I recently purchased a H-bridge board that uses ST L6384d driver chips to drive 4 N mosfets. The input to the driver (pin 1) comes from a dual input NOR gate and the DT (pin3) is pulled low thru a 270k resistor. The gate has a Direction and PWM input. I have tested the board and it functions as advertised, feeding a square wave to the PWM pin and controlling motor direction with the other pin. Both inputs to the board are inverted so the only time the NOR gate outputs a high is when both inputs are high. I would think that when the Direction pin is high the output to the L6384d would be the PWM signal. Why wouldn't this cause the output of the L6384d to change at a PWM rate? Note: The other side of the H-bridge is an identical circuit but the NOR gate output is inverted. Could someone Please explain the operation of this drive IC? The circuit is wired up like the diagram on ST's data sheet with the addition of the 74hc02 NOR gates.

 

I recently purchased a H-bridge board that uses ST L6384d driver chips to drive 4 N mosfets. The input to the driver (pin 1) comes from a dual input NOR gate and the DT (pin3) is pulled low thru a 270k resistor. The gate has a Direction and PWM input. I have tested the board and it functions as advertised, feeding a square wave to the PWM pin and controlling motor direction with the other pin. Both inputs to the board are inverted so the only time the NOR gate outputs a high is when both inputs are high. I would think that when the Direction pin is high the output to the L6384d would be the PWM signal. Why wouldn't this cause the output of the L6384d to change at a PWM rate? Note: The other side of the H-bridge is an identical circuit but the NOR gate output is inverted. Could someone Please explain the operation of this drive IC? The circuit is wired up like the diagram on ST's data sheet with the addition of the 74hc02 NOR gates.

That won't work! The gates need to be AND gates, with an inverter to the direction pin of the second side, and the PWM pins commoned. This is certainly how the bare I.C. needs to be connected as a pair.
Are the NOR gates external ones that you have added? What board is it that you have purchased?

 

Oh but it does work and that leads to my confusion. The ebay link is:
https://www.ebay.com/itm/115978726349?itmmeta=01HW868SBNHFEW7YTR4MJKA48C&hash=item1b00deebcd:g:VJMAAOSwJpZlVZAk&itmprp=enc:AQAJAAAA4PfGN4HiqeDMoIw9rQm8c0D5QoPfopTITt/EIiHJVgROVAeySBVZzfHxK+oiaqM5p4XIpTWMeZOksd+vOnAL/owbuLmN91ynQKksM/VKv7GQYgLuUaDyj2/1IYoQJkqkEKhDaMlfDkW7YLFwQNwx7bbIvVeGfDTtVqrxtiL/rliB3naZm03LmKGr6kPovzOc0NBc9s/XKKCQjtGZ7SfcUecr8UvdCihyAPIdquwMYpWg4JFage8Dvmln3FercPqFn6v0+i6mqSbWDfRLmpl--Kyi6wIDEQz3Tbk4U7swr412|tkp:Bk9SR5CWo4biYw
The board is unchanged except I removed the L6384 and the 74hc02 from 1 side of the bridge to draw a schematic of the board. All parts reinstalled. The price of the board is way cheaper than I could even purchase the components for.

A ruff schematic (Triple Checked) is attached. The schematic is not complete but it shows the logic to the driver.

 

Oh but it does work and that leads to my confusion. The ebay link is:
https://www.ebay.com/itm/115978726349?itmmeta=01HW868SBNHFEW7YTR4MJKA48C&hash=item1b00deebcd:g:VJMAAOSwJpZlVZAk&itmprp=enc:AQAJAAAA4PfGN4HiqeDMoIw9rQm8c0D5QoPfopTITt/EIiHJVgROVAeySBVZzfHxK+oiaqM5p4XIpTWMeZOksd+vOnAL/owbuLmN91ynQKksM/VKv7GQYgLuUaDyj2/1IYoQJkqkEKhDaMlfDkW7YLFwQNwx7bbIvVeGfDTtVqrxtiL/rliB3naZm03LmKGr6kPovzOc0NBc9s/XKKCQjtGZ7SfcUecr8UvdCihyAPIdquwMYpWg4JFage8Dvmln3FercPqFn6v0+i6mqSbWDfRLmpl--Kyi6wIDEQz3Tbk4U7swr412|tkp:Bk9SR5CWo4biYw
The board is unchanged except I removed the L6384 and the 74hc02 from 1 side of the bridge to draw a schematic of the board. All parts reinstalled. The price of the board is way cheaper than I could even purchase the components for.

A ruff schematic (Triple Checked) is attached. The schematic is not complete but it shows the logic to the driver.

Sorry. I thought that you were using the basic I.C., not a ready made board. Since it's a dual motor driver I'll just consider one side, i.e. one motor.
What should happen is that when you have the Direction pin at Low input, one side of the bridge should set it's Low side MOSFET of the pair conducting. Then turning on the PWM signal will switch the opposite bridge pair alternately High and Low at the PWM ratio you have programmed. Swapping the Direction pin to High will have the reverse effect, and the motor will reverse.

 

Sorry. I thought that you were using the basic I.C., not a ready made board. Since it's a dual motor driver I'll just consider one side, i.e. one motor.
What should happen is that when you have the Direction pin at Low input, one side of the bridge should set it's Low side MOSFET of the pair conducting. Then turning on the PWM signal will switch the opposite bridge pair alternately High and Low at the PWM ratio you have programmed. Swapping the Direction pin to High will have the reverse effect, and the motor will reverse.

Thanks much, that makes sense. Here is a second question on the operation of the board. If I set the PWM line to a fixed high level the motor changes direct base on the value of the Direction pin. Isn't the PWM inputs function, not only to control speed but, also to keep a charge on the high side boot cap? I will shortly finish the schematic and re-post it.

Thanks Again.

 

Thanks much, that makes sense. Here is a second question on the operation of the board. If I set the PWM line to a fixed high level the motor changes direct base on the value of the Direction pin. Isn't the PWM inputs function, not only to control speed but, also to keep a charge on the high side boot cap? I will shortly finish the schematic and re-post it.

Thanks Again.

If you are asking whether it's safe to keep the PWM pin High, rather than switching, I would say it's not, because as you rightly surmise, you lose the bootstrap effect. If DIR is High, Q4 will be On and Q1 will try to switch On, but won't have enough Gate boost to switch properly.
If Dir is Low, Q3 will be On, and Q2 will try to switch On, but again won't have enough Gate boost to switch properly.
In either case the upper MOSFETS could overheat and be destroyed. Keep it Off or Switching!
That's how I see it anyway!