Datasheet

LT3959
14
3959fa
For more information www.linear.com/LT3959
It is recommended that the peak repetitive reverse voltage
rating V
RRM
is higher than V
OUT
by a safety margin (a 10V
safety margin is usually sufficient).
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
θ
JA
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 3.
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 charging/discharging
∆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 application,
applicaTions inForMaTion
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 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
OSC
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 3. 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
converter is:
I
RMS(CIN)
= 0.3 • ∆I
L
V
OUT
(AC)
t
ON
∆V
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
∆V
COUT
3959 F03
t
OFF
Figure 3. The Output Ripple Waveform of a Boost Converter