a8735 Allegro MicroSystems, Inc., a8735 Datasheet - Page 10

• Manufacturer Part Number: a8735
• Description: Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger With Igbt Driver
• Manufacturer: Allegro MicroSystems, Inc.

A8735
General Operation Overview
The charging operation is started by a low-to-high signal on the
CHARGE pin, provided that V
strongly recommended to keep the CHARGE pin at logic low
during power-up. After V
to-high transition on the CHARGE pin is required to start the
charging. The ¯ D ¯ ¯ ¯ O ¯ ¯¯ N ¯ ¯ E ¯ open-drain indicator is pulled low when
CHARGE is high and target output voltage is reached.
When a charging cycle is initiated, the transformer primary side
current, I
combined effect of the battery voltage, V
side inductance, L
limit, I
allowing the energy to be pushed into the photoflash capacitor,
C
drops linearly as C
either after the transformer flux is reset, or after a predetermined
time period, t
The A8735 senses output voltage indirectly on primary side. This
eliminates the need for high voltage feedback resistors required
for secondary sensing. Flyback converter stops switching when
output voltage reaches:
Where:
and
Switch On-Time and Off-Time Control
The A8735 implements an adaptive on-time/off-time control. On-
time duration, t
Off-time duration, t
during switch off-time. The A8735 applies two charging modes:
Fast Charging mode and Timer mode, according to the conditions
described in the next section.
K = 31.5 V typically,
V
N is transformer turns ratio.
OUT
d
is the forward drop of the output diode (approximately 2 V),
, from the secondary winding. The secondary side current
SWLIM
PRIMARY
, the internal MOSFET is turned off immediately,
OFF
on
(max) (18 μs), whichever occurs first.
t
, is approximately equal to
, ramps-up linearly at a rate determined by the
on
PRIMARY
OUT
off
= I
, depends on the operating conditions
V
SWlim
charges. The switching cycle starts again,
OUT
IN
. When I
exceeds the UVLO level, a low-
= K × N – V
× L
IN
PRIMARY
is above the V
PRIMARY
BAT
d
/ V
,
BAT
, and the primary
reaches the current
Ultra Small Mobile Phone Xenon Photoflash
UVLO
.
Functional Description
level. It is
Capacitor Charger with IGBT Driver
C2,C3
Timer Mode and Fast Charging Mode
The A8735 achieves fast charging times and high efficiency by
operating in discontinuous conduction mode (DCM) through
most of the charging process. The relationship of Timer mode and
Fast Charging mode is shown in figure 2.
The IC operates in Timer mode when beginning to charge a com-
pletely discharged photoflash capacitor, usually when the output
voltage, V
transformer used). Timer mode is a fixed period, 18 μs, off-time
control. One advantage of having Timer mode is that it limits the
initial battery current surge and thus acts as a “soft-start.” A time-
expanded view of a Timer mode interval is shown in figure 3.
Figure 2. Timer mode and Fast Charging mode: t = 1 s/div;
V
C
Figure 3. Expanded view of Timer mode: V
Ch1: V
Ch4: I
OUT
OUT
C4
C1
= 50 V/div; I
= 100 μF / 330 V; and ILIM = 1.0 A.
SW
OUT
= 500 mA / div., t = 5 μs / div.
= 20 V / div., Ch2: V
OUT
, is less than approximately 30 V (depending on
IN
= 100 mA/div., V
BAT
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
= 5 V / div., Ch3: V
IN
= V
BAT
OUT
= 3.6 V;
≤ 10 V, V
SW
= 5 V / div.,
BAT
= 5.5 V,
V
I
V
OUT
V
V
IN
I
OUT
SW
SW
BAT
10