Datasheet

LT3957
15
3957f
APPLICATIONS INFORMATION
The power dissipated by the diode is:
P
D
= I
O(MAX)
• V
D
where V
D
is diode’s forward voltage drop, and the diode
junction temperature is:
T
J
= T
A
+ P
D
• R
θJA
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 5.
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 ΔV
ESR
and the charging/discharg-
ing ΔV
COUT
. For the purpose of simplicity, we will choose
2% for the maximum output ripple, to be divided equally
between ΔV
ESR
and ΔV
COUT
. This percentage ripple will
change, depending on the requirements of the application,
and the following equations can easily be modifi ed. For a
1% contribution to the total ripple voltage, the ESR of the
output capacitor can be determined using the following
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
•ƒ
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 5. 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 require-
ments. Typically, once the ESR requirement is satisfi ed, the
capacitance is adequate for fi ltering and has the required
RMS current rating. Additional ceramic capacitors in par-
allel 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 1µF 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 • ΔI
L
Figure 5. The Output Ripple Waveform of a Boost Converter
V
OUT
(AC)
t
ON
$V
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
$V
COUT
3957 F05
t
OFF