power consumption of comparator, LMV7239

Hello Hiroshi,

Yes. The supply current will increase with the toggle frequency.

Comparators draw peak currents during the time the output transitions. This is caused by the driving of the internal capacitance of the output stage gates.

Remember:  i = C * dv / dt

These peaks can be as high as 1000% or more of the quiescent current and lasts the duration of the output slew time. This is commonly called comparator "shoot-through current".

These current peaks will be integrated by the supply bypass capacitor and will result in a rise in the average DC supply current.

(This is why even a moderate speed comparator must be treated as a high speed device with proper supply bypassing)

The LMV7239 has a 1.7ns risetime - so the output will transition in about 2ns. This means there will be 2ns peak current pulses on the supplies for every output transition (high to low and low to high). The rise in supply current will be proportional to the duty cycle of the shoot-through peak currents to the input signal rep-rate.

However, The output capacitive load will have the greatest effect on the supply current. Slewing 5V in 2ns into several pF requires milliamps of peak current - which the output must supply. The more "C", the more current required.

I already had a 7239 in the bench from a previous case...so I was able to do a quick measurement:

At Vs=5V, Vin=0-5V 50% square wave, 2.5V Vref:

The shoot-through current followed the output slewing period, as expected, and was about 2ns wide.

The un-loaded shoot-through current spike was about 11.3mA. Loaded with 20pF it was 31mA peak over about 5ns.

 Again, this was a very crude test on a ring-gy protoboard setup - so I am not expecting these numbers to agree perfectly with theory - but they are close.

Regards,

Paul Grohe

SVA Precision Applications