LTC3813 Datasheet, PDF (19/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
APPLICATIONS INFORMATION
Once the output capacitor ESR and bulk capacitance
have been determined, the overall ripple voltage wave-
form should be verified on a dedicated PC board (see PC
Board Layout Checklist section for more information on
component placement). 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 8d. 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.
Manufacturers such as Nichicon, Nippon Chemi-con
and Sanyo should be considered for high performance
throughhole capacitors. The OS-CON (organic semicon-
ductor dielectric) capacitor available from Sanyo has the
lowest product of ESR and size of any aluminum electrolytic
at a somewhat higher price. An additional ceramic capaci-
tor in parallel with OS-CON capacitors is recommended
to reduce the effect of their lead inductance.
In surface mount applications, multiple capacitors placed
in parallel may be required to meet the ESR, RMS current
handling and load step requirements. Dry tantalum, special
polymer and aluminum electrolytic capacitors are available
in surface mount packages. Special polymer capacitors
offer very low ESR but have lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density but it is important to only use types
that have been surge tested for use in switching power
supplies. Several excellent surge-tested choices are the
AVX TPS and TPSV or the KEMET T510 series. Aluminum
electrolytic capacitors have significantly higher ESR, but
can be used in cost-driven applications providing that
consideration is given to ripple current ratings and long
term reliability. Other capacitor types include Panasonic
SP and Sanyo POSCAPs. In applications with V_OUT > 30V,
however, choices are limited to aluminum electrolytic and
ceramic capacitors.
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 8b). 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 though 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
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!
Output Voltage
The LTC3813 output voltage is set by a resistor divider
according to the following formula:
V_OUT
=
0.8V



1+
R_FB1
R_FB2


The external resistor divider is connected to the output as
shown in the Functional Diagram, allowing remote voltage
sensing. The resultant feedback signal is compared with
the internal precision 800mV voltage reference by the
error amplifier. The internal reference has a guaranteed
tolerance of < 1%. Tolerance of the feedback resistors
will add additional error to the output voltage. 0.1% to
1% resistors are recommended.
3813fb
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