Datasheet

LT3758/LT3758A
17
3758afd
ApplicAtions inForMAtion
Figure 6. The Output Ripple Waveform of a Boost Converter
V
OUT
(AC)
t
ON
∆V
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
∆V
COUT
3758 F06
t
OFF
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 6. The RMS ripple
current rating of the output capacitor can be determined
using the following equation:
I
RMS(COUT)
≥I
O(MAX)
•
D
MAX
1− D
MAX
Multiple capacitors are often paralleled to meet ESR
requirements. Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering and has
the required RMS current rating. Additional ceramic capaci-
tors in parallel are commonly used to reduce the effect of
parasitic inductance in the output capacitor, which reduces
high frequency switching noise on the converter output.
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 wave-
form is continuous. 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 boost con-
verter is:
I
RMS(CIN)
= 0.3 • ∆I
L
FLYBACK CONVERTER APPLICATIONS
The LT3758 can be configured as a flyback converter
for the applications where the converters have multiple
outputs, high output voltages or isolated outputs. Figure
7 shows a simplified flyback converter.
The flyback converter has a very low parts count for mul-
tiple
outputs, and with prudent selection of turns ratio, can
have
high output/input voltage conversion ratios with a
desirable duty cycle. However, it has low efficiency due to
the high peak currents, high peak voltages and consequent
power loss. The flyback converter is commonly used for
an output power of less than 50W.
The R
θJA
to be used in this equation normally includes the
R
θJC
for the device plus the thermal resistance from the
board to the ambient temperature in the enclosure. T
J
must
not exceed the diode maximum junction temperature rating.
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 output
capacitors for a given output ripple voltage. The effect of
these three parameters (ESR, ESL and bulk C) on the output
voltage ripple waveform for a typical boost converter is
illustrated in Figure 6.
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 ∆V
ESR
and the charging/discharg-
ing ∆V
COUT
. For the purpose of simplicity, we will choose
2% for the maximum output ripple, to be divided equally
between ∆V
ESR
and ∆V
COUT
. This percentage ripple will
change, depending on the requirements of the applica-
tion, and the following equations 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
OUT
I
D(PEAK)
For the bulk C component, which also contributes 1% to
the total ripple:
C
OUT
≥
I
O(MAX)
0.01• V
OUT
• f