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LM2623-Q1: Suitability of the device and minimum output capacitance

Stephen Thomas
Intellectual 710 points
Part Number: LM2623-Q1
Other Parts Discussed in Thread: TPS61378-Q1, LM2623

Hi,

I need to generate 8.0V/0.3A output for a 2W speaker driver.

The input is from another buck regulator generating 3.8V ( varies from 3.590V to 3.916V ) from automotive battery 14V.

I have referred the datasheet and AN1221. 

1. May i know what is the advantage of Gated  oscillator control scheme? Is it suited for my application here

2.Any simulation model for the PN available?

3. It is not exactly clear to me how this device function the R5 and C3 selection in AN1221 is not straight forward

If i check normal boost equation for duty 1-vin/vout for my application the duty can vary from 51% to 55%. So do i need R5?

4. The output capacitor AN1221 recommends 68uF minimum, is there any calculation for the same. Normally boost regulator the output capacitance depends on Ripple spec and for approximately 25mV ripple with duty 55% and Fsw 1.4MHz, Iout =0.3A im getting 20uF as sufficient using traditional Boost output capacitor equation.

(Equation 12 from following appnote)

  • 0
    TI__Mastermind 34201 points

    Hi Stephen,

    LM2623-Q1 is very old device. You can use TPS61378-Q1 for this application. It has higher switching frequency, smaller Rdson, Higher efficiency. Please see TPS61378-Q1 datasheet on ti.com.

  • 0 in reply to Zack Liu
    Intellectual 710 points

    Hi Zack,

    Thank you for the reply. I understand now that this is not a right solution for my application.

    How ever can you please provide answers to the technical points in my queries.

    Thank you,

    Regards,

    Stephen

  • 0 in reply to Stephen Thomas
    Intellectual 710 points

    Hello,

    Any support on the queries possible?

    If this information is available it will be really helpful for us .

    regards,

    Stephen

  • 0 in reply to Stephen Thomas
    TI__Mastermind 34201 points

    Hi Stephen,

    1. The advantage is: The on/off regulation mode of the LM2623-Q1, along with its ultra-low quiescent current, results in good efficiency over a very wide load range. 

    TPS61378-Q1 has a MODE pin to select working mode between PFM mode and forced PWM mode. The device could also achieves high efficiency at light load range.

    2. No simulation model exists for this device.

    3. The duty cycle of the internal oscillator is programmable with the C3 capacitor. Choosing the correct C3 capacitor to obtain the appropriate duty cycle for a particular application circuit is a trial-and-error process. The non-linear effect that C3 produces is dependent on the input voltage and output voltage values. The correct C3 capacitor for specific input and output voltage values cannot be calculated. Choosing the correct C3 capacitance is best done by trial and error, in conjunction with the checking of the inductor peak current to make sure it is not too close to the current limit of the device. As the C3 capacitor value increases, so does the duty cycle; conversely, as the C3 capacitor value decreases, the duty cycle decreases.

    4. For a boost converter that is working in CCM mode, you can design the Cout with ripple, Iout equations as equation 12. But LM2623 will working in PFM mode so more Cout is needed to achieve low output voltage ripple.

    I suggest you select TPS61378-Q1 because its design process is more easy. There are lots of design tool, such as WEBENCH, excel tool to help you. Each external components selection steps are also very clear.