MIC22400_10 Datasheet, PDF (13/28 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
300kHz to 800kHz Operation
Additionally, the frequency range can be lowered by
adding an additional resistor (R_CF) in parallel with the CF
capacitor. This reduces the amount of current used to
charge the capacitor, reducing the frequency. The
following equation can be used to for frequencies
between 800kHz to 300kHz.:
− R_CF
× C_CF
× ln_{⎜⎜⎝⎛}1 +
1.0V
400μA × R_CF
⎟⎟⎠⎞
=
t
R_CF > 2.9KΩ
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power consumed:
Efficiency
%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞ ^{× 100}
Maintaining high efficiency serves two purposes. It
decreases power dissipation in the power supply,
reducing the need for heat sinks and thermal design
considerations and it decreases consumption of current
for battery powered applications. Reduced current draw
from a battery increases the devices operating time,
critical in hand held devices.
There are mainly two loss terms in switching converters:
static losses and switching losses. Static losses are
simply the power losses due to VI or I²R. For example,
power is dissipated in the high side switch during the on
cycle. Power loss is equal to the high-side MOSFET
RDS_(ON) multiplied by the RMS switch current squared
(I_SW2). During the off cycle, the low-side N-Channel
MOSFET conducts, also dissipating power. Similarly, the
inductor’s DCR and capacitor’s ESR also contribute to
the I²R losses. Device operating current also reduces
efficiency by the product of the quiescent (operating)
current and the supply voltage. The current required to
drive the gates on and in the frequency range from
800kHz to 4MHz and the switching transitions make up
the switching losses.
Figure 3 shows an efficiency curve. The portion, from 0A
to 0.2A, efficiency losses are dominated by quiescent
current losses, gate drive and transition losses. In this
case, lower supply voltages yield greater efficiency in
that they require less current to drive the MOSFETs and
have reduced input power consumption.
MIC22400
Figure 3. Efficiency Curve
The region, 0.2A to 4A, efficiency loss is dominated by
MOSFET RDS_(ON) and inductor DC losses. Higher input
supply voltages will increase the Gate-to-Source voltage
on the internal MOSFETs, reducing the internal RDS_(ON).
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:
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.
Alternatively, under lighter loads, the ripple current due
to the inductance becomes a significant factor. When
light load efficiencies become more critical, a larger
inductor value may be desired. Larger inductances
reduce the peak-to-peak inductor ripple current, which
minimize losses. Figure 4 illustrates the effects of
inductance value at light load.
December 2010
13
M9999-120310-F