LTC1871 Linear Technology, LTC1871 Datasheet - Page 30

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1871
6. The diode for this design must handle a maximum DC
7. The output capacitor usually consists of a high valued
30
output current of 2A and be rated for a minimum
reverse voltage of V
from International Rectifier (30BQ040) is chosen for its
small size, relatively low forward drop and acceptable
reverse leakage at high temp.
bulk C connected in parallel with a lower valued, low
ESR ceramic. Based on a maximum output ripple
voltage of 1%, or 120mV, the bulk C needs to be greater
than:
The RMS ripple current rating for this capacitor needs
to exceed:
To satisfy this high RMS current demand, two 47 F
Kemet capacitors (T495X476K020AS) are required. As
a result, the output ripple voltage is a low 50mV to
60mV. In parallel with these tantalums, two 10 F, low
ESR (X5R) Taiyo Yuden ceramic capacitors
(TMK432BJ106MM) are added for HF noise reduction.
200mV/DIV
V
0.5A/DIV
OUT
I
C
1 5
0 01 12
RMS COUT
(AC)
.
I
OUT
. •
OUT
A
(
•
V
V
IN
OUT
1 5
0 01
= 4.5V
12
.
V
= 12V
. •
5
)
I
OUT MAX
V
A
•
V
300
I
U
O MAX
V
(
(
OUT
2 3
kHz
IN
.
+ V
)
A
U
50 s/DIV
)
•
•
f
OUT
41
V
, or 27V. A 3A, 40V diode
IN MIN
F
V
(
Figure 19. LTC1871 SEPIC Converter Load Step Response
O
W
)
1871 F19a
U
8. The choice of an input capacitor for a SEPIC converter
9. The DC coupling capacitor in a SEPIC converter is
Check the output ripple with a single oscilloscope probe
connected directly across the output capacitor termi-
nals, where the HF switching currents flow.
depends on the impedance of the source supply and the
amount of input ripple the converter will safely tolerate.
For this particular design and lab setup, a single 47 F
Kemet tantalum capacitor (T495X476K020AS) is ad-
equate. As with the output node, check the input ripple
with a single oscilloscope probe connected across the
input capacitor terminals. If any HF switching noise is
observed it is a good idea to decouple the input with a
low ESR, X5R ceramic capacitor as close to the V
GND pins as possible.
chosen based on its RMS current requirement and
must be rated for a minimum voltage of V
ripple voltage. Start with the minimum value which
satisfies the RMS current requirement and then check
the ripple voltage to ensure that it doesn’t exceed the DC
rating.
For this design a single 10 F, low ESR (X5R) Taiyo
Yuden ceramic capacitor (TMK432BJ106MM) is
adequate.
200mV/DIV
V
0.5A/DIV
OUT
I
RMS CI
(AC)
I
OUT
( )
V
V
IN
OUT
= 15V
= 12V
I
O MAX
(
1 5
.
A
)
•
•
50 s/DIV
12
V
V
IN MIN
O
V
(
5
V
V
0 5
D
)
.
V
2 4
1871 F19b
IN
.
A
plus the AC
IN
and