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X R P 6 8 4 0
4 . 3 A S u p e r c a p a c i t o r F l a s h L E D D r i v e r w i t h I 2 C
mode to maintain good current regulation. As
Figure 5 shows, very high efficiency can be
obtained if the system can operate at the
lowest available charge pump gain at very low
V IN levels. Due to the XRP6840’s low drop out
voltage design, good current regulation can
still be achieved at extremely low V LED voltage
levels.
When V LEDX drops below V DO , and V OUT is also
below V OUT_LIMIT , the XRP6840 charge pump
will turn on to charge the V OUT and V LEDX . If
V LEDX is still below V DO after 64 clock cycles or
27μs (at 2.4MHz), the XRP6840 will jump to
the next higher gain. If at any time, V OUT_LIMIT
is reached, then charge pump will turn off.
F AULT P ROTECTION
Although most of these modes of operation
have already been previously described, they
are repeated here to emphasize the
robustness of the XRP6840.
The output voltage is directly monitored and
controlled through the V OUT pin. Should an
open occur on the V OUT pin, the output is
disconnected from the input and no damaging
voltages will be applied to the output super
capacitor due to an open loop condition.
The XRP6840 also has a built-in over current
protection because when the output is
shorted, the XRP6840 will force the part to be
in 1x mode and its output current is always
C OMPONENT S ELECTION
The XRP6840 charge pump circuit requires the
following capacitors:
1. Input Cap: 22μF
2. Output Cap: 10 μ F
3. Charge Pump Flying caps: 2x0.47 μ F
4. Supercapacitor: 0.6F (0.3F to 0.9F).
Input capacitances higher than 10 μ F will help
reduce input voltage ripple and in-rush
current. Refer to Figures 9 and 10 for
comparison.
The input and output capacitors should be
located as close to the V IN and V OUT pins as
possible to obtain best bypassing. Their
returns paths should be connected directly to
the P GND pin or to the thermal pad ground
located under the XRP6840. The flying
capacitors should be located as close to the
C 1P , C 1N and C 2P , C 2N pins as possible.
All the capacitors should be surface mount
ceramic types for low ESR and for low lead
inductances. These capacitors can also
improve bypassing. X5R or X7R temperature
grades are recommended for this application.
The supercapacitor selection depends on LED
current, flash duration, and LED forward
voltage. The minimum super capacitor is
determined as follows:
limited to 600mA regardless of whether or not
the part is in Auto or Programmed Start
Modes.
_
_
The XRP6840 also has thermal protection. If
the junction temperature rises above 150°C,
the part is disabled. Once the temperature
drops below 140°C the part is re-enabled.
O PEN AND S HORT LED P ROTECTION
An open LED has no real effect on the
operation of the XRP6840.
If an LED fails short, that channel will be
disabled. The short condition is asserted when
I OUT is the target flash current, T FLASH is the
flash duration, V OUT_LIMIT is the initial Super
Capacitor voltage programmed by STATUS2
register bits [B7 B6 B5], V F is the LED forward
voltage and V LED is the maximum LED pin
voltage before dropout (360mV at 125°C).
For example, for a 4.3A flash with 110ms
duration, LED V F of 3.5V and V OUT_LIMIT voltage
of 5.3V, the minimum capacitance is:
V OUT to V LEDX is less than 200mV. Only the
driver of the shorted LED will be turned off
and no current will flow. However, the other
5.3
3.5
4.8 0.11
0.36 4.8
0.05Ω
0.44
channels will continue to operate as intended,
independently of the shorted channel.
? 2009 Exar Corporation
14/17
Rev. 1.0.0
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