Datasheet

LTC3862-1
29
38621f
additional power dissipation is important when deciding
on a diode current rating, package type, and method of
heat sinking.
To a close approximation, the power dissipated by the
diode is:
P
D
= I
D(PEAK)
• V
F(PEAK)
• (1 – D
MAX
)
The diode junction temperature is:
T
J
= T
A
+ P
D
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the board to the ambient temperature in the enclosure.
Once the proper diode has been selected and the circuit
performance has been verifi ed, measure the temperature
of the power components using a thermal probe or infrared
camera over all operating conditions to ensure a good
thermal design.
Finally, remember to keep the diode lead lengths short
and to observe proper switch-node layout (see Board
Layout Checklist) to avoid excessive ringing and increased
dissipation.
Output Capacitor Selection
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct combination
of output capacitors for a boost converter application. The
effects of these three parameters on the output voltage
ripple waveform are illustrated in Figure 22 for a typical
boost converter.
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 and the charging/discharging ΔV.
For the purpose of simplicity we will choose 2% for the
maximum output ripple, to be divided equally between the
ESR step and the charging/discharging ΔV. This percentage
ripple will change, depending on the requirements of the
application, and the equations provided below can easily
be modifi ed.
One of the key benefi ts of multi-phase operation is a reduc-
tion in the peak current supplied to the output capacitor
by the boost diodes. As a result, the ESR requirement
of the capacitor is relaxed. 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)
where:
I
D(PEAK)
=
1
n
•1+

2






•
I
O(MAX)
1–D
MAX
The factor n represents the number of phases and the factor
χ represents the percentage inductor ripple current.
APPLICATIONS INFORMATION
Figure 22. Switching Waveforms for a Boost Converter
SW1
100V/DIV
SW2
100V/DIV
V
OUT
100mV/DIV
AC COUPLED
I
L1
2A/DIV
I
L2
2A/DIV
1µs/DIV
38621 F22
V
IN
= 24V
V
OUT
= 72V
350mA LOAD