Connect with us

Varying the output of a buck converter

Discussion in 'Electronic Design' started by Daniel Towner, Nov 15, 2004.

Scroll to continue with content
  1. Hi,

    I am using a buck converter, based on the Zetex 300 to drive a 3V, 1A
    Luxeon LED from a 4*AA battery pack (Zetex design note 73). I want to
    be able to have some control over the output of the buck converter so
    that I can vary the light output. It doesn't need to be that fine
    grained - two different power levels of 1A and 300mA would be
    sufficient. At the moment, the entire circuit is switched on and off
    using a PIC 16F682 controlling a power transistor. Obviously I could
    build a buck circuit based on the PIC itself, but is there some easy
    way of modifying the Zetex circuit itself so that I can vary its
    output? Could I use two current sense resistors of different values,
    and use transistors to determine which one is used as the current
    sense for the Zetex 300?

    thanks,

    dan.
     
  2. John Fields

    John Fields Guest

    ---
    It seems like it would be possible, but because of the low value of
    the sense resistor you'd need to use something like a MOSFET with a
    very low Rds ON as the switch.

    What values of sense resistor do you need for the two different LED
    currents?
     
  3. Genome

    Genome Guest

    Instead of tying the current sense resistor into the ISNS pin directly use
    something like a 100R series resistor. Then inject current into the ISNS pin
    through another resistor from the battery.

    bat
    _____
    |
    |
    |
    \
    /
    \
    / /
    | | |/
    DRV |----|----------------|\
    | | | \
    | | 100R |
    ISNS|----'------/\/\/\-------|
    | |
    /
    \
    /
    \
    |
    '
     
  4. As long as I didn't include the transistor in series with the sense
    resistor (i.e., measure across the sense resistor only, and not across
    the sense resistor and the transistor) does the Rds On matter?

    Would the following work (fixed font required, T = switch transistor,
    is = current sense pin, Rs = current sense resistor). After passing
    through the load the current would go through one or other sense
    resistor, depending upon how the transistors are switched. The isense
    pin then measures the voltage drop across sense resistor only, and the
    transistors RdsON isn't relevent?

    |
    Load
    |
    ---------
    | |
    T T
    | |
    | |
    ---- |
    is--| | |
    ---^-------|
    | |
    Rs Rs
    | |
    ---------
    |
    Gnd
    0.02R and 0.06R for 1A and 300mA respectively.

    thanks,

    dan.
     
  5. Daniel Towner wrote...
    No need to have two transistors and/or two values of current-sense
    resistor. I'd use a single current-sense resistor, yielding 19mV
    for the ZXSC300 sense input at the low LED-current peak, e.g. 0.063
    ohms for 300mA (assuming L is large enough for a ~ steady continuous
    current, see an44), and switch to an attenuated version of the sense
    voltage to obtain your desired higher LED current level. You can
    use a single transistor to select, avoiding a cmos SPDT switch IC.

    ..
    .. Vin one of more LEDs two-setting current-regulated
    .. ----+--+--|>|--|>|--|>|--+--, efficient LED switching supply
    .. | '-----||----------' |
    .. +-----------|<|---------+
    .. | Schottky _|_inductor
    .. +_|_BFC |___|
    .. --- _____ZXSC300 |
    .. | | | |--D
    .. ---|---|Vcc |-------G |<-
    .. | | | |--S Q1
    .. | | | sense | logic-level FET
    .. | |______|---------, | ZXMN2A14F, etc
    .. | | Rs 0.05 | |
    .. '------+---+--/\/\---|--+-----,
    .. Vreturn | R1 100 | R2 200 |
    .. -------------+--/\/\--++--/\/\--+
    .. | |
    .. '--S D--'
    .. G Q2 2n7002
    .. |
    .. control

    In this example, a HI on the control line to the Q2 gate shorts R2
    and the Rs sense voltage appears at the ZXSC300 current sense input.
    And a LO on Q2's gate lets R2 R1 attenuate Rs voltage by 3x, thereby
    increasing the current-peak sense voltage by 3x from 20mV to 60mV.
    The higher 60mV drop on the sense resistor is still an insignificant
    degradation to the circuit's overall efficiency.

    BTW, Zetex's new ZXMN2A14F sot23 MOSFET is better suited for driving
    high-current Luxeons than the ZXMN2A01F they suggested in DN73. It
    is available from DigiKey at 39 cents each, qty 100, 22 cents qty 3k.

    The circuit can be modified to make an adjustable-current LED supply,
    simulating a 600-milliohm pot in the version below. 0.6-ohm pots may
    be available in spice simulations, but they are not in real life. :>)

    ..
    .. Vin one of more LEDs adjustable current-regulated
    .. ----+--+--|>|--|>|--|>|--+--, efficient LED switching supply
    .. | '-----||----------' |
    .. +-----------|<|---------+
    .. | Schottky _|_inductor
    .. +_|_BFC |___|
    .. --- _____ZXSC300 |
    .. | | | |--D
    .. ---|---|Vcc |-------G |<-
    .. | | | |--S Q1
    .. | | | sense |
    .. | |______|---------, +-----, cw
    .. | | Rs 0.10 | | |
    .. '------+---+--/\/\---|--+---> pot
    .. Vreturn | R1 100 | | R2 500
    .. -------------+--/\/\---+--------'
     
  6. Fred Bloggs

    Fred Bloggs Guest

    You may have a problem with your diode placement....
     
  7. Fred Bloggs wrote...
    Thanks, Fred.

    ..
    .. Vin one of more LEDs two-setting current-regulated
    .. ----+--+--|>|--|>|--|>|--+--, efficient LED switching supply
    .. | +-----||----------' _|_
    .. | | |___|
    .. | | Schottky | inductor
    .. | '--------|<|---------+
    .. +_|_BFC |
    .. --- _____ZXSC300 |
    .. | | | |--D
    .. ---|---|Vcc |-------G |<-
    .. | | | |--S Q1
    .. | | | sense | logic-level FET
    .. | |______|---------, | ZXMN2A14F, etc
    .. | | Rs 0.05 | |
    .. '------+---+--/\/\---|--+-----,
    .. Vreturn | R1 100 | R2 200 |
    .. -------------+--/\/\--++--/\/\--+
    .. | |
    .. '--S D--'
    .. G Q2 2n7002
    .. |
    .. control

    and again,

    ..
    .. Vin one of more LEDs adjustable current-regulated
    .. ----+--+--|>|--|>|--|>|--+--, efficient LED switching supply
    .. | +-----||----------' _|_
    .. | | |___|
    .. | | Schottky | inductor
    .. | '--------|<|---------+
    .. +_|_BFC |
    .. --- _____ZXSC300 |
    .. | | | |--D
    .. ---|---|Vcc |-------G |<-
    .. | | | |--S Q1
    .. | | | sense |
    .. | |______|---------, +-----, cw
    .. | | Rs 0.10 | | |
    .. '------+---+--/\/\---|--+---> pot
    .. Vreturn | R1 100 | | R2 500
    .. -------------+--/\/\---+--------'
     
  8. Fred Bloggs

    Fred Bloggs Guest

    View in a fixed-width font such as Courier.


    This is a PFM controller with fixed off time T =1.7us
    off


    and because V must steady state to 0VDC :
    L


    (V - V ) x T = (V - V ) x T
    batt led on led d off


    - or -



    V - V
    led d
    T = ------------- x T
    on V - V off
    batt led





    1 1
    Then Frequency= ------------------ x -----
    V - V T
    led d off
    ----------- + 1
    V - V
    batt led



    Taking V =3V V =0.5V V =6V T = 1.7us makes
    led d batt off



    Frequency= 320KHz which exceeds the maximum F =200KHz
    max



    So that application circuit is not good and it explains


    why their component values don't add up- because they are


    relying on some fudge delays and possibly other things-


    like that high power LED going into the ohmic region. But


    this most likely will NOT work at your dimmed 300mA. This


    means the LED will not dim as expected because the ZXSC300


    cannot turn off fast enough. The most direct fix would be


    to externally extend T by shorting that V pin.
    off drive
     
  9. John Fields

    John Fields Guest

    ---
    I sense a problem in that the OP stated that a 20 milliohm sense
    resistor is required for a 1 amp output and a 60 milliohm resistor for
    300 mA out.

    I assume those values are empirical, they work and, if they're
    accurate, yield a voltage drop of


    E = IR = 1A * 0.02R = 0.02V


    for the 1 amp case, and


    E = IR = 0.3A * 0.06R = 0.018V


    for the 300mA case.



    Your circuit:

    Iin>----+-------+------> |
    | | |
    | [200R] O
    | |R2 |
    [0.05R] +--------+----->Eout
    |R1 |
    | [100R]
    | |R3
    Iout<---+-------+-------------->0V


    which, with the switch closed, should result in a total sense
    resistance of 0.02 ohms for the 1 amp case, doesn't.

    Instead, it yields:

    R1 R3 0.05 * 100
    Rt = --------- = ------------ = 0.04997 ohms
    R1 + R3 0.05 + 100


    which would result in an output current of substantially less that 1A
    WRT the OP's numbers.


    With the switch open and Eout = 0.018V (in order to satisfy the sense
    voltage requirement 300mA case) the voltage at Iin would have to be:



    Eout (R2 + R3) 0.018V * 300R
    V(Iin) = --------------- = --------------- = 0.054V
    R3 100R

    and the current through the string:

    E 0.054V
    I = ---- = -------- = 180µA
    Rt 300R



    Now, in order for 0.054V to be dropped across the 50 milliohm sense
    resistor, the current through it would have to be:


    E 0.054V
    I = ---- = -------- = 1.08A
    R 0.05R


    Since the sense resistor and the R2R3 string are in parallel, and that
    total resistance is in series with the load, the current flowing in
    the load must be equal to the sum of the currents flowing through the
    sense resitor and the R2R3 string, 1.08018A, which is substantially
    higher than 300mA!
     
  10. John Fields

    John Fields Guest

    ---
    That won't work because the two sense resistors are in parallel no
    matter what you do with the switches, so the voltage at 'is' will be
    the same for any particular value of load current whether either or
    both of the switches are on!

    Win's way looks like it may be part of the solution, since it
    essentially gets rid of the switch resistance, but it may have
    problems. (See my reply to his post)
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-