MIC38300_13 Datasheet, PDF (12/15 Pages) Micrel Semiconductor – 3A High-Efficiency Low Dropout Regulator

Micrel, Inc.
There are two types of losses in switching converters; DC
losses and switching losses. DC losses are simply the
power dissipation of I²R. Power is dissipated in the high
side switch during the on cycle. Power loss is equal to the
high-side MOSFET R_DSON multiplied by the switch
current. During the off cycle, the low side N-channel
MOSFET conducts, also dissipating power. Device
operating current also reduces efficiency. The product of
the quiescent (operating) current and the supply voltage
is another DC loss.
Over 100mA, efficiency loss is dominated by MOSFET
R_DSON and inductor losses. Higher input supply voltages
will increase the gate-to-source threshold on the internal
MOSFETs, reducing the internal RD_DSON. This improves
efficiency by reducing DC losses in the device. All but the
inductor losses are inherent to the device. In which case,
inductor selection becomes increasingly critical in
efficiency calculations. As the inductors are reduced in
size, the DC resistance (DCR) can become quite
significant. The DCR losses can be calculated as in
Equation 3:
L_P_D = I_OUT2 × DCR
Eq. 3
From that, the loss in efficiency due to inductor resistance
can be calculated as in Equation 4:
MIC38300
If the current through the current sense of HELDO2 is
less than the current through the current sense of
HELDO1, the inverting pin will be at a higher voltage than
the non-inverting pin and the op-amp will drive the FB of
HELDO2 low. The low voltage sensed on HELDO2 FB
pin will drive the output up until the output voltage of
HELDO2 matches the output voltage of HELDO1. Since
V_OUT will remain constant and both HELDO V_OUT
terminals and sense resistances are matched, the output
currents will be shared equally.
Efficiency
_
Loss
=

_1−





V_OUT × I_OUT
V_OUT × I_OUT + L _
P_D



× 100
Eq. 4
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and
size in this case.
Current-Sharing Circuit
Figure 2 allows two MIC38300 HELDO regulators to
share the load current equally. HELDO1 senses the
output voltage at the load, on the other side of a current
sense resistor. As the load changes, a voltage equal to
the output voltage, plus the load current times the sense
resistor, is developed at the V_OUT terminal of HELDO1.
The op-amp (MIC7300) inverting pin senses this voltage
and compares it to the voltage on the V_OUT terminal of
HELDO2.
April 10, 2013
12
Revision 5.0