MIC22700_10 Datasheet, PDF (12/21 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Figure 2 shows an efficiency curve. The portion, from 0A
to 1A, 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.
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, 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
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.
MIC22700
The following graph in Figure 3 illustrates the effects of
inductance value at light load.
Efficiency
vs. Inductance
95
90
85
L = 1µH
80
75
L = 4.7µH
70
65
60
55
500
200 400 600 800
OUTPUT CURRENT (mA)
Figure 3. Efficiency vs. Inductance
Compensation
The MIC22700 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 100-200 kHz, the MIC22700 is capable of
extremely fast transient responses.
The MIC22700 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
dependant upon the tradeoff between size, cost and
efficiency, keeping the LC natural frequency
(
1
) ideally less than 26 kHz to ensure
2×π × L⋅C
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 2010
12
M9999-031810-B