A8837 查看數據表（PDF） - Allegro MicroSystems
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A8837 Datasheet PDF : 11 Pages
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A8837
Photoflash Capacitor Charger with IGBT Driver
current starts to ramp up to the set ISWLIM. The ISWLIM setting
remains in effect as long as the CHARGE pin is high. To reset the
ILIM counter, pull the CHARGE pin low before clocking in the
new setting.
After the first start-up or an ILIM counter reset, each new current
limit can be set by sending a burst of pulses to the CHARGE pin.
The first rising edge starts the ILIM counter, and up to 8 rising
edges will be counted to set the ISWLIM level. The first pulse
width, tILIM1(H), must be at least 20 μs long. Subsequent pulses
(up to 7 more) can be as short as 0.2 μs. The last low-to-high
edge must arrive within 54 μs from the first edge. The CHARGE
pin will stay high afterwards.
Figure 6 shows the last charging cycle, when the CHARGE pin is
forced low before charging has been completed.
The A8837 implements an adaptive off-time, tOFF , control. After
the switch is turned off, a sensing circuit tracks the flyback volt-
age at the SW node. A special dV/dt detection circuit is used to
allow minimum-voltage switching, even if the SW voltage does
not drop to zero volts. This enables fast-charging to start earlier
than previously possible, thereby reducing the overall charging
time. However, when the photoflash capacitor charger circuit
starts up at low output voltage, a timeout may be triggered to
limit the maximum switch off-time to 18 μs.
VCHARGE
VSW
ISW
VCHARGE
VSW
ISW
VCHARGE
VSW
VCHARGE
VSW
ISW
VCHARGE
VSW
ISWLIM = 2.0 A
ISWLIM = 1.8 A
ISWLIM = 1.6 A
ISW
ISWLIM = 1.4 A
Figure 6. Last charging cycle, when the CHARGE pin is forced low before
charging is complete.
ISW
Figure 7. ILIM programming waveforms for various ISWLIM; VIN =
3.3 V, VBAT = 2.0 V, VCHARGE 5 V/div, VSW 2 V/div, ISW 500 mA/div,
time scale 10 μs/div.
Allegro MicroSystems, Inc.
8
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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