XRP6840 Datasheet, PDF (13/17 Pages) Exar Corporation – 4.3A Supercapacitor Flash LED Driver with I2C

XRP6840
4.3A Supercapacitor Flash LED Driver with I²C
2. Determine if VOUT_LIMIT is reached. If
this condition is met anytime, the charge
pump will turn off, release the control of
the LEDTORCH or LEDFLASH registers, and
will be ready for turning on the LEDs.
protection in 1x mode. Reverse current is
possible in 1.5x or 2x modes and that leads to
interesting applications possibilities.
REVERSING THE POWER FLOW
After system initialization, the XRP6840 will
automatically switch between 1x, 1.5x, or 2x
mode modes to try and keep input current to a
maximum without exceeding 600mA. Auto
Start Mode ends when V_OUT reaches V_{OUT_LIMIT}.
The Auto Start Mode begins in 1x mode or
1.5x mode if V_OUT > V_IN. When the input
current drops to approximately 400mA the
XRP6840 is able to switch from 1x to 1.5x
mode without exceeding 600mA. The switch
from 1.5x mode to 2x mode occurs if V_OUT has
not reached V_{OUT_LIMIT} and when the input
current is below 300mA. After switching to 2x
mode, the charge pump continues to operate
until V_{OUT_LIMIT} is reached.
If V_{OUT_LIMIT} could not be reached, then the
XRP6840 will continuously run the charge
pump in 2x mode. At this point, it is
recommended to shut down through I²C,
RESET_N to ‘0’ or power down.
There are probably 2 main causes for the
XRP6840 to remain in 2x mode: V_OUT
overloaded or V_IN too low. If the output were
shorted, the current in 1x mode would never
drop below 600mA and the XRP6840 would
never move to the higher gains. Removing the
short or overload will allow the XRP6840 to
recover back to normal operation. If V_IN is too
low then it is advisable to power down the
system and change the battery.
PROGRAMMABLE START MODE
Programmable start mode allows the flexibility
to select the maximum gain. Start-up gains
are based strictly on input and output voltage
differences. The 600mA input current limit is
still imposed, but typically is not triggered.
This is why Auto Start Mode is recommended
because it charges the output capacitor the
fastest.
Even though the maximum gain is
programmed, the 5 stage internal control
algorithm on the previous page will still
override the programmed gain to ensure
optimum operation and reverse current
By forcing the charge pump into 1.5x mode
once the output voltage is charged in Flash
Mode, it is possible to reverse the current back
to the input if the input voltage is <3.6V. This
gives one the possibility of using the XRP6840
Flash system to provide power back to the
input when other parts of the system require
unusually high loads. A couple of examples
are; a high power burst transmit, or HDD spin
up.
In Flash Mode, charge the output capacitor to
5.45V (STATUS2 Register B7-B5 = 110) using
Auto Start Mode. When power is required back
at the input, force to 1.5x mode (STATUS1
Register B4-B3 = 10) and change V_{OUT_LIMIT} to
5.60V (STATUS2 Register B7-B5 = 111). This
will have the effect of instantly turning on the
charge pump in 1.5x mode. If the input
voltage is 3V, then approximately 300mA will
flow from the output super capacitor to the
input for 650ms. The current will drop as the
super capacitor discharges, but as much as
150mA is available after 1.5seconds.
A more complete application note will be
forthcoming on this topic.
HIGH EFFICIENCY TORCH MODE
High Efficiency Torch Mode is a special
XRP6840 feature designed to achieve the
highest torch mode power efficiency. This
mode is activated only when STATUS1 [B7 B6
B4 B3] are [0 1 0 0]. LEDTORCH [B7 B6 B5]
can be set according to the number of desired
LED channels.
First, the charge pump will charge V_OUT to
V_{OUT_LIMIT} then turn off. Then when the LEDs
are enabled, the control loop will adjust V_OUT
to the minimum value required to maintain
current regulation. It does this by monitoring
and regulating the V_LEDX voltage with respect
to an adaptive drop out voltage, V_DO.
The adaptive V_DO algorithm will allow V_LEDX to
drop down to 220mV for 1x mode and 180mV
for both 1.5x and 2x modes. This will ensure
the XRP6840 stays in the lowest charge pump
© 2009 Exar Corporation
13/17
Rev. 1.0.0