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TIPD102: Voltage to Current converter, high side, grounded load, ~2A, ~5MHz

David D
Intellectual 580 points
Part Number: TIPD102
Other Parts Discussed in Thread: TIPD120

Hi,

I'm required to design a voltage to current converter with the following characteristics:

  1. Grounded load
  2. DC current: ~1.5A
  3. AC current component: ~0.2Ap-p.
  4. Bandwidth: ~5MHz.

I was looking TIPD102 reference design.

Some questions:

  1. The implementation uses 2 stages in order to "allow the high side current source to accept a ground referenced input". Can you please elaborate regarding this consideration?  Why isn't it sufficient to provide Vin at the input of the second opamp (i.e. removing the first stage)? Is the concern about ripple on the supplies that will directly appear on the output current?
  2. Is it a good topology for these requirements?
  3. Which modifications are required to make this circuit support 5MHz bandwidth and 1.7A current?

Thanks,

David.

www.ti.com/.../slau502.pdf

  • 0
    TI__Guru* 93105 points

    Hello David,

    Here are some answers to your questions about the TIPD120 circuit.

    1. The implementation uses 2 stages in order to "allow the high side current source to accept a ground referenced input". Can you please elaborate regarding this consideration?  Why isn't it sufficient to provide Vin at the input of the second opamp (i.e. removing the first stage)? Is the concern about ripple on the supplies that will directly appear on the output current? If the A1 stage is removed from the circuit and the non-inverting input of A2 is driven directly from a voltage source the currrent through Rs2 will change as the input voltage is varied. The voltage applied to the non-inverting input will have to have a high dc offset because A2 operates with a common-mode voltage very close to the V+ voltage level. That A2 arrangement allows for a wide voltage output range from 0 V on the low end, to the MOSFET Vsat voltage + the drop across Rs3 on the high end.
    2. Is it a good topology for these requirements? The two stage design is a a good design that is logical in its implementation. It provides a resonable input voltage range that starts at 0 V on the low end. 
    3. Which modifications are required to make this circuit support 5MHz bandwidth and 1.7A current? Circuits like this one presented in TIPD120 are intended for precision analog applications and aren't optimized for wide bandwidth, high frequency operation. It could prove to be a sizable task to successfully transform the circuit for 5 MHz operation. Wideband op amps, and likley RF power MOSFETs, and effective compensation techniques would have to be applied. Precision Amplifiers, where TIPD120 originated, does not produce op amps with GBW greater than 50 MHz, and it is highly likely that a 5 MHz implementation will require much higher GBW for the op amps. TI's High-Speed Amplifiers group has op amps that have GBW rating that exceed 1 GHz and some may be be able to drive the high gate capacitance of power MOSFETs. If you decide to try and implement the TIPD120 circuit at 5 MHz the High-Speed Amplifiers E2E forum may be of assistance. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering