View Full Version : BS2p40 and Cascading TPIC6595N devices

Bill Chennault
04-26-2007, 08:29 PM

How may I calculate the number of TPIC6595N shift registers I can cascade from a single pin of a BS2p40 without exceeding the current draw of the BS2p40's pin?

If this question needs refinement, will you help me do so?



You are what you write.

phil kenny
04-26-2007, 09:18 PM
The TI data sheet shows the equivalent input circuit for the logic signals
as driving essentially gates of DMOS devices. In order to calculate the
transient input current, you would have to know the risetime of the Basic
Stamp output signals as well as the input capacitance of the TPIC6595N
logic inputs.

I couldn't find any input capacitance listed in the TI data sheet. Maybe a
call to a nearby TI Fields Sales office could help pin down this parameter.

In any event, I think you could connect quite a few TPIC6595N logic
inputs to the BS2P40 output pins without damage since you will be driving
essentially only the gate capacitor of a DMOS device.

phil kenny
04-26-2007, 09:50 PM
Making a few assumptions about the signals involved:

ristetime of BS2p40 Stamp output voltage = dv/dt = 20 volts/microsecond

input capacitance of TPIC6595N logic input = C = 50 picofarads

Then the peak transient current drawn from the Stamp output pin will be:

I = C*(dv/dt)

I = (20*10^6)* 50*10^(-12) = 1 milliamp

Since each output pin of the BS2P40 can source 30 milliamps, you
could connect 30 TPIC6595N devices to the BS2P40.

These numbers are probably in the ballpark.

Bill Chennault
04-26-2007, 10:31 PM

It was very gracious of you to go to all that work. Thank you very much!

I have no end-goal in mind other than experimentation. However, my experimentation numbers involve 8 TPIC6595 per pin for a maximum of 8 mA, well within your calculations. Now, I just need to look up the maximum current draw allowed by the entire BS2p40 and multiply by 32.


You are what you write.

phil kenny
04-26-2007, 11:58 PM
Only one more comment, Bill.

The 1 ma current I calculated is a transient current and it ONLY flows when a
Stamp output pin is rising or falling. No current would be drawn while the
Stamp output pin is a constant hi or low. So you only have to add up the
currents delivered by pins that change state simultaneously. Even then, it
is only a pulsed load current.

To be really on the safe side, put a 220 ohm resistor in series with each Stamp
output pin.

Bill Chennault
04-27-2007, 01:01 AM


And thank you again.


You are what you write.