LT1766 Linear Technology, LT1766 Datasheet - Page 10

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1766/LT1766-5
APPLICATIO S I FOR ATIO
where:
Peak-to-peak ripple current (I
and into the output capacitor is typically chosen to be
between 20% and 40% of the maximum load current. It is
approximated by:
Example: with V
and ESL = 10nH, output ripple voltage can be approxi-
mated as follows:
To reduce output ripple voltage further requires an in-
crease in the inductor value or a reduction in the capacitor
ESR. The latter can effect loop stability since the ESR
forms a useful zero in the overall loop response. Typically
the inductor value is adjusted with the trade-off being a
physically larger inductor with the possibility of increased
component height and cost. Choosing a smaller inductor
with lighter loads may result in discontinuous operation
but the LT1766 is designed to work well in both continu-
ous or discontinuous mode.
Peak Inductor Current and Fault Current
To ensure that the inductor will not saturate, the peak
inductor current should be calculated knowing the maxi-
mum load current. An appropriate inductor should then
be chosen. In addition, a decision should be made whether
10
ESR = equivalent series resistance of the output
capacitor
ESL = equivalent series inductance of the output
capacitor
dI/dt = slew rate of inductor ripple current = V
I
I
V
dI
dt
LP P
P-P
RIPPLE
0 0465 0 0085 55
-
.
47 10
40 47 10
V
•
40
OUT
0 465
IN
.
V
.
6
IN
= 40V, V
•
5 40 5
V
U
IN
A
10
f L
–
6
0 1
6
V
.
U
OUT
• .
OUT
200 10
0 85
mV
LP-P
= 5V, L = 47 H, ESR = 0.1
10 10
P-P
•
) through the inductor
•
W
3
9
0 465
.
10
6
U
A
0 85
IN
.
/L
or not the inductor must withstand continuous fault
conditions.
If maximum load current is 0.5A, for instance, a 0.5A
inductor may not survive a continuous 2A overload condi-
tion. Dead shorts will actually be more gentle on the
inductor because the LT1766 has frequency and current
limit foldback.
Peak switch and inductor current can be significantly
higher than output current, especially with smaller induc-
tors and lighter loads, so don’t omit this step. Powdered
iron cores are forgiving because they saturate softly,
whereas ferrite cores saturate abruptly. Other core
Table 2
VENDOR/
PART NO.
Coiltronics
CTX15-1P
CTX15-1
CTX33-2P
CTX33-2
UP2-330
UP2-470
UP2-680
UP2-101
Sumida
CDRH6D28-150M
CDRH6D38-150M
CDRH6D28-330M
CDRH104R-330M
CDRH125-330M
CDRH104R-470M
CDRH125-470M
CDRH6D38-680M
CDRH104R-680M
CDRH125-680M
CDRH104R-101M
CDRH125-101M
Coilcraft
DT3316P-153
DT3316P-333
DT3316P-473
VALUE
( H)
100
100
100
15
15
33
33
33
47
68
15
15
33
33
33
47
47
68
68
68
15
33
47
(Amps)
0.97
0.75
1.35
I
1.4
1.1
1.3
1.4
2.4
1.9
1.7
1.4
1.4
1.6
2.1
2.1
2.1
1.8
1.5
1.5
1.3
1.8
1.3
DC
1
(Ohms)
0.087
0.126
0.106
0.099
0.146
0.277
0.076
0.062
0.122
0.069
0.044
0.095
0.058
0.173
0.158
0.093
0.225
0.120
DCR
0.08
0.19
0.06
0.09
0.11
HEIGHT
(mm)
4.2
4.2
5.9
5.9
5.9
5.9
3.8
3.8
3.8
3.8
6
6
3
4
3
6
6
4
6
6
5
5
5
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