MIC22950 Datasheet, PDF (11/22 Pages) Micrel Semiconductor – 10A Integrated Switch Synchronous Buck Regulator with Frequency Programmable to 2MHz

Micrel, Inc.
drive the gates on and off at a constant 400kHz to 2MHz
frequency and the switching transitions make up the
switching losses.
Figure 2 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 vs. Load Current
100
95
90
85
80
75
70
3.6V to 1.8V
65
60
0
2
4
6
8
10
LOAD CURRENT (A)
Figure 1. 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 voltage on the internal MOSFETs, thus 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
MIC22950
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. The following graph, in Figure 2,
illustrates the effects of inductance value at light load.
Efficiency vs. Inductance
100
95 L = 4.7µH
90
85
L = 1µH
80
75
70
65
60
55
50
0
200 400 600 800 1000
OUTPUT CURRENT (mA)
Figure 2. Efficiency vs. Inductance
Compensation
The MIC22950 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
which is derived from the CF current charging an
external capacitor. This ramp is compared to 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 – 200kHz, the MIC22950
is capable of fast transient responses.
The MIC22950 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 F_SW/17 to ensure
2 × π × L × ⋅C
stability can be achieved. The minimum recommended
inductor value is 0.39µH and minimum recommended
output capacitor value is 10µF.
February 2010
11
M9999-021910-B