LT3506/LT3506A
U U
W U
APPLICATIO S I FOR ATIO
Finally,theremustbeenoughcapacitanceforgoodtransient
performance.Thelastequationgivesagoodstartingpoint.
Alternatively, you can start with one of the designs in this
datasheetandexperimenttogetthedesiredperformance.
This topic is covered more thoroughly in the section on
loop compensation.
Table 2. Low-ESR Surface Mount Capacitors
VENDOR
Taiyo-Yuden
AVX
TYPE
SERIES
Ceramic
Ceramic
Tantalum
TPS
Kemet
Tantalum
Tantalum
Organic
Aluminum
Organic
T491, T494, T495, T520
For 5V and 3.3V outputs with greater than 1A output, a
22μF 6.3V ceramic capacitor (X5R or X7R) at the output
results in very low output voltage ripple and good tran-
sient response. For lower voltages, 22μF is adequate but
A700
Sanyo
Tantalum or Aluminum
Organic
POSCAP
increasing C
will improve transient performance. For
OUT
Panasonic
TDK
Aluminum
Organic
SP
CAP
the LT3506A, 10μF of output capacitance is sufficient at
between 3.3V and 5V. Other types and values can be
Ceramic
V
OUT
used. The following discusses tradeoffs in output ripple
and transient performance.
Catch Diode
The high performance (low ESR), small size and robust-
ness of ceramic capacitors make them the preferred type
for LT3506 applications. However, all ceramic capacitors
are not the same. As mentioned above, many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and temperature extremes. Because
the loop stability and transient response depend on the
The catch diode (D1 in Figure 2) must have a reverse volt-
age rating greater than the maximum input voltage. The
average current of the catch diode is given by:
I
=I (1-DC
DAVE OUT
)
MIN
A Schottky diode with a 1A average forward current rating
will suffice for most applications. The ON Semiconductor
MBRM120LT3 (20V) and MBRM130LT3 (30V) are good
choices;theyhaveatinypackagewithgoodthermalproper-
ties.Manyvendorshavesuitablesurfacemountversionsof
the 1N5817 (20V) and 1N5818 (30V) 1A Schottky diodes
such as the Microsemi UPS120.
value of C , you may not be able to tolerate this loss.
OUT
Use X7R and X5R types.
Youcanalsouseelectrolyticcapacitors. TheESRsofmost
aluminum electrolytics are too large to deliver low output
ripple. Tantalumandnewer, lowerESRorganicelectrolytic
capacitors intended for power supply use are suitable,
and the manufacturers will specify the ESR. The choice of
capacitor value will be based on the ESR required for low
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 perfor-
mance. One benefit is that the larger capacitance may give
bettertransientresponseforlargechangesinloadcurrent.
Table 2 lists several capacitor vendors.
Applications with large step down ratios and high output
currents may have more than 1A of average diode current.
TheONSemiconductorMBRS230LT3orInternationalRec-
tifier 20BQ030 (both 2A, 30V) would be good choices.
BOOST Pin Considerations
The capacitor and diode tied to the BOOST pin generate
a voltage that is higher than the input voltage. In most
cases a 0.1μF capacitor and fast switching diode (such
as the CMDSH-3 or FMMD914) will work well. Figure 3
3506afb
ꢀꢀ
Linear [ Linear ]
