Datasheet

LT8302
14
Rev E
For more information www.analog.com
APPLICATIONS INFORMATION
The equations below calculate output power:
P
OUT
= η • V
IN
• D • I
SW(MAX)
• 0.5
η = Efficiency = ~85%
D= Duty Cycle =
V
OUT
+ V
F
( )
•N
PS
V
OUT
+ V
F
( )
•N
PS
+ V
IN
I
SW(MAX)
= Maximum switch current limit = 3.6A (MIN)
Primary Inductance Requirement
The LT8302 obtains output voltage information from the
reflected output voltage on the SW pin. The conduction of
secondary current reflects the output voltage on the pri-
mary SW pin. The sample-and-hold error amplifier needs
a minimum 350ns to settle and sample the reflected out
-
put voltage. In order to ensure proper sampling, the sec-
ondary 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 (TYP)
I
SW(MIN)
= Minimum switch current limit = 0.87A (TYP)
In addition to the primary inductance requirement for
the minimum switch-off time, the LT8302 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 (TYP)
In general, choose a transformer with its primary mag
-
netizing inductance about 40% to 60% 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 LT8302. In addi-
tion to the usual list of guidelines dealing with high fre-
quency isolated power supply transformer design, the
following information should be carefully considered.
Analog Devices has worked with several leading magnetic
component manufacturers to produce pre-designed fly-
back transformers for use with the LT8302. Table1 shows
the details of these transformers.
Table1. Predesigned Transformers–Typical Specifications
TRANSFORMER
PART NUMBER
DIMENSIONS
(W × L × H) (mm)
L
PRI
(µH)
L
LKG
(µH) N
P
:N
S
R
PRI
(mΩ)
R
SEC
(mΩ) VENDOR
TARGET APPLICATION
V
IN
(V) V
OUT
(V) I
OUT
(A)
750311625 17.75 × 13.46 × 12.70 9 0.35 4:1 43 6 Würth Elektronik 8 to 32 3.3 2.1
750311564 17.75 × 13.46 × 12.70 9 0.12 3:1 36 7 Würth Elektronik 8 to 32 5 1.5
750313441 15.24 × 13.34 x 11.43 9 0.6 2:1 75 18 Würth Elektronik 8 to 32 5 1.3
750311624 17.75 × 13.46 × 12.70 9 0.18 3:2 34 21 Würth Elektronik 8 to 32 8 0.9
12387-TO79 15.5 × 12.5 × 11.5 9 0.5 1:1:1 55 90 Sumida 8 to 36 ±12 0.3
750313445 15.24 × 13.34 × 11.43 9 0.25 1:2 85 190 Würth Elektronik 8 to 36 24 0.3
750313457 15.24 × 13.34 × 11.43 9 0.25 1:4 85 770 Würth Elektronik 8 to 36 48 0.15
750313460 15.24 × 13.34 × 11.43 12 0.7 4:1 85 11 Würth Elektronik 4 to 18 5 0.9
750311342 15.24 × 13.34 × 11.43 15 0.44 2:1 85 22 Würth Elektronik 4 to 18 12 0.4
750313439 15.24 × 13.34 × 11.43 12 0.6 2:1 115 28 Würth Elektronik 18 to 42 3.3 2.1
750313442 15.24 × 13.34 × 11.43 12 0.75 3:2 150 53 Würth Elektronik 18 to 42 5 1.6
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