LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
9
1767fb
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applicaTions inForMaTion
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
Maximum output current for a buck converter is equal
to the maximum switch rating (I
P
) minus one half peak
to peak inductor current. In past designs, the maximum
switch current has been reduced by the introduction of
slope compensation. Slope compensation is required at
duty cycles above 50% to prevent an affect called sub
-
harmonic oscillation
(
see Application Note 19 for details).
The LT1767 has a new circuit technique that maintains a
constant switch current rating at all duty cycles.
For most applications, the output inductor will be in the
1µH to 10µH range. Lower values are chosen to reduce
the physical size of the inductor, higher values allow
higher output currents due to reduced peak to peak ripple
current, and reduces the current at which discontinuous
operation occurs. The following formula gives maximum
output current for continuous mode operation, implying
that the peak to peak ripple (2x the term on the right) is
less than the maximum switch current.
input capacitor is reduced so values in the range of 1µF
to 4.7µF are suitable for most applications.
Y5V or similar
type ceramics can be used since the absolute value of ca-
pacitance is
less important and has no significant effect on
loop
stability. If operation is required close to the minimum
input required by the output of the LT1767, a larger value
may be required. This is to prevent excessive ripple causing
dips below the minimum operating voltage, resulting in
erratic operation.
If tantalum capacitors are used, values in the 22µF to 470µF
range are generally needed to minimize ESR and meet ripple
current and surge ratings. Care should be taken to ensure
the ripple and surge ratings are not exceeded. The AVX TPS
and Kemet T495 series are surge rated. AVX recommends
derating capacitor operating voltage by 2:1 for high surge
applications.
OUTPUT CAPACITOR
Unlike the input capacitor, RMS ripple current in the output
capacitor is normally low enough that ripple current rating
is not an issue. The current waveform is triangular, with
an RMS value given by:
I
RIPPLE RMS
( )
=
0.29 V
OUT
( )
V
IN
− V
OUT
( )
L
( )
f
( )
V
IN
( )
The LT1767 will operate with both ceramic and tantalum
output capacitors. Ceramic capacitors are generally chosen
for their small size, very low ESR (effective series resistance),
and good high frequency operation, reducing output ripple
voltage. Typical ceramic output capacitors are in the 4.7µF
to 47µF range. Since the absolute value of capacitance
defines the pole frequency of the output stage, an X7R or
X5R type ceramic, which have good temperature stability,
is recommended.
Tantalum capacitors are usually chosen for their bulk capac
-
itance properties,
useful in high transient load applications.
ESR rather than capacitive value defines output ripple at
1.25MHz. Typical LT1767 applications require a tantalum
capacitor with less than 0.3Ω ESR at 22µF to 500µF.
Figure 3. Output Ripple Voltage Waveform
V
OUT
USING 47µF, 0.1Ω
TANTALUM CAPACITOR
(10mV/DIV)
0.2µs/DIV
1767 F03
V
OUT
USING 2.2µF
CERAMIC CAPACITOR
(10mV/DIV)
V
SW
(5V/DIV)
Figure 3 shows a comparison of output ripple for a ceramic
and tantalum capacitor at 200mA ripple current.
