LT1912 Linear Technology Corporation, LT1912 Datasheet - Page 12

• Manufacturer Part Number: LT1912
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LT1912
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
where I
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. Table
3 lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
12
Table 2. Capacitor Vendors
PART NUMBER
On Semicnductor
MBRM120E
MBRM140
Diodes Inc.
B220
B230
DFLS240L
International Rectifi er
10BQ030
20BQ030
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
I
D(AVG)
OUT
= I
is the output load current. The only reason to
OUT
PHONE
[phone]
[phone]
[phone]
[phone]
[phone]
(V
IN
– V
(V)
V
20
40
20
30
40
30
30
R
OUT
)/V
I
(A)
AVE
1
1
2
2
2
1
2
IN
V
F
(mV)
530
550
420
AT 1A
URL
www.panasonic.com
www.kemet.com
www.sanyovideo.com
www.murata.com
www.avxcorp.com
www.taiyo-yuden.com
V
F
(mV)
595
500
500
500
470
470
AT 2A
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 simplifi es 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
and a resistor (R
tion, there may be lower value capacitor in parallel. This
capacitor (C
is used to fi lter 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 ap-
plication and in particular the type of output capacitor. A
C
pin, as shown in Figure 2. Generally a capacitor (C
PART SERIES
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
F
) is not part of the loop compensation but
C
) in series to ground are used. In addi-
COMMANDS
EEF Series
T494, T495
POSCAP
TPS Series
1912f
C
)