A8439 ALLEGRO [Allegro MicroSystems], A8439 Datasheet - Page 9

• Manufacturer Part Number: A8439
• Description: Photoflash Capacitor Charger with IGBT Driver and Refresh
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A8439
Overview
The A8439 is a photoflash capacitor charger control IC with
adjustable input current limiting and automatic refresh. It also
integrates an IGBT driver for strobe operation of the flash tube,
dramatically saving board space in comparison to discrete solu-
tions for strobe flash operation. The control logic is shown in the
functional block diagram.
The charging operation of the A8439 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. The primary peak current is set by input
clock signals from the CHARGE pin. When a charging cycle is
initiated, the transformer primary side current, I
linearly at a rate determined by the combined effect of the battery
voltage, V
I
is turned off immediately, allowing the energy to be pushed into
the photoflash capacitor, C
The secondary side current drops linearly as C
recharging cycle starts again, either after the transformer flux
is reset, or after a predetermined time period, t
whichever occurs first.
The output voltage, V
and R
the positive terminal of the output capacitor 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 volt-
age level based on a typical value of 1.205 V at the FB pin. As
soon as V
terminated. The A8439 automatically starts a new charging cycle
when the internal voltage sensing circuit detects a 10 % drop in
the output voltage. Toggling the CHARGE pin can also start a
refresh operation.
Primary
3
(see application circuit diagrams), connected between
reaches the current limit, I
OUT
BATT
reaches the desired value, the charging process is
, and the primary side inductance, L
OUT
, is sensed by a resistor string, R
OUT
, from the secondary winding.
Photoflash Capacitor Charger with IGBT Driver and Refresh
SWLIM
, the internal MOSFET
IN
IN
reaches V
OFF(Max)
OUT
is above V
Primary
Functional Description
charges. The
Primary
, ramps up
(18 μs),
UVLO
. When
UVLO
1
, R
,
2
,
Figure 3. Auto-refresh waveform of A8439.
Auto-Refresh
The A8439 features auto-refresh when the feedback resistor
network is connected at the output. Auto-refresh initiates when
the output voltage drops to ≈90 % of the set stop voltage of the
resistor network. The operation is shown in figure 3.
Input Current Limiting
The peak current limit can be adjusted to eight different levels,
from 270 mA to 1.4 A, by clocking the CHARGE pin. An internal
digital circuit decodes the input clock signals to a counter, which
sets the charging time. This flexible scheme allows the user
to operate the flash circuit according to different battery input
voltages. The battery life can be effectively extended by setting a
lower current limit at low battery voltages.
Figure 4 shows the ILIM clock timing scheme protocol. The
total ILIM setup time, t
ILIM(SU)
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
, denotes the time needed for the
9