LTC1871EMS-7 Linear Technology, LTC1871EMS-7 Datasheet - Page 17

• Manufacturer Part Number: LTC1871EMS-7
• Description: IC MULTI CONFIG SYNC ADJ 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871EMS-7

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 6 ~ 36 V
• Operating Temperature: -40°C ~ 85°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: -

APPLICATIONS INFORMATION
For some designs it may be possible to choose a single
capacitor type that satisfi es both the ESR and bulk C require-
ments for the design. In certain demanding applications,
however, the ripple voltage can be improved signifi cantly
by connecting two or more types of capacitors in paral-
lel. For example, using a low ESR ceramic capacitor can
minimize the ESR step, while an electrolytic capacitor can
be used to supply the required bulk C.
Once the output capacitor ESR and bulk capacitance have
been determined, the overall ripple voltage waveform
should be verifi ed on a dedicated PC board (see Board
Layout section for more information on component place-
ment). Lab breadboards generally suffer from excessive
series inductance (due to inter-component wiring), and
these parasitics can make the switching waveforms look
signifi cantly worse than they would be on a properly
designed PC board.
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 13. 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.
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. Also, ceramic
capacitors are now available with extremely low ESR, ESL
and high ripple current ratings.
I
RMS(COUT)
I
O(MAX)
•
V
O
V
– V
IN(MIN)
IN(MIN)
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input and the input current waveform
is continuous (see Figure 13b). The input voltage source
impedance determines the size of the input capacitor,
which is typically 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 boost con-
verter is:
I
RMS(CIN)
Figure 13. Switching Waveforms for a Boost Converter
V
13e. Output Voltage Ripple Waveform
(AC)
I
OUT
SW
I
I
D
L
13b. Inductor and Input Currents
13d. Diode and Output Currents
V
IN
= 0.3 •
t
ON
13a. Circuit Diagram
13c. Switch Current
L
SW
ΔV
V
t
IN(MIN)
OFF
ESR
L • f
ΔV
D
COUT
• D
MAX
C
OUT
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
LTC1871-7
V
OUT
I
IN
R
L
I
O
18717 F13
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