XRP6840 Exar Corporation, XRP6840 Datasheet - Page 13

• Manufacturer Part Number: XRP6840
• Description: 4.3a Supercapacitor Flash Led Driver With I2c
• Manufacturer: Exar Corporation

2. Determine if VOUT_LIMIT is reached. If
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
The Auto Start Mode begins in 1x mode or
1.5x mode if V
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
not reached V
current is below 300mA. After switching to 2x
mode, the charge pump continues to operate
until V
If V
XRP6840 will continuously run the charge
pump in 2x mode. At this point, it is
recommended to shut down through I
RESET_N to ‘0’ or power down.
There are probably 2 main causes for the
XRP6840
overloaded or V
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
low then it is advisable to power down the
system and change the battery.
P
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
programmed, the 5 stage internal control
algorithm on the previous page will still
override the programmed gain to ensure
optimum
© 2009 Exar Corporation
ROGRAMMABLE
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.
OUT_LIMIT
OUT_LIMIT
though
to
operation
could not be reached, then the
is reached.
remain
OUT_LIMIT
IN
OUT
S
the
too low. If the output were
TART
> V
and
OUT
and when the input
maximum
in
M
IN
ODE
. When the input
reaches V
2x
reverse
mode:
OUT_LIMIT
gain
4
4
IN
.
current
.
OUT
3
3
is too
A
A
V
has
2
OUT
.
C,
S
S
is
u
u
13/17
p
p
e
e
r
r
protection in 1x mode. Reverse current is
possible in 1.5x or 2x modes and that leads to
interesting applications possibilities.
R
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
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.
H
High
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
V
are enabled, the control loop will adjust V
to the minimum value required to maintain
current regulation. It does this by monitoring
and regulating the V
to an adaptive drop out voltage, V
The adaptive V
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
c
c
OUT_LIMIT
a
a
EVERSING THE
IGH
p
p
a
a
E
c
c
Efficiency
i
FFICIENCY
i
t
t
o
then turn off. Then when the LEDs
o
r
r
F
F
l
l
DO
a
a
s
P
s
Torch
algorithm will allow V
h
h
OWER
T
ORCH
L
L
LEDX
E
E
D
D
F
Mode
voltage with respect
LOW
D
M
D
r
r
ODE
i
i
v
v
e
e
X
X
is
r
DO
r
R
R
w
w
.
OUT_LIMIT
a
P
P
Rev. 1.0.0
i
i
6
6
t
t
special
h
h
LEDX
OUT
8
8
I
I
4
4
2
2
OUT
0
to
to
to
0
C
C