LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
APPLICATIONS INFORMATION
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size
AVX TPS, Sprague 593D
AVX TAJ
D Case Size
AVX TPS, Sprague 593D
C Case Size
AVX TPS
0.2 (typ)
0.5 (typ)
0.1 to 0.3
0.7 to 1.1
ESR (Max, Ω)
0.1 to 0.3
0.7 to 0.9
Ripple Current (A)
0.7 to 1.1
0.4
I
OUT
(
MAX
)
=
Continuous Mode
I
P
–
( V
OUT
)( V
IN
– V
OUT
)
2(L)( f)( V
IN
)
For V
IN
= 8V, V
OUT
= 5V and L = 3.3μH,
I
OUT(MAX )
=
3
−
2 3.3 • 10
−
6
1.25 • 10
6
(
8
)
=
3
−
0.23
=
2.77 A
(
(
5
) (
8
−
5
)
)(
)
Figure 3 shows a comparison of output ripple for a ceramic
and tantalum capacitor at 200mA ripple current.
Note that the worst case (minimum output current avail-
able) condition is at the maximum input voltage. For the
same circuit at 15V, maximum output current would be
only 2.6A.
Inductor Selection
The output inductor should have a saturation current rating
greater than the peak inductor current set by the current
comparator of the LT1765. The peak inductor current will
depend on the output current, input and output voltages
and the inductor value:
V
OUT
USING 47μF, 0.1Ω
TANTALUM CAPACITOR
(10mV/DIV)
V
OUT
USING 2.2μF
CERAMIC CAPACITOR
(10mV/DIV)
V
SW
(5V/DIV)
0.2μs/DIV
1765 F03
I
PEAK
=
I
OUT
+
V
OUT
(
V
IN
−
V
OUT
)
2
(
L
)(
f
)
(
V
IN
)
Figure 3. Output Ripple Voltage Waveform
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
Maximum output current for an LT1765 buck converter is
equal to the maximum switch rating (I
P
) minus one half
peak to peak inductor ripple current. The LT1765 main-
tains a constant switch current rating at all duty cycles.
(Patent Pending)
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.
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.
V
IN
= Maximum input voltage
f = Switching frequency, 1.25MHz
If an inductor with a peak current lower than the maximum
switch current of the LT1765 is chosen a soft-start circuit
in Figure 10 should be used. Also, short-circuit conditions
should not be allowed because the inductor may saturate
resulting in excessive power dissipation.
Also, consideration should be given to the resistance
of the inductor. Inductor conduction loses are directly
proportional to the DC resistance of inductor. Sometime,
the manufacturers will also provide maximum current
rating based on the allowable losses in the inductor. Care
should be taken, however. At high input voltages and low
DCR, excessive switch current could flow during shorted
output condition.
Suitable inductors are available from Coilcraft, Coiltronics,
Dale, Sumida, Toko, Murata, Panasonic and other
manufacturers.
1765fd
9
LINER [ LINEAR TECHNOLOGY ]
