MIC22405 Datasheet, PDF (15/30 Pages) Micrel Semiconductor – 4A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
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 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
in that they require less current to drive the MOSFETs
and have reduced input power consumption.
Efficiency (V_IN = 5.0V)
vs. Output Current
100
95
3.3V
90
1.8V
85
80
V_IN = 5.0V
75
70
0
1
2
3
4
OUTPUT CURRENT (A)
Figure 2. Efficiency Curve
From 0.5A 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, 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 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
MIC22405
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 maybe desired. Larger inductances
reduce the peak-to-peak inductor ripple current, which
minimize losses.
Compensation
The MIC22405 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 MIC22405 is capable of
extremely fast transient response.
The MIC22405 is designed to be stable with a typical
application using a 1µH inductor and a 100µF ceramic
(X5R) output capacitor. These values can be varied
dependent upon the tradeoff between size, cost and
efficiency, keeping the LC natural frequency
⎜⎜⎝⎛
1
2×π×
L×C
⎟⎟⎠⎞
ideally
less
than
26
kHz
to
ensure
stability can be achieved. The minimum recommended
inductor value is 0.47µH and minimum recommended
output capacitor value is 22µF. The tradeoff between
changing these values is that with a larger inductor,
there is a reduced peak-to-peak current which yields a
greater efficiency at lighter loads. A larger output
capacitor will improve transient response by providing a
larger hold up reservoir of energy to the output.
June 2011
15
M9999-061511-A