LT3468 Linear Technology, LT3468 Datasheet - Page 7

• Manufacturer Part Number: LT3468
• Description: Photoflash Capacitor Charger in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Choosing The Right Device (LT3468/LT3468-1)
The only difference between the two versions of the
LT3468 is the peak current level. For the fastest possible
charge time, use the LT3468. The LT3468-1 has a lower
peak current capability, and is designed for applications
that need a more limited drain on the batteries. Due to the
lower peak current, the LT3468-1 can use a physically
smaller transformer.
Transformer Design
The flyback transformer is a key element for any LT3468/
LT3468-1 design. It must be designed carefully and checked
that it does not cause excessive current or voltage on any
pin of the part. The main parameters that need to be
designed are shown in Table 1.
The first transformer parameter that needs to be set is the
turns ratio N. The LT3468/LT3468-1 accomplish output
voltage detection by monitoring the flyback waveform on
the SW pin. When the SW voltage reaches 31.5V higher
than the V
the choice of N sets the target output voltage as it changes
Table 1. Recommended Transformer Parameters
PARAMETER
L
L
N
V
I
R
R
SAT
PRI
LEAK
ISO
PRI
SEC
IN
voltage, the part will halt power delivery. Thus,
NAME
Primary Inductance
Primary Leakage Inductance
Secondary: Primary Turns Ratio
Secondary to Primary Isolation Voltage
Primary Saturation Current
Primary Winding Resistance
Secondary Winding Resistance
U
U
W
U
the amplitude of the reflected voltage from the output to
the SW pin. Choose N according to the following equation:
Where: V
2 in the numerator is used to include the effect of the
voltage drop across the output diode(s).
Thus for a 320V output, N should be 322/31.5 or 10.2.
For a 300V output, choose N equal to 302/31.5 or 9.6.
The next parameter that needs to be set is the primary
inductance, L
formula:
Where: V
the transformer turns ratio. I
(LT3468-1)
L
that the LT3468/LT3468-1 has adequate time to respond
to the flyback waveform.
PRI
N
L
PRI
needs to be equal or larger than this value to ensure
V
OUT
OUT
31 5 .
OUT
V
OUT
is the desired output voltage. The number
PRI
2
is the desired output voltage. N is
. Choose L
•
N I
200 10
•
TYPICAL RANGE
PK
100 to 300
LT3468
•
8 to 12
LT3468/LT3468-1
>500
<300
>1.6
<40
>5
PRI
9
according to the following
PK
is 1.4 (LT3468), 0.7
TYPICAL RANGE
200 to 500
LT3468-1
8 to 12
>500
<500
>0.8
>10
<80
UNITS
34681i
7
m
nH
H
V
A