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LT3759HMSE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P32
LT3759
19
3759fc
For more information www.linear.com/3759
APPLICATIONS INFORMATION
L
LK
is the leakage inductance of the primary winding, which
is usually specified in the transformer characteristics. L
LK
can be obtained by measuring the primary inductance with
the secondary windings shorted. The snubber capacitor
value (C
CN
) can be determined using the following equation:
C
CN
=
V
SN
ΔV
SN
•R
CN
• f
OSC
where ΔV
SN
is the voltage ripple across C
CN
. A reasonable
ΔV
SN
is 5% to 10% of V
SN
. The reverse voltage rating of
D
SN
should be higher than the sum of V
SN
and V
IN(MAX)
.
Flyback Converter: Sense Resistor Selection
In a flyback converter, when the power switch is turned
on, the current flowing through the sense resistor
(I
SENSE
) is:
I
SENSE
= I
LP
Set the sense voltage at I
LP(PEAK)
to be the minimum of
the SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
R
SENSE
=
40mV
I
LP(PEAK)
Flyback Converter: Power MOSFET Selection
For the flyback configuration, the MOSFET is selected with
a V
DC
rating high enough to handle the maximum V
IN
, the
reflected secondary voltage and the voltage spike due to
the leakage inductance. Approximate the required MOSFET
V
DC
rating using:
BV
DSS
> V
DS(PEAK)
where:
V
DS(PEAK)
= V
IN(MAX)
+ V
SN
The power dissipated by the MOSFET in a flyback con-
verter is:
P
FET
= I
2
M(RMS)
• R
DS(ON)
+ 2 • V
2
DS(PEAK)
• I
L(MAX)
•
C
RSS
• f
OSC
/1A
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
RSS
is the reverse transfer capacitance, which is
usually specified in the MOSFET characteristics.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
equation:
T
J
= T
A
+ P
FET
• θ
JA
= T
A
+ P
FET
• (θ
JC
+ 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.
Flyback Converter: Output Diode Selection
The output diode in a flyback converter is subject to large
RMS current and peak reverse voltage stresses. A fast
switching diode with a low forward drop and a low reverse
leakage is desired. Schottky diodes are recommended if
the output voltage is below 100V.
Approximate the required peak repetitive reverse voltage
rating V
RRM
using:
V
RRM
>
N
S
N
P
• V
IN(MAX)
+ V
OUT
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.
Flyback Converter: Output Capacitor Selection
The output capacitor of the flyback converter has a similar
operation condition as that of the boost converter. Refer to
the Boost Converter: Output Capacitor Selection section
for the calculation of C
OUT
and ESR
COUT
.
LT3759
20
3759fc
For more information www.linear.com/3759
The RMS ripple current rating of the output capacitors
in discontinuous operation can be determined using the
following equation:
I
RMS(COUT),DISCONTINUOUS
≥ I
O(MAX)
•
4−(3 • D2)
3 • D2
Flyback Converter: Input Capacitor Selection
The input capacitor in a flyback converter is subject to
a large RMS current due to the discontinuous primary
current. To prevent large voltage transients, use a low
ESR input capacitor sized for the maximum RMS current.
The RMS ripple current rating of the input capacitors in
discontinuous operation can be determined using the
following equation:
I
RMS(CIN),DISCONTINUOUS
≥
P
OUT(MAX)
V
IN(MIN)
•η
•
4−(3 • D
MAX
)
3 • D
MAX
SEPIC CONVERTER APPLICATIONS
The LT3759 can be configured as a SEPIC (single-ended
primary inductance converter), as shown in Figure 1. This
topology allows for the input to be higher, equal, or lower
than the desired output voltage. The conversion ratio as
a function of duty cycle is:
V
OUT
+ V
D
V
IN
=
D
1−D
In continuous conduction mode (CCM).
In a SEPIC converter, no DC path exists between the input
and output. This is an advantage over the boost converter
for applications requiring the output to be disconnected
from the input source when the circuit is in shutdown.
SEPIC Converter: Switch Duty Cycle and Frequency
For a SEPIC converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (V
OUT
), the input voltage (V
IN
) and diode forward
voltage (V
D
).
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum input voltage:
D
MAX
=
V
OUT
+ V
D
V
IN(MIN)
+ V
OUT
+ V
D
SEPIC Converter: Inductor and Sense Resistor
Selection
As shown in Figure 1, the SEPIC converter contains two
inductors: L1 and L2. L1 and L2 can be independent, but can
also be wound on the same core, since identical voltages
are applied to L1 and L2 throughout the switching cycle.
For the SEPIC topology, the current through L1 is the
converter input current. Based on the fact that, ideally, the
output power is equal to the input power, the maximum
average inductor currents of L1 and L2 are:
I
L1(MAX)
= I
IN(MAX)
= I
O(MAX)
D
MAX
1–D
MAX
I
L2(MAX)
= I
O(MAX)
In a SEPIC converter, the switch current is equal to I
L1
+
I
L2
when the power switch is on, therefore, the maximum
average switch current is defined as:
I
SW(MAX)
= I
L1(MAX)
+I
L2(MAX)
= I
O(MAX)
•
1
1–D
MAX
and the peak switch current is:
I
SW(PEAK)
= 1+
χ
2
• I
O(MAX)
•
1
1–D
MAX
The constant
χ
in the preceding equations represents the
percentage peak-to-peak ripple current in the switch, rela-
tive to I
SW(MAX)
, as shown in Figure 8. Then, the switch
ripple current ΔI
SW
can be calculated by:
D
I
SW
= c
• I
SW(MAX)
APPLICATIONS INFORMATION
LT3759
21
3759fc
For more information www.linear.com/3759
The inductor ripple currents ΔI
L1
and ΔI
L2
are identical:
DI
L1
= DI
L2
= 0.5 • D I
SW
By making L1 = L2, and winding them on the same core, the
value of inductance in the preceding equation is replaced
by 2L, due to mutual inductance:
L =
V
IN(MIN)
DI
SW
• f
OSC
• D
MAX
=
R
SENSE
• V
IN(MIN)
0.01V • f
OSC
• D
MAX
In a SEPIC converter, when the power switch is turned on,
the current flowing through the sense resistor (I
SENSE
) is
the switch current.
Set the sense voltage at I
SENSE(PEAK)
to be minimum of
the SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
R
SENSE
=
40mV
I
SW(PEAK)
SEPIC Converter: Power MOSFET Selection
For the SEPIC configuration, choose a MOSFET with a
V
DC
rating higher than the sum of the output voltage and
input voltage by a safety margin (a 10V safety margin is
usually sufficient).
The power dissipated by the MOSFET in a SEPIC converter
is:
P
FET
= I
2
SW(MAX)
• R
DS(ON)
• D
MAX
+ (V
IN(MIN)
+ V
OUT
)
2
• I
SW(MAX)
• C
RSS
•
f
OSC
1A
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
RSS
is the reverse transfer capacitance, which is
usually specified in the MOSFET characteristics.
Figure 8. The Switch Current Waveform of a SEPIC Converter
3759 F08
∆I
SW
=
χ
• I
SW(MAX)
I
SW
t
DT
S
I
SW(MAX)
T
S
APPLICATIONS INFORMATION
The inductor ripple current has a direct effect on the
choice of the inductor value. Choosing smaller values of
ΔI
L
requires large inductances and reduces the current
loop gain (the converter will approach voltage mode).
Accepting larger values of ΔI
L
allows the use of low in-
ductances, but results in higher input current ripple and
greater core losses. It is recommended that
χ
falls in the
range of 0.2 to 0.4.
Choose an inductor value based on operating frequency,
input and output voltage to provide a current mode ramp
on SENSE during the switch on-time of approximately
10mV magnitude. The inductor value (L1 and L2 are
independent) of the SEPIC converter can be determined
using the following equation:
L1= L2 =
V
IN(MIN)
0.5 • DI
SW
• f
OSC
• D
MAX
=
R
SENSE
• V
IN(MIN)
0.5 • 0.01V • f
OSC
• D
MAX
For most SEPIC applications, the equal inductor values
will fall in the range of 1µH to 100µH.
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LT3759HMSE#PBF

Mfr. #:
Buy LT3759HMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Boost, Flyback, SEPIC, and Inverting Controller
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