LTC1871IMS Linear Technology, LTC1871IMS Datasheet - Page 28

• Manufacturer Part Number: LTC1871IMS
• Description: IC MULTI CONFIG SYNC ADJ 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871IMS

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 72 V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 2.5 ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LTC1871
APPLICATIONS INFORMATION
The output capacitor in a SEPIC regulator experiences
high RMS ripple currents, as shown in Figure 17. The
RMS output capacitor ripple current is:
Note that the ripple current ratings from capacitor manu-
facturers are often based on only 2000 hours of life. This
makes it advisable to further derate the capacitor or to
choose a capacitor rated at a higher temperature than
required. Several capacitors may also be placed in parallel
to meet size or height requirements in the design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest product of
ESR and size of any aluminum electrolytic, at a somewhat
higher price.
In surface mount applications, multiple capacitors may
have to be placed in parallel in order to meet the ESR or
RMS current handling requirements of the application.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount packages. In the case of
tantalum, it is critical that the capacitors have been surge
tested for use in switching power supplies. An excellent
choice is AVX TPS series of surface mount tantalum. Also,
ceramic capacitors are now available with extremely low
ESR, ESL and high ripple current ratings.
SEPIC Converter: Input Capacitor Selection
The input capacitor of a SEPIC converter is less critical
than the output capacitor due to the fact that an inductor
is in series with the input and the input current waveform
is triangular in shape. The input voltage source impedance
determines the size of the input capacitor which is typi-
cally in the range of 10μF to 100μF . A low ESR capacitor
is recommended, although it is not as critical as for the
output capacitor.
The RMS input capacitor ripple current for a SEPIC con-
verter is:
28
I
I
RMS(CIN)
RMS(COUT)
=
=I
1
12
O(MAX)
• I
L
•
V
IN(MIN)
V
O
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to
the input of the converter and solid tantalum capacitors
can fail catastrophically under these conditions. Be sure
to specify surge-tested capacitors!
SEPIC Converter: Selecting the DC Coupling Capacitor
The coupling capacitor C1 in Figure 16 sees nearly a rectan-
gular current waveform as shown in Figure 17. During the
switch off-time the current through C1 is I
approximately – I
waveform creates a triangular ripple voltage on C1:
The maximum voltage on C1 is then:
which is typically close to V
through C1 is:
The value chosen for the DC coupling capacitor normally
starts with the minimum value that will satisfy 1) the RMS
current requirement and 2) the peak voltage requirement
(typically close to V
capacitors work well here.
SEPIC Converter Design Example
The design example given here will be for the circuit shown
in Figure 18. The input voltage is 5V to 15V and the output
is 12V at a maximum load current of 1.5A (2A peak).
1. The duty cycle range is:
2. The operating mode chosen is pulse skipping, so the
MODE/SYNC pin is shorted to INTV
I
V
RMS(C1)
C1(MAX)
V
D =
C1(P P)
V
=I
= V
IN
=
O(MAX)
V
+ V
O
IN
I
O(MAX)
C1• f
O
+ V
+
O
fl ows during the on-time. This current
+ V
IN
D
•
V
). Low ESR ceramic and tantalum
C1(P P)
D
•
2
V
V
V
IN(MIN)
O
IN
= 45.5% to 71.4%
+ V
+ V
IN(MAX)
V
D
O
O
+ V
. The ripple current
D
CC
.
O
(V
O
/V
IN
) while
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