Driving a CMOS driver when VDD is not present

[Kam]

I need to drive 10 Microchip TC4432 high voltage drivers
(http://ww1.microchip.com/downloads/en/DeviceDoc/21424c.pdf) 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

 

[Winfield Hill]

Kam wrote...

I need to drive 10 Microchip TC4432 high voltage drivers
(http://ww1.microchip.com/downloads/en/DeviceDoc/21424c.pdf) from
3.3V logic (Xilinx Spartan 3). Each of the 10 TC4432 is driven by
an independent 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?

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.

 

[[email protected]]

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 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...

 

[Kam]

Kam wrote:

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...

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

 

[Jim Thompson]

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

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

...Jim Thompson

 

[Joerg]

Hello Jim,

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

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