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LM2903B-Q1: Response Time H to L ＝ L to H

cafain
Mastermind 6365 points
Part Number: LM2903B-Q1
Other Parts Discussed in Thread: LM2903B, LM2903, LM339, TLV1812

Hi Staff

I have a question regarding the contents of the LM2903B-Q1 data sheet (SLCS141L).

A time summary of TR and TF is shown on page21 Response Time.
• The rise time (τR) is approximately τR ⇒ RPULLUP × CL
• The fall time (τF) is approximately τF ⇒ RCE × CL
– RCE can be determine by taking the slope of Section 6.14 in its linear region at the desired temperature,
　or by dividing the VOL by Iout

Question 1.(LM2903B-Q1)
Figure 3-38 L to H rse time takes approximately 1us.
I think it is necessary to set Rpullup and CL to small values in order to make this rise time steep.
What do you think?
Figure 3-38 I want to set the L to H time to the same rise time as Figure 3-37 H to L.

Question 2.(LM2903B SLCS005AF)
I think this device is an open collector output comparator, and the output rise and fall times are determined by the sink current, Rpullup, and CL.
Figure 6-4 and Figure 6-5 transition at the same time.
Please tell me the power supply voltage, Rpullup, and CL to achieve this waveform.
The waveform in the figure looks like a push-pull waveform.

best regards
cafain

  • 0
    Guru 244370 points

    1. Yes; a stronger pull-up resistor will reduce the rise time. It will also increase the power useage, and increase VOL. If you want an output that behaves like a push/pull output, you should use a comparator with a push/pull output.

    2. Figure 6-5 appears to be wrong. (The LM2903 and LM2903B should be very much similar.)

  • +1
    TI__Guru 50016 points

    Hi Cafain,

    Please see section 4.2 of the LM339 family application note:

    Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    Risetime is purely dependent on the RC time constant on the output. Once the output "releases", it is up to the pull-up resistor and stray capacitance to determine the rise-time.

    If you want a faster risetime, then the resistor must be reduced to increase the risetime.

    Fall-time is dependent on the output capacitence and the equivalent output impedance (RDSon) of the output device (typically ~16-20 ohms for the LM339 family)

    If you want symmetrical edges, then you will need a push-pull output device, such as the TLV1812.

    Keep in mind that the rise/fall time is in addition to the response time (tpHL/LH) of the comparator. You cannot make the comparator "faster" by loading the output. All you are doing is sharpening the edge rate.

    The conditions for the figures are at the top of the page:

     TA= 25°C, VS= 5V, RPULLUP=5.1k, CL = 15 pF, VCM=0V unless otherwise noted.