MIC23050_10 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – 4MHz PWM Buck Regulator with HyperLight Load Switching Scheme

Micrel, Inc.
MIC23050
Figure 1. MIC23050 Efficiency Curve
Figure 1 illustrates an efficiency curve for the MIC23050.
From no load to 100mA, efficiency losses are dominated
by quiescent current losses, gate drive and transition
losses. By using the HyperLight Load^™ mode the
MIC23050 is able to maintain high efficiency at low output
currents.
Over 100mA, efficiency loss is dominated by MOSFET
RDSON and inductor losses. Higher input supply voltages
will increase the Gate to Source threshold on the internal
MOSFETs, reducing the internal RDSON. 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 follows:
HyperLight Load Mode™
MIC23050 uses a minimum on and off time proprietary
control loop. When the output voltage falls below the
regulation threshold, the error comparator begins a
switching cycle that turns the PMOS on and keeps it on for
the duration of the minimum-on-time. When the output
voltage is over the regulation threshold, the error
comparator turns the PMOS off for a minimum-off-time.
The NMOS acts as an ideal rectifier that conducts when
the PMOS is off. Using a NMOS switch instead of a diode
allows for lower voltage drop across the switching device
when it is on. The asynchronous switching combination
between the PMOS and the NMOS allows the control loop
to work in discontinuous mode for light load operations. In
discontinuous mode MIC23050 works in pulse frequency
modulation (PFM) to regulate the output. As the output
current increases, the switching frequency increases. This
improves the efficiency of MIC23050 during light load
currents. As the load current increases, the MIC23050
goes into continuous conduction mode (CCM) at a
constant frequency of 4MHz. The equation to calculate the
load when the MIC23050 goes into continuous conduction
mode may be approximated by the following formula:
I_LOAD
=
⎜⎛
⎝
(V_IN
− V_OUT )
2L × f
×
D
⎟⎞
⎠
L Pd = I_OUT2 × DCR
From that, the loss in efficiency due to inductor resistance
can be calculated as follows:
Efficiency
Loss
=
⎡
⎢^{1 −}
⎢⎣
⎜⎜⎝⎛
V_OUT × I_OUT
V_OUT × I_OUT + L
Pd
⎟⎟⎠⎞⎥⎥⎦⎤
× 100
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.
January 2010
10
M9999-012010-D