MIC22601 Datasheet, PDF (11/18 Pages) Micrel Semiconductor – 4 MHz, 6A Integrated Switch Synchronous Buck Regulator

Micrel, Inc.
diodes between switching transitions. The MOSFET
body diode is less efficient for these short current pulses.
Figure 3 shows an efficiency curve. The non-shaded
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.
Efficiency 3.6V to 1.8V
L = 470nH (3mm x 3mm)
100
90
80
70
60
50
400 _{1 2 3 4 5 6}
OUTPUT CURRENT (A)
Figure 3. Efficiency Curve
The dashed region, 1A to 6A, efficiency loss is
dominated by MOSFET RDS_(ON) and inductor DC losses.
Higher input supply voltages will increase the Gate-to-
Source threshold 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 maybe desired. Larger inductances
reduce the peak-to-peak inductor ripple current, which
minimize losses.
MIC22601
Compensation
The MIC22601 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 4Mhz ramp
signal and using the output of the error amplifier to
modulate the pulse width of the switch node, maintaining
output voltage regulation. With a typical gain bandwidth
of 100-200 kHz, the MIC22601 is capable of extremely
fast transient responses.
The MIC22601 is designed to be stable with a typical
application using a 0.22µH inductor and a 47µF ceramic
(X5R) output capacitor. These values can be varied
dependant on the tradeoff between size, cost and
efficiency, keeping the LC natural frequency
(
1
) ideally less than 34kHz to ensure stability
2⋅Π⋅ L⋅C
can be achieved. The minimum recommended inductor
value is 0.22µH 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.
The integration of one pole-zero pair within the control
loop greatly simplifies compensation. The optimum
values for C_COMP (in series with a 20k resistor) are shown
below.
CÆ 22-47µF
47µF-
100µF-
LÈ
100µF
470µF
0.22µH
4.7pF
10pF
15pF
0.47µH
1µH
0*-10pF
0^†-15pF
22pF
15-22pF
33pF
33pF
2.2µH
15-33pF 33-47pF 100-220pF
* V_OUT > 1.2V, ^† V_OUT > 1V
Feedback
The MIC22601 provides a feedback pin to adjust the
output voltage to the desired level. This pin connects
internally to an error amplifier. The error amplifier then
compares the voltage at the feedback to the internal
0.7V reference voltage and adjusts the output voltage to
maintain regulation. To calculate the resistor divider
network for the desired output is as follows:
R2 = ^R1
⎜⎜⎝⎛
V_OUT
V_REF
− 1_{⎟⎟⎠⎞}
May 2009
11
M9999-050509-A