Datasheet

LT1912
12
1912fa
APPLICATIONS INFORMATION
VENDOR PHONE URL PART SERIES COMMANDS
Panasonic [phone] www.panasonic.com Ceramic,
Polymer,
Tantalum
EEF Series
Kemet [phone] www.kemet.com Ceramic,
Tantalum T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic
Table 2. Capacitor Vendors
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 neces-
sary 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. Table3 lists several Schottky diodes and their
manufacturers.
Table 3. Diode Vendors
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(mV)
V
F
AT 2A
(mV)
On Semicnductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B220
B230
DFLS240L
20
30
40
2
2
2
500
500
500
International Rectifier
10BQ030
20BQ030
30
30
1
2
420 470
470
Ceramic Capacitors
A precaution regarding ceramic capacitors concerns the
maximum input voltage rating of the LT1912. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT1912 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT1912’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
Frequency Compensation
The LT1912 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT1912 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. Frequency
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 addi-
tion, 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.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application