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LM3478: power in drop

bar greidy
Intellectual 460 points
Part Number: LM3478

Hello,

We are trying to figure how to solve an issue with power drop using a lm3478 to step up from vin 12v to 20v, Vout 27v, up to 2A. it uses as a 24v battery charger.

When connecting a power source for example 12v, max 1A or 2A . the voltage at Cin drops immediately to appx 3.5v, the power source shows and kept at 3.5v, 2.5A and gives no output current tough the output voltage at vout is 26v, 0A.

Attached is the circuit schematic, we tried a few things as bellow to solve this but none of them made any changes:

1. Added another Cin bulk cap (C1) 100uf in parallel so its 2 pcs Cin caps.but it doesn't reacts.

2. Added a 20oHm (R7) resistor in series as suggested in the data sheet 8.2.1.2.8 and changed also the byp cap (C2) to 1uF.

when changing to bigger power source (12v, 5A), it works fine. But purpose of this circuit is to work mainly with lower input power up to 20w and to output even minimum currents. 

Please assist and advice how it can be solved? maybe changing R7 / C2 values or maybe or other Cin bulk values? or other suggest or solution?

Thank you in advance

Barg

  • 0
    TI__Intellectual 2655 points
    Hi Bar
    Let’s me confirm your application first.
    1 LM3478: vin 12v to 20v, Vout 27v, up to 2A and it will be used as battery charger. Is the load of LM3478 a battery?
    2 when you do the test, what is your load? battery? resistor? E-load?
    You can change your load to a resistor or E-load to have a try.
    3 If the Vout 27v, up to 2A, the output power will up to 54W, why do you say the input power is only 20W?
    Waiting for your reply.
  • 0 in reply to Jing Ji
    Intellectual 460 points
    Hi Jing,

    Thank you f or your reply.

    As for your questions,

    Load is battery. I also test it with e-load before, it output current but it behaves differently with the battery.

    the test is done from 12v, 2.5A max power supply as an Input power. so i expect the output to be appx 20w, as i noted for the output current to be upto 2A and can be appx 0.5A at 27+-vout but somehow it doesn't output ant current..
  • 0 in reply to bar greidy
    TI__Intellectual 2655 points
    Hi Bar
    For your LM3478 SCH, it ONLY can regulate output voltage to 27V. You can’t set the output current by Vin power supply or LM3478.
    If you connect a battery (for example 26V) for the load, LM3478 want to increase output voltage to 27V, it will increase duty cycle, the inductor current will be increased. If you set a lower input power supply current limit like 2A, the power supply can’t give big current; the input voltage will be decrease, and the IC will shut down.
    Another thing, you can set R4 to control the highest charging current.
    Any question please tell me.
  • 0 in reply to Jing Ji
    Intellectual 460 points
    Hi Jing,

    Well noted, so you are saying to limit the output current so it will work with low input power such as bellow 20w?

    Please advice if adding or increasing the bulk CIN will affect the ability to step up even in low input power?

    Also if Changing the value of R7?

    Thanks
  • 0 in reply to bar greidy
    TI__Intellectual 2655 points
    Hi Bar
    I think increasing input cap is no help for low input power.
    What is your battery voltage range? If the minimum voltage of the battery is lower than the maximum input voltage 20V, you can’t use this circuit, boost can’t control at this condition, it will be broken.
    Another advice, why don’t you choose a charger IC?
  • 0 in reply to Jing Ji
    Intellectual 460 points
    Hi Jing,

    The battery lower voltage is 22v so this circuit.

    can you offer other solutions? or other ic that can boost from 12v in low power input?
  • 0 in reply to bar greidy
    TI__Intellectual 2655 points
    Hi Bar
    There are two ways.
    1 Continue to use LM3478, you can increase R7 to about 200mohm to decrease input current to 2A.
    2 If you want a charger IC suggestion, you can ask it in charger forum, they will help you.
  • 0 in reply to Jing Ji
    Intellectual 460 points
    Hi Jing,

    Did you meant A 200 ohm instead of mohm?

    You noted for 2A input limit, can you please advice for calculating formula to get R7 values vs input current limit? for example 2.5A?

    Thanks,

    Barg
  • 0 in reply to bar greidy
    TI__Intellectual 2655 points
    Hi Bar
    LM3478 has a current limit; you can read the datasheet 8.2.1.2.4 Setting the Current Limit. We can set a current limit to restrict input current. But LM3478 only can control the inductor peak current, so it is hard to get an accuracy formula for input limit. You can take 200m resistor to test first and change it.