1. Home
  2. Electronics Design
  3. Driving a CMOS driver when VDD is not present

Driving a CMOS driver when VDD is not present

I need to drive 10 Microchip TC4432 high voltage drivers

formatting link
from 3.3V logic (Xilinx Spartan 3). Each of the 10 TC4432 is driven by an independnet signal.

The VDD of the TC4432 is normally at +24V level. The problem is that the +24V that powers the TC4432 and the +3.3V that powers the logic are from different power supplys which can be switched on and off independelty. As a result, the +3.3V logic may drive the TC4432 while the VDD of the TC4432 is at zero level.

This will violate the absolute max input voltage of the TC4432 which is specified as VDD + 0.3V.

Is it safe to drive the TC4432 through a current limiting resistor (e.g. 1K) and relay on the internal parasitic diode at the TC4432 input to limit the input voltage?

Is there a better solution to drive the TC4432 or a better choice for high voltage driver?

I am trying to avoid discrete components to reduce component count or using open collector drivers such as the ULNXXXX which result in slow rise time (the TC4432 has fast fall and rise times).

Thanks,

Kam

Reply to
Kam
Loading thread data ...

I'd use the 1k resistor solution if I were you. The '4432 will be powered from the logic inputs via the protection diode, but the logic current will be low at 3.3-0.6 = 2.7V, and the '4432 will be disabled by its under-voltage lockout. The protection diode design is meant to handle high currents without causing bad chip behavior, such a supply latchup, but it's a good idea to limit the current with a 1k resistor anyway.

--
 Thanks,
    - Win
Reply to
Winfield Hill

This is a classic problem. Can you buffer the inputs of the TC4432 with a VHC or LCX family chip with a 3.3V produced from the 24V?

74LCX541 for instance...
Reply to
a7yvm109gf5d1

But if the 24V supply is off (and therefore the 3.3V supply of the LCX), the problem is then shifted from the TC4432 input to the LCX input.

I must be missing something.

Kam

Reply to
Kam

The 74LCX541 is built to handle inputs while no VDD is present.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Hello Jim,

Then it would be a perfect candidate for a customer support position. Lots of input, not enough VDD, not much output ;-)

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.