A8438(2005) 查看數據表（PDF） - Allegro MicroSystems
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A8438 Datasheet PDF : 13 Pages
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A8438 Photoflash Capacitor Charger with IGBT Driver
Functional Description
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. When a charging cycle is initiated,
the transformer primary side current, IPrimary, ramps up linearly at
a rate determined by the combined effect of the battery voltage,
VBATT , and the primary side inductance, LPrimary. When IPrimary
reaches the current limit, ISWLIM , set by configuring the ILIM
pin, the internal MOSFET is turned off immediately, allowing the
energy to be pushed into the photoflash capacitor, COUT, from the
secondary winding. The secondary side current drops linearly as
COUT charges.
While the internal MOSFET switch is turned off, the output
voltage, VOUT, is sensed by a resistor string, R1 through R3, con-
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 VOUT reaches the desired value, the charging process
is terminated. The user may toggle the CHARGE pin to refresh
the photoflash capacitor.
Switch On-Time and Off-Time Control
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
output voltage, VOUT, is less than approximately 10 to 20 V.
Timer 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.
To understand the Timer mode, it is noted that the secondary
winding charge current, ISecondary, decreases linearly at a rate of:
dISecondary =
dt
VOUT
LPrimary N 2
(1)
where:
ISecondary is the secondary side current,
LPrimary is the primary side inductance, and
N is the transformer turns ratio (NSecondary / NPrimary).
Timer
Mode
Fast Charging Mode
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
duration, tON , is determined by input voltage, VIN, transformer
primary inductance, LPrimary, and the set current limit, ISWLIM .
Off-time duration, tOFF , depends on the operating conditions
during switch off-time. The A8438 applies its two charging
modes, Fast Charging mode and Timer mode, according to those
conditions.
IBATT(Avg)
VOUT
Fast Charging and Timer Modes
The IC operates in the Fast Charging mode when the photoflash
capacitor, COUT, is only partially discharged. In Fast Charging
mode, the converter operates near the discontinuous boundary,
Figure 3. Sequencing of Timer mode and Fast Charging mode (time
axis scale is 1 s per division)
A8438-DS, Rev. 1
Allegro MicroSystems, Inc.
9
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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