lt1765es8-trpbf Linear Technology Corporation, lt1765es8-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt1765es8-trpbf
• Description: Monolithic 3a, 1.25mhz Step-down Switching Regulators
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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
Figure 3 shows a comparison of output ripple for a ceramic
and tantalum capacitor at 200mA ripple current.
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
Maximum output current for an LT1765 buck converter is
equal to the maximum switch rating (I
peak to peak inductor ripple current. The LT1765 maintains
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.
TANTALUM CAPACITOR
V
OUT
CERAMIC CAPACITOR
USING 47µF, 0.1Ω
V
OUT
Figure 3. Output Ripple Voltage Waveform
USING 2.2µF
(10mV/DIV)
(10mV/DIV)
(5V/DIV)
V
SW
U
ESR (Max, Ω)
U
0.1 to 0.3
0.7 to 0.9
0.1 to 0.3
0.2 (typ)
0.2µs/DIV
W
P
) minus one half
Ripple Current (A)
0.7 to 1.1
0.7 to 1.1
0.5 (typ)
0.4
U
1765 F03
For V
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:
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 pro-
portional 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.
LT1765/LT1765-1.8/LT1765-2.5/
I
I
Continuous Mode
V
f = Switching frequency, 1.25MHz
OUT MAX
OUT MAX
I
PEAK
IN
IN
( )
(
= Maximum input voltage
= 8V, V
=
I
)
OUT
= −
=
= −
OUT
3
3 0 23 2 77
+
V
= 5V and L = 3.3µH,
2 3 3 10
LT1765-3.3/LT1765-5
OUT
(
.
2
I
P
. •
( )( )( )
L f V
(
–
=
V
IN
(
V
( )
.
OUT
−
−
5 8 5
6
2
V
IN
( )
)(
A
( )( )(
OUT
)(
L f V
1 25 10
V
−
.
IN
)
–
•
IN
V
OUT
)
6
)
( )
8
)
1765fb
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