RT8059 step-down DC/DC converter - can't get it to be stable, annyone with experience?

Discussion in 'Power Electronics' started by Andy_C_, Sep 24, 2016.

  1. Andy_C_

    Thread Starter New Member

    Sep 24, 2016
    2
    0
    Hi all,

    I have an RT8059 step-down DC/DC converter. I want to use it to regulate the voltage from a li-ion battery (seeing as the battery voltage drops from 4.2V to ~3.4V as its charge level decreases). I've constructed the circuit from the IC's datasheet, using suggested component values and component types. However, the circuit doesn't provide a stable output - the output voltage wanders and doesn't stay at a fixed level. I've tried several identical IC's, different components around the IC, and tried constructing the circuit on a breadboard, and copper stripboard, but still no stability.

    Does anyone have any experience of this regulator, or any suggestions for how to get it to perform in a stable fashion?

    Many thanks.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    What type of board did you use to construct the circuit?
    An improper breadboard layout can cause oscillations and erratic behavior of the converter.
     
  3. Andy_C_

    Thread Starter New Member

    Sep 24, 2016
    2
    0
    Hi Crutschow, thanks for your reply

    Here's the link to the RT8059 datasheet
    http://www.richtek.com/assets/product_file/RT8059/DS8059-05.pdf


    I've tried constructing the circuit on a breadboard and also a copper stripboard. Both have unstable outputs.

    Here's the schematic of the constructed circuit
    [​IMG]

    Here's a picture of the breadboard circuit
    [​IMG]

    Here's a picture of the copper stripboard circuit
    [​IMG]

    The circuit feedback network resistors are R1 = 1M ohm; and R2 = 200k ohm.
    From the formula in the datasheet, this should give an output voltage of 3.6V.
    I have an LED and 100 ohm resistor in series as the load (basic 5mm indicator LED, forward voltage ~2.4V)

    The circuit is powered by an 18650 li-ion battery, which I've measured regularly and it has a stable output.
    I've been using a digital multimeter to measure the output voltage from the circuit. The output voltage typically starts at 1.4V and drops steadily at a rate of approximately 100mV per second. The voltage sporadically rises and falls, without settling at a stable level. I've noticed that touching the ground wire with my finger causes the output voltage to rise significantly.

    I suspect that the type of components and their layout are the important factor for stability - the datasheet discusses this, however, I don't have the ability to make a PCB with the appropriate layout! Any suggestions about how to construct the circuit with the components in the required layout but without a custom made PCB?

    Thanks again for your reply.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    All connections need to be short and direct, not the long dangling wires and capacitor leads you show.
    Following the Figure 3 basic layout shown in page 9 of this spec sheet as close as possible would be good.
    You should be able to reasonably approximate that using a copper backed stripboard by cutting breaks in the connection strips as required.
    If the copper strips are narrow you can solder several together in parallel to make a more robust connection to approximate the size shown in the figure.
     
    Last edited: Sep 26, 2016
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,972
    389
    This chip operates at 1.5MHz and as it is a square wave it produces lots of higher harmonic frequencies so it best to think of it as an RF circuit. The pulsed currents flowing in the diode and storage capacitor flowing in tracks/wires, because of the inductance will produce a voltage drop and if the layout is not good, that voltage can be injected into parts of the circuit where it should not be.
     
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