Datasheet

LT3759
15
3759fc
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to choose a MOSFET whose BV
DSS
is higher than V
OUT
by a safety margin (a 10V safety margin is usually
sufficient).
The power dissipated by the MOSFET in a boost
converter is:
P
FET
= I
2
L(MAX)
•R
DS(ON)
•
D
MAX
+V
2
OUT
•
I
L(MAX)
•
C
RSS
•
f
1A
The first term in the preceding equation represents the
conduction losses in the devices, and the second term, the
switching loss. C
RSS
is the reverse transfer capacitance,
which is usually specified in the MOSFET characteristics.
For maximum efficiency, R
DS(ON)
and C
RSS
should be
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
T
J
=
T
A
•
P
F ET
•
θ
JA
= T
A
+P
FET
•(θ
J C
+θ
CA
)
T
J
must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
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+
χ
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
θJ A
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
APPLICATIONS INFORMATION
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
3759 F05
t
OFF
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 charging/discharging
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
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