Datasheet

LT3957A
14
3957afa
APPLICATIONS INFORMATION
ΔI
SW
increases output current capability, but requires
large inductances and reduces the current loop gain (the
converter will approach voltage mode). Accepting larger
values of ΔI
SW
provides fast transient response and
allows the use of low inductances, but results in higher
input current ripple and greater core losses, and reduces
output current capability.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value of the boost converter can be determined
using the following equation:
L =
V
IN(MIN)
ΔI
SW
•ƒ
•D
MAX
The peak inductor current is the switch current limit (5.9A
typical), and the RMS inductor current is approximately
equal to I
L(MAX)
. The user should choose the inductors
having sufficient saturation and RMS current ratings.
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.
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
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 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
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
3957A F05
t
OFF
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