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LT8300IS5#TRPBF

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LT8300
10
8300f
applicaTions inForMaTion
Selecting Actual R
FB
Resistor Value
The LT8300 uses a unique sampling scheme to regulate
the isolated output voltage. Due to the sampling nature,
the scheme contains repeatable delays and error sources,
which will affect the output voltage and force a re-evaluation
of the R
FB
resistor value. Therefore, a simple two-step
process is required to choose feedback resistor R
FB
.
Rearrangement of the expression for V
OUT
in the Output
Voltage section yields the starting value for R
FB
:
R
FB
=
N
PS
• V
OUT
+ V
F
( )
100µA
V
OUT
= Output voltage
V
F
= Output diode forward voltage = ~0.3V
N
PS
= Transformer effective primary-to-secondary
turns ratio
Power up the application with the starting R
FB
value and
other components connected, and measure the regulated
output voltage, V
OUT(MEAS)
. The final R
FB
value can be
adjusted to:
R
FB(FINAL)
=
V
OUT
V
OUT(MEAS)
•R
FB
Once the final R
FB
value is selected, the regulation accuracy
from board to board for a given application will be very
consistent, typically under ±5% when including device
variation of all the components in the system (assuming
resistor tolerances and transformer windings matching
within ±1%). However, if the transformer or the output
diode is changed, or the layout is dramatically altered,
there may be some change in V
OUT
.
Output Power
A flyback converter has a complicated relationship between
the input and output currents compared to a buck or a
boost converter. A boost converter has a relatively constant
maximum input current regardless of input voltage and a
buck converter has a relatively constant maximum output
current regardless of input voltage. This is due to the
continuous non-switching behavior of the two currents. A
flyback converter has both discontinuous input and output
currents which make it similar to a non-isolated buck-boost
converter. The duty cycle will affect the input and output
currents, making it hard to predict output power. In ad-
dition, the winding ratio can be changed to multiply the
output current at the expense of a higher switch voltage.
The graphs in Figures 1 to 4 show the typical maximum
output power possible for the output voltages 3.3V, 5V,
12V, and 24V. The maximum output power curve is the
calculated output power if the switch voltage is 120V dur-
ing the switch-off time. 30V of margin is left for leakage
inductance voltage spike. To achieve this power level at
a given input, a winding ratio value must be calculated
to stress the switch to 120V, resulting in some odd ratio
values. The curves below the maximum output power
curve are examples of common winding ratio values and
the amount of output power at given input voltages.
One design example would be a 5V output converter with
a minimum input voltage of 36V and a maximum input
voltage of 72V. A six-to-one winding ratio fits this design
example perfectly and outputs equal to 2.44W at 72V but
lowers to 1.87W at 36V.
The following equations calculate output power:
P
OUT
= η • V
IN
•D •I
SW(MAX)
• 0.5
η = Efficiency = 85%
D = DutyCycle =
V
OUT
+ V
F
( )
•N
PS
V
OUT
+ V
F
( )
•N
PS
+ V
IN
I
SW(MAX)
= Maximum switch current limit = 260mA
LT8300
11
8300f
applicaTions inForMaTion
Figure 4. Output Power for 24V Output
Figure 1. Output Power for 3.3V Output Figure 2. Output Power for 5V Output
Figure 3. Output Power for 12V Output
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F01
100
N = 12:1
MAXIMUM
OUTPUT
POWER
N = 8:1
N = 6:1
N = 4:1
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F02
100
MAXIMUM
OUTPUT
POWER
N = 2:1
N = 4:1
N = 6:1
N = 8:1
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F03
100
N = 4:1
MAXIMUM
OUTPUT
POWER
N = 3:1
N = 2:1
N = 1:1
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F04
100
N = 1:1
N = 3:2
N = 1:2
MAXIMUM
OUTPUT
POWER
N = 2:1
Primary Inductance Requirement
The LT8300 obtains output voltage information from the
reflected output voltage on the SW pin. The conduction
of secondary current reflects the output voltage on the
primary SW pin. The sample-and-hold error amplifier needs
a minimum 350ns to settle and sample the reflected output
voltage. In order to ensure proper sampling, the second-
ary winding needs to conduct current for a minimum of
350ns. The following equation gives the minimum value
for primary-side magnetizing inductance:
L
PRI
≥
t
OFF(MIN)
•N
PS
• V
OUT
+ V
F
( )
I
SW(MIN)
t
OFF(MIN)
= Minimum switch-off time = 350ns
I
SW(MIN)
= Minimum switch current limit = 52mA
In addition to the primary inductance requirement for
the minimum switch-off time, the LT8300 has minimum
switch-on time that prevents the chip from turning on
the power switch shorter than approximately 160ns. This
minimum switch-on time is mainly for leading-edge blank-
ing the initial switch turn-on current spike. If the inductor
current exceeds the desired current limit during that time,
oscillation may occur at the output as the current control
loop will lose its ability to regulate. Therefore, the following
equation relating to maximum input voltage must also be
followed in selecting primary-side magnetizing inductance:
L
PRI
≥
t
ON(MIN)
• V
IN(MAX)
I
SW(MIN)
t
ON(MIN)
= Minimum Switch-On Time = 160ns
LT8300
12
8300f
applicaTions inForMaTion
In general, choose a transformer with its primary mag-
netizing inductance about 20% to 40% larger than the
minimum values calculated above. A transformer with
much larger inductance will have a bigger physical size
and may cause instability at light load.
Selecting a Transformer
Transformer specification and design is perhaps the most
critical part of successfully applying the LT8300. In addition
to the usual list of guidelines dealing with high frequency
isolated power supply transformer design, the following
information should be carefully considered.
Linear Technology has worked with several leading mag-
netic component manufacturers to produce pre-designed
flyback transformers for use with the LT8300. Table 1
shows the details of these transformers.
Turns Ratio
Note that when choosing the R
FB
resistor to set output
voltage, the user has relative freedom in selecting a trans-
former turns ratio to suit a given application. In contrast,
the use of simple ratios of small integers, e.g., 4:1, 2:1,
1:1, provides more freedom in settling total turns and
mutual inductance.
Table 1. Predesigned Transformers — Typical Specifications
TRANSFORMER
PART NUMBER
L
PRI
(µH)
L
LEAKAGE
(µH) NP:NS:NB VENDOR TARGET APPLICATIONS
750312367 400 4.5 8:1 Würth Elektronik 48V to 3.3V/0.51A, 24V to 3.3V/0.37A, 12V to 3.3V/0.24A
750312557 300 2.5 6:1 Würth Elektronik 48V to 3.3V/0.42A, 24V to 3.3V/0.32A, 12V to 3.3V/0.22A
48V to 5V/0.38A, 24V to 5V/0.27A, 12V to 5V/0.17A
750312365 300 1.8 4:1 Würth Elektronik 48V to 5V/0.29A, 24V to 5V/0.22A, 12V to 5V/0.15A
750312558 300 1.75 2:1:1 Würth Elektronik 48V to ±12V/67mA, 24V to ±12V/50mA, 12V to ±12V/33mA
48V to ±15V/62mA, 24V to ±15V/44mA, 12V to ±15V/28mA
750312559 300 2 1:1 Würth Elektronik 48V to 24V/67mA, 24V to 24V/50mA, 12V to 24V/33mA
750311019 400 5 6:1:2 Würth Elektronik 48V to 3.3V/0.42A, 24V to 3.3V/0.32A, 12V to 3.3V/0.22A
48V to 5V/0.38A, 24V to 5V/0.27A, 12V to 5V/0.17A
750311558 300 1.5 4:1:1 Würth Elektronik 48V to 5V/0.29A, 24V to 5V/0.22A, 12V to 5V/0.15A
750311660 350 3 2:1:0.33 Würth Elektronik 48V to 12V/0.134A, 24V to 12V/0.1A, 12V to 12V/0.066A
48V to 15V/0.124A, 24V to 15V/0.088A, 12V to 15V/0.056A
750311838 350 3 2:1:1 Würth Elektronik 48V to ±12V/67mA, 24V to ±12V/50mA, 12V to ±12V/33mA
48V to ±15V/62mA, 24V to ±15V/44mA, 12V to ±15V/28mA
750311659 300 2 1:1:0.2 Würth Elektronik 48V to 24V/67mA, 24V to 24V/50mA, 12V to 24V/33mA
10396-T026 300 2.5 6:1:2 Sumida 48V to 3.3V/0.42A, 24V to 3.3V/0.32A, 12V to 3.3V/0.22A
48V to 5V/0.38A, 24V to 5V/0.27A, 12V to 5V/0.17A
10396-T024 300 2 4:1:1 Sumida 48V to 5V/0.29A, 24V to 5V/0.22A, 12V to 5V/0.15A
10396-T022 300 2 2:1:0.33 Sumida 48V to 12V/0.134A, 24V to 12V/0.1A, 12V to 12V/0.066A
48V to 15V/0.124A, 24V to 15V/0.088A, 12V to 15V/0.056A
10396-T028 300 2.5 2:1:1 Sumida 48V to ±12V/67mA, 24V to ±12V/50mA, 12V to ±12V/33mA
48V to ±15V/62mA, 24V to ±15V/44mA, 12V to ±15V/28mA
L10-0116 500 7.3 6:1 BH Electronics 48V to 3.3V/0.42A, 24V to 3.3V/0.32A, 12V to 3.3V/0.22A
48V to 5V/0.38A, 24V to 5V/0.27A, 12V to 5V/0.17A
L10-0112 230 3.38 4:1 BH Electronics 48V to 5V/0.29A, 24V to 5V/0.22A, 12V to 5V/0.15A
L11-0067 230 2.16 4:1 BH Electronics 48V to 5V/0.29A, 24V to 5V/0.22A, 12V to 5V/0.15A
* All the transformers are rated for 1.5kV Isolation.
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LT8300IS5#TRPBF

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Buy LT8300IS5#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 150V, 300mA Micropower Isolated Flyback Converter
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