Datasheet

LT3980
12
3980fa
For more information www.linear.com/LT3980
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 2 lists several capacitor
vendors.
Table 2. Capacitor Vendors
VENDOR URL PART SERIES COMMANDS
Panasonic www.panasonic.com Ceramic, Polymer,
Tantalum
EEF Series
Kemet www.kemet.com Ceramic, Tantalum T494, T495
Sanyo www.sanyovideo.com Ceramic, Polymer,
Tantalum
POSCAP
Murata www.murata.com Ceramic SOT-23
AVX www.avxcorp.com Ceramic, Tantalum SOT-23
Taiyo Yuden www.taiyo-yuden.com Ceramic TPS Series
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
I
D(AVG)
= I
OUT
(V
IN
– V
OUT
)/V
IN
where I
OUT
is the output load current. The only reason to
consider a diode with a larger current rating than necessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3980 will keep the
switch off when V
IN
> 64V which allows the use of 64V
rated Schottky even when V
IN
ranges up to 80V.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3980 due to their piezoelectric nature.
When in Burst Mode operation, the LT3980’s switching
frequency depends on the load current, and at very light
loads the LT3980 can excite the ceramic capacitor at
audio frequencies, generating audible noise. Since the
LT3980 operates at a lower current limit during Burst
Mode operation, the noise is nearly silent to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Frequency Compensation
The LT3980 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3980 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequen
-
cy compensation is provided by the components tied to
the V
C
pin, as shown in Figure 2. Generally a capacitor
(C
C
) and a resistor (R
C
) in series to ground are used. In
addition, there may be lower value capacitor in parallel.
This capacitor (C
F
) is not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
–
+
0.79V
SW
V
C
g
m
=
500µmho
GND
3M
LT3980
3980 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 5.3mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 2. Model for Loop Response
applicaTions inFormaTion