A8437ECGLT Datasheet, PDF (12/22 Pages) Allegro MicroSystems – Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver

A8437
Mobile Phone Xenon Photoflash Capacitor Charger
with IGBT Driver
Smart Current Limit (Optional)
With the help of some simple external logic, the
user can change the charging current according to
the battery voltage. For example, assume that I_SET is
normally 36 μA (for I_SWlim = 1.0 A). Referring to the
following illustration, when the battery voltage drops
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
the following figure.
Timer Mode
BL RBL
ISET
RSET
below 2.5 V, the signal at BL (battery-low) goes high.
The resistor RBL, connecting BL to the ISET pin,
then injects 10 μA into RSET. This effectively reduces
ISET current to 26 μA (for I_SWLIM = 0.73 A). A disad-
vantage of the above method is that the 10 μA current
is always flowing whenever the BL signal goes high.
Timer Mode and Fast Charging Mode
The A8437 achieves fast charging times and high effi-
ciency 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 the following figure.
V_OUT
V_SW
V_BAT
V_OUT
I_SW
V_OUT≤ 14 V; t =2 μs/div; V_BAT =3.6 V; R_SET=33.2 kΩ
As soon as a sufficient voltage has built up at the
output capacitor, the IC enters Fast-Charging Mode.
In this mode, the next switching cycle starts after the
secondary side current has stopped flowing, and the
switch voltage has dropped to a minimum value. A
proprietary circuit is used to allow minimum-voltage
switching, even if the SW pin voltage does not drop to
0 V. This enables Fast-Charging Mode to start earlier
than previously possible, thereby reducing the overall
charging time. Minimum-voltage switching is shown
in the following figure.
Timer Mode
Fast Charging Mode
V_BAT
Minimum Voltage
Switching
I_IN
t =200 ms/div; V_OUT =50 V/div; V_BAT =1 V/div.; I_IN =100 mA/div.
V_BAT =3.6 V; C_OUT =20 μF/330 V; R_SET=46 kΩ (I_SWlim≈0.75 A)
The IC operates in Timer Mode when beginning to
charge a completely discharged photoflash capaci-
tor, usually when the output voltage, V_OUT, is less
than approximately 15 to 20 V. Timer Mode is a fixed
V_SW
V_BAT
V_OUT
I_SW
V_OUT ≥ 15 V; t =1 μs/div; V_BAT =3.6 V; R_SET=33.2 kΩ
During Fast-Charging Mode, when V_OUT is high
enough (over 50 V), true zero-voltage switching
Allegro MicroSystems, Inc.
11
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com