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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
THEORY OF OPERATION
When V IN reaches 2.7V during initial power up,
a power on reset signal will be issued to reset
all the registers and the internal logic, and the
system will be ready for operation.
To be in operation the XRP6840 must be
enabled through RESET_N, pin 6. The LEDs
can be activated through LEDFLASH or
LEDTORCH registers.
C HARGE P UMP M ODES 1 X , 1.5 X AND 2 X
The XRP6840 charge pump can operate in 1x,
1.5x and 2x modes to ensure desired current
regulation. Once the output reaches V OUT_LIMIT ,
the charge pump will turn off.
In 1x mode, the input is simply connected to
the output through an internal 0.5 Ω MOSFET.
An internal in-rush current limit will keep the
charging current controlled to a maximum of
600mA.
In 1.5x mode, during the first clock phase the
2 flying capacitors, C 1 and C 2 , are series
connected between V IN and ground, charging
each capacitor to ? V IN . In the second clock
phase, the flying capacitors are parallel
connected and placed in series with V IN ,
producing 1.5xV IN to be discharged across the
output capacitor. Note the maximum total
output current is now 600mA÷1.5 or 400mA.
In 2x mode, during the first clock phase, one
flying capacitors is charged to V IN . In the
second clock phase, this capacitor is placed in
series with V IN , producing 2xV IN to be
discharged across the output capacitor. Note
further in the High Efficiency Torch Mode
section.
The XRP6840 can operate in either “Auto Gain
Mode” or “Programmable Gain Mode”.
However, either mode will be overridden
during start-up based on the following internal
control algorithm:
1. If V OUT > VOUT_LIMIT then the charge pump is
OFF.
2. If V OUT < V IN – 1V then the charge pump
can only operate in 1x mode.
3. If V IN – 1V < V OUT < V IN then the charge
pump is allowed to operate in either 1x or
1.5x mode but not in 2x mode.
4. If V OUT > V IN then the charge pump can
operate either in 1.5x or 2x mode but not
in 1x mode (there is no reverse current
limit on the 0.5 Ω bypass MOSFET).
5. If V OUT > V OUT_LIMIT , and all LEDs are
deactivated then XRP6840 will operate in
standby mode with 30μA supply current.
V OUT_LIMIT is 4.5V for Torch Mode and High
Efficiency Torch Mode; for Flash mode
V OUT_LIMIT is shown in Table 4.
Standby mode is set by either LEDFLASH or
LEDTORCH registers [B7 B6 B5] to [0 0 0].
This will deactivate all the LED channels.
A UTO G AIN S TART - UP
The Auto Gain Start mode is the fastest way to
charge the output toward VOUT_LIMIT. It is
initiated with the following conditions; as
shown in Table 7 (a sub-set of Table 2), in
STATUS1 Register.
the maximum total output current is now
600mA÷2 or 300mA.
C HARGE P UMP C ONTROLLER
B7
0
1
1
B6
1
0
1
B4
0
0
0
B3
0
0
0
Operation Mode
High Efficiency Torch Mode
Auto Gain Start Torch Mode
Auto Gain Start Flash Mode
The charge pump controller regulates the
output by turning the charge pump off once
the output reaches V OUT_LIMIT . For both Torch
and Flash Modes, there is 50mV of output
voltage hysteresis before the charge pump is
re-enabled. However, in High Efficiency Torch
mode, it will only turn on again when V LEDX is
below the drop out voltage of approximately
0.36V. Once the charge pump turns on again,
it will then monitor both V OUT and V LEDX
voltages. This operation will be explained
Table 7: Auto Start Mode from STATUS1 Register
When in Auto Gain Start Mode, the XRP6840
will charge up V OUT to V OUT_LIMIT with maximum
available current within the constraints defined
above for start-up.
In this mode the XRP6840 will initialize the
system as follows:
1. Turn off all the LEDs that were
programmed through LEDTORCH or
LEDFLASH registers.
? 2009 Exar Corporation
12/17
Rev. 1.0.0
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