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LDO and DC-DC Converter

lee peng kiat
Intellectual 450 points

Hi,

The DC-DC converter will operate at PFM mode at light load in order to maintain efficiency.

However, the efficiency at PFM mode will be lower if compare to PWM mode (at normal or heavy load).

 My question is whether the efficiency of DC-DC Converter (at PFM mode) will be comparable to LDO ?                   

If the answer is 'Yes', then I can use LDO in my design instead of DC-DC Converter which is more noisy & expensive. (For low current application < 0.2A , Vin=5V & Vout=3.3V)

What is the range of the output current which cause DC-DC Converter to operate at PFM mode ?

what is the range of oscillation frequency at PFM mode for DC-DC Converter ?

Thanks.

 

  • 0
    TI__Guru**** 150770 points

    This is a very complicated question whose exact answer depends entirely on which dc-dc converter you are talking about.  So, let me first say that modern dc-dc converters will give better efficiency than an LDO down to very low load currents (< 1 mA).  Let me also give an example.

    For your power specs, the TPS62237 would be an ideal dc-dc converter.  At a 100uA load, it would have about 60% efficiency.  At load currents higher than this, it has better efficiency.

    An ideal LDO would have 66% efficiency at any load, ignoring its ground or Iq current.  But at very low loads, like 100uA, the ground current becomes significant and the efficiency reduces further.  So, it seems that a dc-dc converter is almost always more efficient than an LDO.

    The output noise of a dc-dc converter is entirely dependent on which converter you are talking about.  The TPS62237 uses DCS-Control, which is explained in more detail here: http://www.ti.com/lit/an/slyt531/slyt531.pdf  DCS-Control is a very low noise switching topology.