Datasheet

LT1941
12
1941fb
APPLICATIONS INFORMATION
You can estimate output ripple with the following
equations:
V
RIPPLE
= ΔI
L
/(8 • f • C
OUT
) for ceramic capacitors
and
V
RIPPLE
= ΔI
L
• ESR for electrolytic capacitors (tantalum
and aluminum)
where ΔI
L
is the peak-to-peak ripple current in the inductor.
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
I
C(RMS)
= ΔI
L
/√12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
C
OUT
> 10 • L • (I
LIM
/V
OUT
)
2
The low ESR and small size of ceramic capacitors make
them the preferred type for LT1941 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coeffi cients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes.
Because loop stability and transient response depend on
the value of C
OUT
, this loss may be unacceptable. Use
X7R and X5R types.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Tantalum, as well as newer,
lower-ESR organic electrolytic capacitors intended for
power supply use are suitable. Chose a capacitor with a
low enough ESR for the required output ripple. Because
the volume of the capacitor determines its ESR, both the
size and the value will be larger than a ceramic capacitor
that would give similar ripple performance. One benefi t
is that the larger capacitance may give better transient
response for large changes in load current. Table 2 lists
several capacitor vendors.
Table 2. Low ESR Surface Mount Capacitors
VENDOR TYPE SERIES
Taiyo-Yuden Ceramic
AVX Ceramic
Tantalum
TPS
Kemet Tantalum
Tantalum Organic
Aluminum Organic
T491,T494,T495
T520
A700
Sanyo Tantalum or Aluminum Organic POSCAP
Panasonic Aluminum Organic SP CAP
TDK Ceramic
Diode Selection
The catch diode (D1 from Figure 2) 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
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 diode with a reverse voltage rating greater than the
input voltage. Table 3 lists several Schottky diodes and
their manufacturers.
Table 3. Schottky Diodes
PART
NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(mV)
V
F
AT 2A
(mV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B120
B130
B220
B230
20
30
20
30
1
1
2
2
500
500
500
500
International Rectifi er
10BQ030
20BQ030
30
30
1
2
420 470
470