AT1454 Datasheet, PDF (12/18 Pages) Aimtron Technology – Photoflash Capacitor Charger for DSC

Application Information
AT1454/AT1454A/AT1454B
Preliminary Product Information
Photoflash Capacitor Charger for DSC
AT1454 series is designed to precise control of switching current to charge photoflash
capacitors quickly and efficiently. When the Charge pin is first driven high, a one shot
set SR latches in the correct state. The built-in power MOSFET turns on and primary
current ramps up linearly according to V_BAT and primary inductance. Comparator
monitors the switch On-time and when the switch on-time reaches limit value,
MOSFET is turned off. During this phase, stored energy of primary transformer is
delivered to the photoflash capacitor via the secondary and diode. The secondary
current is decreased linearly at a rate determined by the secondary inductance. The
secondary current is monitored by the Isec pin. When the secondary current drops
below 10mA, the fires a one shot will turn on MOSFET. The charging cycle repeats to
deliver power to the output.
AT1454 series can adjust external resistance R_SENSE to set N-MOS switch on-time, but
switch on-time value must match with electrical characteristic of transformer. It is
usually equal to or less than saturation current of transformer otherwise transformer
will be saturated. Therefore, efficiency will be reduced.
The output voltage is detected by a voltage divider connecting to output capacitor.
When the output voltage reaches the desired voltage set by resistive voltage-divider,
comparator output goes high which resets the master latch and disable the charging
block. The N-channel MOSFET is turned off pulling the Ready pin high, indicating
that the part has finished charging. Power delivery can be restarted automatically
when feedback voltage is less than 90% of reference voltage. Power delivery can only
be restarted by toggling the Charge pin. The Charge pin gives full control of the part
to the user. The IGBT buffer will provide a sufficient current to drive IGBT to ignite
flash tube.
Transformer Design
The Fly-back transformer is a key element for AT1454 series. It must be designed
carefully and checked that it does not cause excessive current or voltage on any pin of
the part. The first transformer parameter that needs to be set is the turns ratio N,
should be high enough so that the absolute maximum voltage rating for the NMOS
drain to source voltage is not exceeded. Choose the minimum turns ratio according to
following formula:
N _MIN
≥
V_OUT
V_{DS (MAX )} − V_BAT
7F, No.9,PARK AVENUE. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Tel: [phone] Fax: [phone]
WWW: http://www.aimtron.com.tw
3/24/2006 REV:1.2
Email: [e-mail]
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