Datasheet

LT3573
16
3573fd
For more information www.linear.com/LT3573
APPLICATIONS INFORMATION
The transformer turns ratio is selected such that the con-
verter has adequate current capability and a switch stress
below 50V. T
able 6 shows the switch voltage stress and
output current capability at different transformer turns
ratio.
Table 6. Switch Voltage Stress and Output Current Capability vs
Turns-Ratio
N
V
SW(MAX)
AT V
IN(MAX)
(V)
I
OUT(MAX)
AT V
IN(MIN)
(A)
DUTY CYCLE
(%)
1:1 33.5 0.39 16~22
2:1 39 0.65 28~35
3:1 44.5 0.825 37~45
4:1 50 0.96 44~52
BIAS winding turns ratio is selected to program the BIAS
voltage to 3V~5V. The BIAS voltage shall not exceed the
input voltage.
The turns ratio is then selected as primary: secondary:
BIAS = 3:1:1.
2. Select the transformer primary inductance for target
switching frequency.
The LT3573 requires a minimum amount of time to sample
the output voltage during the off-time. This off-time,
t
OFF(MIN)
, shall be greater than 350ns over all operating
conditions. The converter also has a minimum current limit,
L
MIN
, of 250mA to help create this off-time. This defines
the minimum required inductance as defined as:
L
MIN
=
N(V
OUT
+ V
F
)
I
MIN
• t
OFF(MIN)
The transformer primary inductance also affects the
switching frequency which is related to the output ripple. If
above the minimum inductance, the transformer’s primary
inductance may be selected for a target switching frequency
range in order to minimize the output ripple.
The following equation estimates the switching frequency.
f
SW
=
1
t
ON
+ t
OFF
=
1
I
PK
V
IN
L
+
I
PK
N
PS
(V
OUT
+ V
F
)
L
Table 7.Switching Frequency at Different Primary
Inductance at I
PK
L (µH)
f
SW
AT V
IN(MIN)
(kHz)
f
SW
AT V
IN(MAX)
(kHz)
25 236 305
50 121 157
100 61 80
Note: The switching frequency is calculated at maximum output.
In this design example, the minimum primary inductance is
used to achieve a nominal switching frequency of 275kHz
at full load. The PA2454NL from Pulse Engineering is
chosen as the flyback transformer.
Given the turns ratio and primary inductance, a custom
-
ized transformer can be designed by magnetic component
manufacturer or a multi-winding transformer such as a
Coiltronics V
ersa-Pac may be used.
3.
Select the output diodes and output capacitor.
The output diode voltage stress V
D
is the summation of
the output voltage and reflection of input voltage to the
secondary side. The average diode current is the load
current.
V
D
= V
OUT
+
V
IN
N
The output capacitor should be chosen to minimize the
output voltage ripple while considering the increase in
size and cost of a larger capacitor. The following equation
calculates the output voltage ripple.
DV
MAX
=
LI
2
PK
2CV
OUT
4. Select the snubber circuit to clamp the switch
voltage spike.
A flyback converter generates a voltage spike during switch
turn-off
due to the leakage inductance of the transformer.
In order to clamp the voltage spike below the maximum
rating of the switch, a snubber circuit is used. There are
many types of snubber circuits, for example R-C, R-C-D and