Datasheet

LT3757/LT3757A
16
3757afd
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
3757 F05
t
OFF
Boost Converter: Output Diode Selection
To maximize efficiency, a fast switching diode with low
forward drop and low reverse leakage is desirable. The
peak reverse voltage that the diode must withstand is
equal to the regulator output voltage plus any additional
ringing across its anode-to-cathode during the on-time.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to:
I
D(PEAK)
=I
L(PEAK)
= 1+
c
2






• I
L(MAX)
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
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
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 application,
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)
these three parameters (ESR, ESL and bulk C) on the output
voltage ripple waveform for a typical boost converter is
illustrated in Figure 6.