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LTC1871EMS-7#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34
LTC1871-7
16
18717fd
applicaTions inForMaTion
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P
FET
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(CA)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
Boost Converter: Output Diode Selection
To maximize efficiency, a fast switching diode with low
forward drop and low reverse leakage is desired. The output
diode in a boost converter conducts current during the
switch off-time. The peak reverse voltage that the diode
must withstand is equal to the regulator output voltage.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to the
peak inductor current.
I
D(PEAK)
= I
L(PEAK)
= 1+
χ
2
•
I
O(MAX)
1– D
MAX
The power dissipated by the diode is:
P
D
= I
O(MAX)
• V
D
and the diode junction temperature is:
T
J
= T
A
+ P
D
• R
TH(JA)
Figure 12. Normalized R
DS(ON)
vs Temperature
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the board to the ambient temperature in the enclosure.
Remember to keep the diode lead lengths short and to
observe proper switch-node layout (see Board Layout
Checklist) to avoid excessive ringing and increased dis-
sipation.
Boost Converter: Output Capacitor Selection
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct component
for a given output ripple voltage. The effects of these three
parameters (ESR, ESL and bulk C) on the output voltage
ripple waveform are illustrated in Figure 13 for a typical
boost converter.
The choice of component(s) begins with the maximum
acceptable ripple voltage (expressed as a percentage of
the output voltage), and how this ripple should be divided
between the ESR step and the charging/discharging ∆V.
For the purpose of simplicity we will choose 2% for the
maximum output ripple, to be divided equally between the
ESR step and the charging/discharging ∆V. This percentage
ripple will change, depending on the requirements of the
application, and the equations provided below can easily
be modified.
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor can be determined using the fol-
lowing equation:
ESR
COUT
≤
0.01• V
O
I
IN(PEAK)
where:
I
IN(PEAK)
= 1+
χ
2
•
I
O(MAX)
1– D
MAX
For the bulk C component, which also contributes 1% to
the total ripple:
C
OUT
≥
I
O(MAX)
0.01• V
O
• f
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
18717 F12
0.5
0
0
50
100
2.0
LTC1871-7
17
18717fd
applicaTions inForMaTion
For some designs it may be possible to choose a single
capacitor type that satisfies both the ESR and bulk C require-
ments for the design. In certain demanding applications,
however, the ripple voltage can be improved significantly
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 verified 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
significantly 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:
I
RMS(COUT)
≈ I
O(MAX)
•
V
O
– V
IN(MIN)
V
IN(MIN)
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.
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)
= 0.3 •
V
IN(MIN)
L • f
•D
MAX
Figure 13. Switching Waveforms for a Boost Converter
V
IN
L D
SW
13a. Circuit Diagram
13b. Inductor and Input Currents
C
OUT
V
OUT
R
L
I
IN
I
L
13c. Switch Current
I
SW
t
ON
13d. Diode and Output Currents
13e. Output Voltage Ripple Waveform
I
O
18717 F13
I
D
V
OUT
(AC)
t
OFF
ΔV
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔV
COUT
LTC1871-7
18
18717fd
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!
Burst Mode Operation and Considerations
The choice of sense resistor and inductor value also deter-
mines the load current at which the LTC1871-7 enters Burst
Mode operation. When bursting, the controller clamps the
peak inductor current to approximately:
I
BURST(PEAK)
=
30mV
R
SENSE
which represents about 20% of the maximum 150mV
SENSE pin voltage. The corresponding average current
depends upon the amount of ripple current. Lower inductor
values (higher ∆I
L
) will reduce the load current at which
Burst Mode operations begins, since it is the peak current
that is being clamped.
The output voltage ripple can increase during Burst
Mode operation if ∆I
L
is substantially less than I
BURST
.
This can occur if the input voltage is very low or if a very
large inductor is chosen. At high duty cycles, a skipped
cycle causes the inductor current to quickly decay to
zero. However, because ∆I
L
is small, it takes multiple
cycles for the current to ramp back up to I
BURST(PEAK)
.
Table 1. Recommended Component Manufacturers
VENDOR COMPONENTS TELEPHONE WEB ADDRESS
AVX Capacitors (207) 282-5111 avxcorp.com
BH Electronics Inductors, Transformers (952) 894-9590 bhelectronics.com
Coilcraft Inductors (847) 639-6400 coilcraft.com
Coiltronics Inductors (407) 241-7876 coiltronics.com
Diodes, Inc Diodes (805) 446-4800 diodes.com
Fairchild MOSFETs (408) 822-2126 fairchildsemi.com
General Semiconductor Diodes (516) 847-3000 generalsemiconductor.com
International Rectifier MOSFETs, Diodes (310) 322-3331 irf.com
IRC Sense Resistors (361) 992-7900 irctt.com
Kemet Tantalum Capacitors (408) 986-0424 kemet.com
Magnetics Inc Toroid Cores (800) 245-3984 mag-inc.com
Microsemi Diodes (617) 926-0404 microsemi.com
Murata-Erie Inductors, Capacitors (770) 436-1300 murata.co.jp
Nichicon Capacitors (847) 843-7500 nichicon.com
On Semiconductor Diodes (602) 244-6600 onsemi.com
Panasonic Capacitors (714) 373-7334 panasonic.com
Sanyo Capacitors (619) 661-6835 sanyo.co.jp
Sumida Inductors (847) 956-0667 sumida.com
Taiyo Yuden Capacitors (408) 573-4150 t-yuden.com
TDK Capacitors, Inductors (562) 596-1212 component.tdk.com
Thermalloy Heat Sinks (972) 243-4321 aavidthermalloy.com
Tokin Capacitors (408) 432-8020 nec-tokinamerica.com
Toko Inductors (847) 699-3430 tokoam.com
United Chemicon Capacitors (847) 696-2000 chemi-com.com
Vishay/Dale Resistors (605) 665-9301 vishay.com
Vishay/Siliconix MOSFETs (800) 554-5565 vishay.com
Vishay/Sprague Capacitors (207) 324-4140 vishay.com
Zetex Small-Signal Discretes (631) 543-7100 zetex.com
applicaTions inForMaTion
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34

LTC1871EMS-7#TRPBF

Mfr. #:
Buy LTC1871EMS-7#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Controllers No Rsense DC/DC Controller Boost, Flyback & SEPIC
Lifecycle:
New from this manufacturer.
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