MIC22705_11 Datasheet, PDF (15/29 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Reduced current demand 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 off at a constant 1MHz frequency
and the switching transitions make up the switching
losses.
Figure 2 illustrates an efficiency curve. The portion, from
0A to 0.4A, 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 (VIN = 3.3V)
vs. Output Current
100
95
90
85
80
V_IN = 3.3V
75
I_OUT = 1.8V
70
01234567
OUTPUT CURRENT (A)
Figure 2. Efficiency Curve
The region, 1A to 7A, 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.
MIC22705
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 maybe desired. Larger inductances
reduce the peak-to-peak inductor ripple current, which
minimize losses.
Compensation
The MIC22705 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 1MHz 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 MIC22705 is
capable of extremely fast transient responses.
The MIC22705 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
⎜⎜⎝⎛
2×π
1
×
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.
March 2011
15
M9999-033111-A