive current

[杨思奇]

    Good afternoon ST engineer。I'm developing a DC motor control hardware using the L9945 in H brigde mode 。But facing a problem: the datasheet haven't any description about the Mosfet Pre-driver peak drive current when using external drive resistors. Without the currunt,I can't select a right drive resistors to limte the drive current for each channel。

    I will be grateful for you to tell me how much it is. thank you.

[GiuliaNicotra]

Hello Yang,

i will suggest you to see the paragraph 2.5 of the Application note AN5311 that you can download from the st.com page related to L9945.

It's possible to choose different dead time values for both H-bridges independently to prevent cross-conduction between HS and LS channels.

It's possible to choose different gate charge / discharge currents for every channel via SPI through GCC_CONGIG_XX bit. Gate charge/discharge currents can be either constant (with specific value) or limited by external resistor. They should be selected with respect to the selected dead time to avoid cross-conduction between HS and LS channels.

if you want to use the H-bridge configuration the max current of the driver are listed in the paragraph 5.4.4 of the L9945 datasheet.

thanks

[杨思奇]

    I'm glad to hear from you，your imformation is really halpful But this problem still bothers me: The lised drive current in the table 15 of the L9945 datasheet is the average drive current or the peak drive current ? What I want to known is the drive peak current which will leads me to design external drive resistor. This  external drive resistor  on the one hand is to fit for mosfet to let the mosfet swich on and off at a adequat  rise and fall time；One the other, it also limts the drive current  to prevent L9945 gate drive pin from the damage of over current.

If The lised drive current in the table 15 of the L9945 datasheet is the peak drive current ,I'll face anther problem: According to the table 15 ,The  gate charge / discharge currents are less than 100mA, which is too small to let the mosfets swiching fast ,resulting the power dissipation  increasing and mosfets over hiting . So, to drive power  mosfets  L9945's dvrive ability maybe not enough？

[GiuliaNicotra]

Hello,

The gate current is internally limited to value reported in table 15 that can be considered as peak value, so also applying 0Ohm resistor to gate there is no risk to damage the device.

In fact, with configuration GCC[1:0] = [1, 1], [1, 0], [0, 1], there is no need to add external resistor and mosfet gate can be connected directly to L9945 pin.

With GCC[1:0] = [1, 1] the current is anyway internally limited to ICh0Gx/ IDCh0Gx, while the external resistor is used to limit the current to lower value if necessary.

I confirm that the gate charge / discharge currents are less than 100mA even if this value fits with most of the power MOSFETs used in automotive environment.