LT3420EMS-1 Linear Technology, LT3420EMS-1 Datasheet - Page 10

• Manufacturer Part Number: LT3420EMS-1
• Description: IC PHOTOFLASH CHARGER CAP 10MSOP
• Manufacturer: Linear Technology

Specifications of LT3420EMS-1

• Applications: Photoflash Capacitor Charger
• Current - Supply: 90µA
• Voltage - Supply: 2.2 V ~ 16 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LT3420/LT3420-1
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
Choosing the Right Transformer
The flyback transformer plays a key role in any LT3420/
LT3420-1 application. A poorly designed transformer can
result in inefficient operation. Linear Technology Corpora-
tion has worked with a number of transformer manufactur-
ers to develop specific transformers for use with the
LT3420/LT3420-1. These predesigned transformers are
sufficient for a large majority of the applications that may
be encountered. In some cases, the reader may choose to
design his own transformer or may simply be curious about
the issues involved in designing the transformer. The fol-
lowing is a brief discussion of the issues relating to trans-
former design.
Transformer Turns Ratio
The turns ratio for the transformer, N, should be high
enough so that the absolute maximum voltage rating for
the NPN power switch is not exceeded. When the power
switch turns off, the voltage on the collector of the switch
(SW Pin) will “fly” up to the output voltage divided by N
plus the battery voltage (neglecting the voltage drop
across the rectifying diodes). This voltage should not
exceed the 38V (LT3420) or 50V (LT3420-1) breakdown
rating of the power switch.
Choose the minimum N by the following formula.
For an LT3420 design, a 5V battery voltage and a 330V
output results in a N
should be used.
10
N
N
MIN
MIN
≥
≥
38
50
V
V
–
–
OUT
OUT
V
V
BAT
BAT
MIN
U
(
(
of 10 so a turns ratio of 10 or greater
LT
LT
U
3420
3420 1
)
−
)
W
U
Transformer Primary Inductance
A flyback transformer needs to store substantial amounts
of energy in the core during each switching cycle. The
transformer, therefore, will generally require an air gap.
The use of an air gap in the core makes the energy storage
ability, or inductance, much more stable with temperature
and variations in the core material. Most core manufactur-
ers will supply standard sizes of air gaps with a given type
of core, resulting in different A
tance of a particular core per square turns of winding. To
get a certain inductance, simply divide the desired induc-
tance by the A
to find the number of turns needed on the primary of the
transformer.
The LT3420/LT3420-1 detect the output voltage via the
flyback pulse on the SW pin. Since this can only occur
while the power switch is off, an important criteria is that
the value of the primary inductance of the transformer be
larger than a certain minimum value. The switch off time
should be 500ns or larger for the LT3420 and 350ns or
larger for the LT3420-1. The minimum inductance can be
calculated with the following formula:
Transformer Leakage Inductance
The leakage inductance of the transformer must be care-
fully minimized for both proper and efficient operation of
the part. The DC voltage rating of the SW pin on the LT3420
is 38V while on the LT3420-1 it is 50V. These ratings are
for DC blocking voltages only and additional precautions
V
N: Transformer Turns Ratio
L
L
OUT
PRI
PRI
: Target Output Voltage
≥
≥
500 10
350 10
N
N
• ( . – .
• ( . – .
L
1 0 0 015
•
•
1 4 0 04
value and take the square root of the result
–
–
9
9
•
•
V
V
OUT
OUT
N
N
)
)
(
(
L
LT
LT
values. A
3420
3420 1
)
−
L
)
is the induc-
3420fb