a8438eej-t Allegro MicroSystems, Inc., a8438eej-t Datasheet - Page 11

• Manufacturer Part Number: a8438eej-t
• Description: A8438 Photoflash Capacitor Charger With Igbt Driver
• Manufacturer: Allegro MicroSystems, Inc.

A8438
Overview
The A8438 is a photoflash capacitor charger control IC with
adjustable input current limiting. It also integrates an IGBT
driver for strobe operation of the flash tube, dramatically saving
board space in comparison to discrete solutions for strobe flash
operation. The control logic is shown in the functional block
diagram.
The charging operation of the A8438 is started by a low-to-high
signal on the CHARGE pin, provided that V
level. If CHARGE is already high before V
another low-to-high transition on the CHARGE pin is required
to start the charging. When a charging cycle is initiated, the
transformer primary side current, I
rate determined by the combined effect of the battery voltage,
V
reaches the current limit, I
pin, the internal MOSFET is turned off immediately, allowing the
energy to be pushed into the photoflash capacitor, C
secondary winding. The secondary side current drops linearly as
C
While the internal MOSFET switch is turned off, the output
voltage, V
nected between the anode of the output diode, D1, and ground.
This resistor string forms a voltage divider that feeds back to the
FB pin. The resistors must be sized to achieve a desired output
voltage level based on a typical value of 1.205 V at the FB pin.
As soon as V
is terminated. The user may toggle the CHARGE pin to refresh
the photoflash capacitor.
Switch On-Time and Off-Time Control
The A8438 implements an adaptive on-time/off-time control. (For
circuit details, please refer to the the Control Logic block in the
simplified Functional Block Diagram on page 2.) On-time dura-
tion, t
inductance, L
duration, t
off-time. The A8438 applies its two charging modes, Fast Charg-
ing mode and Timer mode, according to those conditions.
OUT
BATT
ON
charges.
, and the primary side inductance, L
, is determined by input voltage, V
OFF
OUT
OUT
Primary
, depends on the operating conditions during switch
, is sensed by a resistor string, R
reaches the desired value, the charging process
, and the set current limit, I
SWLIM
, set by configuring the ILIM
Primary
, ramps up linearly at a
Primary
IN
IN
IN
, transformer primary
SWLIM
reaches V
1
is above V
through R
Photoflash Capacitor Charger with IGBT Driver
. When I
Functional Description
OUT
. Off-time
, from the
UVLO
Primary
UVLO
3
, con-
,
Figure 3. Sequencing of Timer mode and Fast Charging mode (time
axis scale is 1 s per division)
Fast Charging and Timer Modes
The IC operates in the Fast Charging mode when the photoflash
capacitor, C
mode, the converter operates near the discontinuous boundary,
and a sensing circuit tracks the fly-back voltage at the SW node.
As soon as this voltage swings below 1.2 V, the internal MOS-
FET switch is turned on again, starting the next charging cycle.
The IC operates in the Timer mode when beginning to charge a
completely discharged photoflash capacitor, usually when the out-
put voltage, V
mode is a fixed 18 μs off-time control. One advantage of the
A8438 watchdog timer control scheme is that it limits the initial
current surge and thus acts as a “soft-start.” As shown in figure 3,
the timer mode only lasts a small fraction of a second (usually <
100 ms). It can be recognized by its lower initial input charging
current as a result of a lower duty cycle. As output voltage rises to
more than 10 to 20 V, the adaptive Fast Charging mode takes over
the control, raising the average input current level.
I
V
BATT(Avg)
OUT
OUT
OUT
, is only partially discharged. In Fast Charging
, is less than approximately 10 to 20 V. Timer
Timer
Mode
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
Fast Charging Mode
11