LT8471
18
8471fd
For more information www.linear.com/8471
Capacitor Selection (Primary Channels)
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 4.7μF to 20μF output capaci
-
tor is
sufficient for most applications, but systems with
ver
y low output currents may need only a 1μF or 2.2μF
output capacitor. Always use a capacitor with a sufficient
voltage rating. Many capacitors rated at 2.2μF to 20μF,
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 ceramic ones and will have higher
ESR with greater output ripple.
Low ESR capacitors should also be used as the input de
-
coupling capacitors,
which should be placed as closely as
possible to the LT8471. Ceramic capacitors make a good
choice for this purpose. A 2.2μF to 4.7μF 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 capacitors.
Table 2. Ceramic Capacitor Manufacturers
VENDOR WEB
Kemet www.kemet.com
Murata www.murata.com
Taiyo-Yuden www.t-yuden.com
TDK www.tdk.com
Compensation Theory (Primary Channels)
Like
all other current mode switching regulators, the
primary channels of LT8471 need to be compensated for
stable and efficient operation. For each primary channel,
two feedback loops are used—a fast current loop which
does not require compensation, and a slower voltage
loop which does. In order to reduce the PCB footprint, the
voltage loop compensation network is integrated inside
the LT8471. Therefore, only the inductor and the output
capacitor are available for adjusting the loop stability.
Standard bode plot analysis can be used to analyze and
adjust the voltage feedback loop.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 3 shows the key equivalent elements of a boost/
buck/inverting converter. Because of the fast current con
-
trol loop, the power stage of the IC, inductor and diode
have
been replaced by a combination of the equivalent
transconductance amplifier g
mp
and the current controlled
current source (which converts I
VIN
to η • V
IN
•
I
VIN
/V
OUT
for boost converters, I
VIN
to η • V
IN
• DC/V
OUT
for buck
and single inductor inverting converters). g
mp
acts as
a current source where the peak input current, I
VIN
, is
proportional to the VC voltage. η is the efficiency of the
switching regulator, and is typically about 88%.
Note that the maximum output currents of g
mp
and g
ma
are finite. The limits for g
mp
are in the Electrical Character-
istics section (
switch current limit), and g
ma
is nominally
limited to about ±5μA.
applicaTions inForMaTion
Figure 3. Boost/Buck/Inverting Converter Equivalent Model
–
+
–
+
g
ma
R
C
R
O
BOOST:
BUCK AND SINGLE
INDUCTOR INVERTING:
R
B
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
g
ma
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
DIVIDED BY I
LOAD(MAX)
R
O
: OUTPUT RESISTANCE OF g
ma
R
A
, R
B
: FEEDBACK RESISTOR DIVIDER NETWORK
R
ESR
: OUTPUT CAPACITOR ESR
8471 F03
R
A
C
OUT
C
PL
R
L
R
ESR
I
VIN
V
OUT
V
C
C
C
C
F
g
mp
REFERENCE
FB
• I
VIN
η • V
IN
V
OUT
η • V
IN
• I
VIN
• DC
V
OUT
