LT1946 LINER [Linear Technology], LT1946 Datasheet - Page 6

• Manufacturer Part Number: LT1946
• Description: 1.2MHz Boost DC/DC Converter with 1.5A Switch and Soft-Start
• Manufacturer: LINER [Linear Technology]

LT1946
APPLICATIO S I FOR ATIO
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 dielectrics are preferred, followed by
X7R, as these materials retain the capacitance over wide
voltage and temperature ranges. A 4.7 F to 20 F output
capacitor is sufficient for most applications, but systems
with very low output currents may need only a 1 F or 2.2 F
output capacitor. 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. Always use a capacitor
with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT1946. A 2.2 F to 4.7 F input capacitor is
sufficient for most applications. Table 2 shows a list of
several ceramic capacitor manufacturers. Consult the
manufacturers for detailed information on their entire
selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
Taiyo Yuden
AVX
Murata
Compensation—Adjustment
To compensate the feedback loop of the LT1946, a series
resistor-capacitor network should be connected from the
COMP pin to GND. For most applications, a capacitor in
the range of 220pF to 680pF will suffice. A good starting
value for the compensation capacitor, C
compensation resistor, R
to 100k. A good technique to compensate a new applica-
tion is to use a 100k potentiometer in place of R
use a 470pF capacitor for C
eter while observing the transient response, the optimum
value for R
process for the circuit of Figure 1 with a load current
stepped from 250mA to 300mA. Figure 3a shows the tran-
sient response with R
6
C
can be found. Figures 3a to 3c illustrate this
U
C
equal to 7.5k. The phase margin is
C
U
, is usually in the range of 20k
C
. By adjusting the potentiom-
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W
C
, is 470pF. The
U
C
, and
poor as evidenced by the excessive ringing in the output
voltage and inductor current. In Figure 3b, the value of R
is increased to 18k, which results in a more damped re-
sponse. Figure 3c shows the results when R
further to 49.9k. The transient response is nicely damped
and the compensation procedure is complete. The COMP
pin provides access to an internal resistor (120k) and
capacitor (90pF). For some applications, these values will
suffice and no external R
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
Figure 3a. Transient Response Shows Excessive Ringing
20mV/DIV
20mV/DIV
20mV/DIV
0.5A/DIV
0.5A/DIV
0.5A/DIV
V
V
V
Figure 3c. Transient Response is Well Damped
OUT
OUT
OUT
I
I
I
LI
LI
LI
R
R
R
Figure 3b. Transient Response is Better
C
C
C
= 7.5k
= 49.9k
= 18k
200 s/DIV
200 s/DIV
200 s/DIV
C
and C
C
will be needed.
1946 F03a
1946 F03b
1946 F03b
C
is increased
sn1946 1946fs
C