MIC2155_0911 Datasheet, PDF (21/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
Each supply contributes approximately half the power:
P_IN1 = P_IN2 = 27.5W
For V_IN1 = 12V and V_IN2 = 3.3V
I_IN1
=
27.5W
12V
=
2.3A
and
I_IN2
=
27.5W
3.3V
= 8.3A
Component Selection, Guidelines and Design
Example
The following section outlines a procedure for designing
a two-phase synchronous buck converter using the
MIC2155.
This example will use the following parameters:
V_IN = 12V
V_OUT = 1.8V
I_OUT = 30A
Switching frequency (f_S) = 500kHz/channel
(MIC2155)
Output Filter Selection
The output filter is comprised of the output capacitors
and the output inductors. The filter is designed to
attenuate the output voltage ripple to the desired value.
The output filter components also determine how well
the supply responds to output current transients. If
output transients are significant, the output capacitors
should be chosen first to meet the transient specification.
The output inductor is then selected to insure the filter
attenuates the output ripple to meet the specification.
A second, commonly used method of designing the filter
is to select the inductor value to keep the ripple current
between 20% and 30% of the output current for that
channel. Then select the output capacitance to meet the
output voltage ripple specification and output current
transient specification.
Values for inductance, peak and RMS currents are
required to choose the output inductors. The input and
output voltages and the inductance value determine the
peak to peak inductor ripple current. Output capacitor
selection requires calculation of transient current, RMS
capacitor current and output voltage.
There are several tradeoffs to be made when selecting
the output inductor. Generally, higher inductance values
are used with higher input voltages. Larger peak to peak
ripple currents will increase the power dissipation in the
inductor and MOSFETs. Larger output ripple currents
MIC2155/2156
will also require more output capacitance to smooth out
the larger ripple current. Smaller peak to peak ripple
currents require a larger inductance value and therefore
a larger and more expensive inductor.
Higher switching frequencies allow the use of a small
inductance but increase power dissipation in the inductor
core and MOSFET switching loss. The MIC2155
switches at 500kHz/channel and is designed to use a
smaller inductor at the expense of higher switching
losses and slightly lower efficiency. While the 300kHz
MIC2156 was optimized for higher efficiency and higher
output current but its lower switching frequency requires
a larger output inductance to maintain the same peak-to-
peak output ripple current.
The peak output ripple current for a two-phase converter
is shown in Figure 19. The graph shows that peak ripple
current is a function of duty cycle. Since each channel is
180° out of phase with the other, at 50% duty cycle, the
output ripple currents from each channel cancel and
output ripple current is close to zero.
1.0
0.9
0.8
0.7
0.6
Single Phase
0.5
0.4
0.3
2 Phase
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY CYCLE
Figure 19. 2 Phase Output Ripple Current vs. Duty Cycle
For this example, with V_IN = 12V, V_OUT = 1.8V and
efficiency = 88%, the duty cycle is:
D = ^VOUT = ^1.8V = 0.17
η × V_IN 0.88 ×12
Figure 19 shows the peak-to-peak output ripple current
normalized to:
V_OUT
f_S × L_OUT
The peak-to-peak output ripple current is less than for a
single phase conversion. If V_IN varies, the input voltage
that generated the highest ripple current should be used
for the calculation.
November 2009
21
M9999-111209-B