LT3468 Linear Technology, LT3468 Datasheet - Page 8

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

APPLICATIO S I FOR ATIO
LT3468/LT3468-1
All other parameters need to meet or exceed the recom-
mended limits as shown in Table 1. A particularly impor-
tant parameter is the leakage inductance, L
power switch of the LT3468/LT3468-1 turns off, the
leakage inductance on the primary of the transformer
causes a voltage spike to occur on the SW pin. The height
of this spike must not exceed 40V, even though the
absolute maximum rating of the SW Pin is 50V. The 50V
absolute maximum rating is a DC blocking voltage speci-
fication, which assumes that the current in the power NPN
is zero. Figure 4 shows the SW voltage waveform for the
circuit of Figure 1(LT3468). Note that the absolute maxi-
mum rating of the SW pin is not exceeded. Figure 5 shows
the SW voltage waveform for the circuit of Figure 2(LT3468-
1). Again, the absolute maximum rating of the SW pin is
Table 2. Pre-Designed Transformers - Typical Specifications Unless Otherwise Noted.
FOR USE WITH
LT3468
LT3468-1
LT3468
LT3468-1
LT3468/LT3468-1
LT3468-1
8
10V/DIV
V
SW
Figure 4. LT3468 SW Voltage Waveform
V
V
IN
OUT
TRANSFORMER NAME
= 5V
= 320V
LDT565630T-001
LDT565630T-002
U
SBL-5.6S-1
SBL-5.6-1
T-15-089
T-15-083
100ns/DIV
U
W
(W L H) mm
5.6 8.5 4.0
5.6 8.5 3.0
6.4 7.7 4.0
8.0 8.9 2.0
5.8 5.8 3.0
5.8 5.8 3.0
SIZE
3468 G18
LEAK
. When the
U
( H)
L
14.5
10
24
12
20
PRI
6
LPRI-LEAKAGE
200 Max
400 Max
200 Max
500 Max
400 Max
500 Max
(nH)
not compromised. Make sure to check the SW voltage
waveform with V
is the worst case condition for SW voltage.
It is important not to minimize the leakage inductance to
a very low level. Although this would result in a very low
leakage spike on the SW pin, the parasitic capacitance of
the transformer would become large. This will adversely
effect the charge time of the photoflash circuit.
Linear Technology has worked with several leading mag-
netic component manufacturers to produce pre-designed
flyback transformers for use with the LT3468/LT3468-1.
Table 2 shows the details of several of these transformers.
10.2
10.2
10.4
10.2
10.2
10.2
10V/DIV
N
V
Figure 5. LT3468-1 SW Voltage Waveform
SW
100 Max 10 Max
240 Max 16.5 Max
211 Max 27 Max
675 Max 35 Max
V
V
(m )
IN
OUT
R
103
305
PRI
= 5V
OUT
= 320V
near the target output voltage, as this
R
( )
26
55
SEC
100ns/DIV
[e-mail] (email)
www.components.tdk.com
Tokyo Coil Engineering
www.tokyo-coil.co.jp
[phone] (ph)
852-2489-8266 (ph)
Chicago Sales Office
0426-56-6336 (ph)
Hong Kong Office
Kijima Musen
Japan Office
VENDOR
3468 G19
TDK
34681i