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TL499A: Need to transform 3.7 V to 20V and data sheet is either not helpful or too complicated.

Clayton Dittman
Prodigy 80 points
Part Number: TL499A


So I have the TL 499A and I want to take a samsung limo battery, then use it to create 20V. I tried to follow the guide lines in the data sheet but it basically just says, "connect the circuit shown in figure one, unless specified in the tables shown, and choose a value for Re1. I don't know what I'm doing wrong but I do know what I'm doing so I'm going to list that. 

1) Connected a LiPo battery with a 3.8V rating and 4V measured to the Series 2 input of the TL499A dip. 

2) Connect the exact same circuit as shown in the diagram, except re1 was a 22k resistor, not a potentiometer. 

3) try a bacagillion different combinations to see if I got different results... I didn't really. 

Any help would be appreciated. I didn't see an equaition for Vout using the step up configuration so if you have one that would be wonderful as well. 

  • 0
    TI__Intellectual 2655 points

    Hi Clayton

      TL499A has two part: the primary power is LDO(Vin is SERIES IN1 pin) the secondary power is BOOST(Vin is SW REG IN2 pin), when the primary power source is remove, the secondary power source is stepped up, you can use Vout=Vin/(1-D)

  • 0 in reply to Jing Ji
    Prodigy 80 points
    Okay I should have said that series in1 power has been grounded.

    Vout=Vin/(1-D), what is D is this equation?

    Thank you for your help btw
  • 0 in reply to Clayton Dittman
    TI__Intellectual 2655 points

    Hi Clayton

      this is the PWM waveform

    D=Pluse width/ Pulse Period

  • 0 in reply to Jing Ji
    Prodigy 80 points

    Okay I see now. So I am using DC power from a Lithium battery. Meaning that I won't have a Duty cycle. I'm getting a straight 3.7-4V at all times on Series 2. So do I need to create the PWM signal before delivering to the series in2 pin or do I control the duty cycle using the resistor value Re1?

  • 0 in reply to Clayton Dittman
    TI__Intellectual 2655 points
    you can change Re1 to set output voltage. TL499A could generates PWM.
  • 0 in reply to Jing Ji
    Prodigy 80 points

    So Here is what I got... a .25watt 47uH Inductor, 220 Ohm connected at Rc, 47uF connected at Vout, and 4V DC provided to Series In 2. Series 1 is grounded, Re2 = 4.7k Ohm, Re1=27k Ohm, and Cp=.01uF. My voltage is less than 1 and I don't understand why. I thought that the REF voltage would be compared to 1.26 volts and that would affect the duty cycle but nothing I've done thus far to that circuit really changes anything.

    How can I get to 20V? I guess I don't understand how to set the duty cycle appropriately. 

  • 0 in reply to Clayton Dittman
    Prodigy 80 points

    Okay I finally Figured it out. For anyone else who comes across this issue.

    1) forget about Rc... you don't want to limit the current since we are stepping up our voltage. Inductors store Current so by using Rc we are actually limiting the amount of current in the circuit. 


    2) 99% of the time you can forget about Cf as well. Its a voltage sucker... its going to lower your output voltage and is pretty unnecessary if you are doing a large step up. 


    3) Use Re1/Re2*Vin For a rough estimate of the Output voltage. Obviously, your value of Cp will make a difference as to the voltage across the resistor and capacitor in parallel but once the cap is charged which takes almost no time at all, then its just a wire basically. 


    4) forget about the feedback REF wire, we don't need it. Its used to bias the switching transistor but we don't need to bias the transistor because we want that thing stepped-up as soon as possible. 


    5) Watch your inductor temperature. As the temp rises your voltage efficiency decreases and output Voltage decreases as a result. Voltage Lost due to heat. I decided to. use 3 15uH inductors to expose more surface area to air and help keep the inductors cool. 


    6) Don't attempt to skip #3 and attempt grounding the output to drain the current... you will burn up your capacitors, inductors, and circuit possibly. 


    So for me, RE1=33K Ohm, RE2=4.7K Ohm, L=(3)15uH, Cp=.01uF, Vin=4V, and finally Vout= 19.5-20V. Fairly stable around 19.77.

    This should have been really simple but the datasheet only shows 1 diagram and it's for voltage dealing with series 1. I had to figure out what I didn't need in that diagram to come up with the actual step-up circuit design. 

    I Attached a photo in case anyone needs to reference the setup for the circuit.