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LTC3704 数据表(PDF) 18 Page - Linear Technology

部件名 LTC3704
功能描述  Wide Input Range, No RSENSE??Current Mode Boost, Flyback and SEPIC Controller
Download  32 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC3704 数据表(HTML) 18 Page - Linear Technology

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LTC1871X
18
1871xf
For more information www.linear.com/LTC1871X
APPLICATIONS INFORMATION
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 11b). 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:
IRMS(CIN) = 0.3•
VIN(MIN)
L • f
•DMAX
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 MOSFET RDS(ON) and inductor value also
determines the load current at which the LTC1871X en-
ters Burst Mode operation. When bursting, the controller
clamps the peak inductor current to approximately:
IBURST(PEAK) =
30mV
RDS(ON)
which represents about 20% of the maximum 150mV
SENSE pin voltage. The corresponding average current
dependsupontheamountofripplecurrent.Lowerinductor
values (higher
∆IL) 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
∆IL is substantially less than IBURST. 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
∆IL is small, it takes multiple cycles
for the current to ramp back up to IBURST(PEAK). Dur-
ing this inductor charging interval, the output capacitor
must supply the load current and a significant droop in
the output voltage can occur. Generally, it is a good idea
to choose a value of inductor
∆IL between 25% and 40%
of IIN(MAX). The alternative is to either increase the value
of the output capacitor or disable Burst Mode operation
using the MODE/SYNC pin.
Burst Mode operation can be defeated by connecting the
MODE/SYNC pin to a high logic-level voltage (either with
a control input or by connecting this pin to INTVCC). In
this mode, the burst clamp is removed, and the chip can
operateatconstantfrequencyfromcontinuousconduction
mode (CCM) at full load, down into deep discontinuous
conduction mode (DCM) at light load. Prior to skipping
pulses at very light load (i.e., <5% of full load), the control-
ler will operate with a minimum switch on-time in DCM.
VIN
L
D
SW
11a. Circuit Diagram
11b. Inductor and Input Currents
COUT
VOUT
RL
IIN
IL
11c. Switch Current
ISW
tON
11d. Diode and Output Currents
11e. Output Voltage Ripple Waveform
IO
ID
VOUT
(AC)
tOFF
ΔVESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔVCOUT
1871x F11
Figure 11. Switching Waveforms for a Boost Converter


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