LT8580
11
8580fa
For more information www.linear.com/LT8580
applicaTions inForMaTion
For dual dual inductor topologies, the peak output inductor
current is given by:
I
L2-PEAK
=I
OUT
+
OUT
−
For the dual inductor topologies, the total peak current is:
I
L-PEAK
=I
OUT
1+
V
OUT
h • V
⎡
⎣
⎢
⎤
⎦
⎥
+
V
IN
• DC
2 • L • f
Note: Peak inductor current is limited by the switch current
limit. Refer to the Electrical Characteristics table and to
the Switch Current Limit vs Duty Cycle plot in the Typical
Performance Characteristics.
Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice, as
they have an extremely low ESR and are available in very
small packages. X5R or X7R dielectrics are preferred, as
these materials retain their capacitance over wider voltage
and temperature ranges. A 0.47µF to 10µF output capacitor
is sufficient for most applications. Always use a capacitor
with a sufficient voltage rating. Many ceramic capacitors,
particularly 0805 or 0603 case sizes, have greatly reduced
capacitance at the desired output voltage. Solid tantalum
or OS-CON capacitors can be used, but they will occupy
more board area than a ceramic and will have a higher
ESR with greater output ripple.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as closely
as possible to the V
IN
pin of the LT8580 as well as to the
inductor connected to the input of the power path. If it is
not possible to optimally place a single input capacitor,
then use one at the V
IN
pin of the chip (C
VIN
) and one at
the input of the power path (C
PWR
). See equations in
Table 4, Table 5 and Table 6 for sizing information. A 1µF
to 2.2µF input capacitor is sufficient for most applications.
Table 2 shows a list of several ceramic capacitor manufac-
turers. Consult the manufacturers for detailed information
on their entire selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
TDK www.tdk.com
Compensation—Adjustment
To compensate the feedback loop of the LT8580, a series
resistor-capacitor network in parallel with a single capacitor
should be connected from the VC pin to GND. For most
applications, the series capacitor should be in the range
of 470pF to 2.2nF with 1nF being a good starting value.
The parallel capacitor should range in value from 10pF to
100pF with 47pF a good starting value. The compensation
resistor, R
C
, is usually in the range of 5k to 50k. A good
technique to compensate a new application is to use a
100kΩ potentiometer in place of series resistor R
C
. With
the series capacitor and parallel capacitor at 1nF and 47pF
respectively, adjust the potentiometer while observing
the transient response and the optimum value for R
C
can
be found. Figure 3 (3a to 3c) illustrates this process for
the circuit of Figure 4 with a load current stepped be
-
tween 60mA and 160mA. Figure 3a shows the transient
response with R
C
equal to 2k. The phase margin is poor,
as evidenced by the excessive ringing in the output
voltage and inductor current. In Figure 3b, the value of
R
C
is increased to 3k, which results in a more damped
response. Figure 3c shows the results when R
C
is increased
further to 6.04k. The transient response is nicely damped
and the compensation procedure is complete.
Compensation—Theory
Like all other current mode switching regulators, the
LT8580 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT8580—
a fast current loop which does not require compensation,
and a slower voltage loop which does. Standard bode plot
analysis can be used to understand and adjust the voltage
feedback loop.
