MIC22400 Datasheet, PDF (13/19 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
Figure 2 shows an efficiency curve. The portion, from 0A
to 0.2A, 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 V_O = 1.2V
100
95
90
85
80
75
70
65
V_IN = 3.3V
60
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
OUTPUT CURRENT (A)
Figure 2. Efficiency Curve
The region, 0.2A 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, 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 may be desired. Larger inductances
reduce the peak-to-peak inductor ripple current, which
minimize losses. The following graph in Figure 3
illustrates the effects of inductance value at light load.
MIC22400
Efficiency
vs. Inductance
94 _4.7µH
92
90
88
86
84
1µH
82
80
78
V_IN = 3.3V
760 _{0.2 0.4 0.6 0.8 1.0 1.2}
OUTPUT CURRENT (A)
Figure 3. Efficiency vs. Inductance
Compensation
The MIC22400 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 100-200kHz, the MIC22400 is capable of
extremely fast transient responses.
The MIC22400 is designed to be stable with a typical
application using a 1µH inductor and a 47µF 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 stability
2⋅Π⋅ L⋅C
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.
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
LÈ
0.47µH
1µH
0*-10pF
0^†-15pF
2.2µH
15-33pF
* VOUT > 1.2V, ^† VOUT > 1V
47µF-
100µF
22pF
15-22pF
33-47pF
100µF-
470µF
33pF
33pF
100-220pF
February 2008
13
M9999-022108-A