MIC22205 Datasheet, PDF (15/29 Pages) Micrel Semiconductor – 2A, Integrated, Switch, High-Efficiency

Micrel, Inc.
Maintaining high efficiency serves two purposes. First, it
decreases power dissipation in the power supply, which
reduces the need for heat sinks and thermal design
considerations; also, it decreases the consumption of
current for battery-powered applications. Reduced
current demand from a battery increases the device’s
operating time, which is critical in hand-held devices.
There are mainly two loss terms in switching converters:
static losses and switching losses. Static losses are 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, which also dissipates power. Similarly, the
inductor’s DCR and capacitor’s ESR also contribute to
I²R losses. A device’s 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 off at a constant 1MHz frequency,
and the switching transitions make up the switching
losses.
Figure 2 illustrates a typical efficiency curve. From 0A to
0.2A, efficiency losses are dominated by quiescent
current losses, gate drive, transition, and core losses. In
this case, lower supply voltages yield greater efficiency
because they require less current to drive the
MOSFETs, and have reduced input power consumption.
Efficiency (V_IN = 5V)
100
vs. Output Current
95
3.3V
90
85
80
1.8V
75
70
65
60
VIN = 5V
55
50
0
0.5
1
1.5
2
OUTPUT CURRENT (A)
Figure 2. Efficiency Curve
MIC22205
From 0.5A to 2A, 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, thereby reducing the internal
RDS_(ON). This improves efficiency by decreasing DC
losses in the device. All but the inductor losses are
inherent to the device. In this case, inductor selection is
critical for 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 in losses.
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.
Compensation
The MIC22205 has a combination of internal and
external stability compensation to simplify the circuit for
small, high efficiency designs. In such designs, voltage
mode conversion is often the optimum solution. Voltage
mode is achieved by creating an internal ramp signal,
and using the output of the error amplifier to modulate
the pulse width of the switch node, thereby maintaining
output voltage regulation. With a typical gain bandwidth
of 100kHz-200kHz, the MIC22205 is capable of
extremely fast transient response.
August 2011
15
M9999-082511-A