XRP6840 Exar Corporation, XRP6840 Datasheet - Page 12

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

THEORY OF OPERATION
When V
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
LEDTORCH registers.
C
The XRP6840 charge pump can operate in 1x,
1.5x and 2x modes to ensure desired current
regulation. Once the output reaches V
the charge pump will turn off.
In 1x mode, the input is simply connected to
the output through an internal 0.5Ω 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
connected between V
each capacitor to ½ V
phase,
connected and placed in series with V
producing 1.5xV
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
second clock phase, this capacitor is placed in
series
discharged across the output capacitor. Note
the maximum total output current is now
600mA÷2 or 300mA.
C
The charge pump controller regulates the
output by turning the charge pump off once
the output reaches V
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
below the drop out voltage of approximately
0.36V. Once the charge pump turns on again,
it will then monitor both V
voltages. This operation will be explained
© 2009 Exar Corporation
HARGE
HARGE
be
IN
with
the
P
P
reaches 2.7V during initial power up,
activated
UMP
UMP
flying
V
M
C
IN
IN
ONTROLLER
,
ODES
to be discharged across the
producing
capacitors
through
IN
OUT_LIMIT
IN
1
1
and ground, charging
. In the second clock
X
and C
, 1.5
. For both Torch
X AND
OUT
LEDFLASH
2xV
2
, are series
are
IN
IN
and V
. In the
4
4
2
OUT_LIMIT
parallel
.
.
to
X
3
3
LEDX
A
A
LEDX
be
IN
S
or
S
is
,
,
u
u
12/17
p
p
e
e
r
r
further in the High Efficiency Torch Mode
section.
The XRP6840 can operate in either “Auto Gain
Mode”
However, either mode will be overridden
during start-up based on the following internal
control algorithm:
1. If V
2. If V
3. If V
4. If V
5. If V
V
Efficiency
V
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
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.
When in Auto Gain Start Mode, the XRP6840
will charge up V
available current within the constraints defined
above for start-up.
In this mode the XRP6840 will initialize the
system as follows:
1. Turn
c
c
OUT_LIMIT
OUT_LIMIT
a
a
UTO
B7
0
1
1
p
p
OFF.
can only operate in 1x mode.
pump is allowed to operate in either 1x or
1.5x mode but not in 2x mode.
operate either in 1.5x or 2x mode but not
in 1x mode (there is no reverse current
limit on the 0.5Ω bypass MOSFET).
deactivated then XRP6840 will operate in
standby mode with 30µA supply current.
programmed
LEDFLASH registers.
Table 7: Auto Start Mode from STATUS1 Register
a
a
c
c
G
OUT
B6
OUT
IN
OUT
i
i
1
0
1
OUT
AIN
t
t
or
is shown in Table 4.
o
o
is 4.5V for Torch Mode and High
– 1V < V
>
off
< V
r
r
> V
Torch
S
> V
B4
F
VOUT_LIMIT
F
0
0
0
“Programmable
TART
l
l
IN
a
a
IN
OUT
all
s
s
OUT_LIMIT
– 1V then the charge pump
then the charge pump can
h
h
B3
-
0
0
0
through
to V
Mode;
OUT
UP
L
L
the
then the charge pump is
E
E
High Efficiency Torch Mode
Auto Gain Start Torch Mode
Auto Gain Start Flash Mode
< V
OUT_LIMIT
D
D
, and all LEDs are
D
D
LEDs
Operation Mode
IN
for
r
r
i
i
then the charge
LEDTORCH
v
v
with maximum
e
Gain
e
X
X
Flash
r
r
R
R
that
w
w
P
P
Rev. 1.0.0
i
i
6
6
t
t
Mode”.
h
h
8
8
mode
were
I
I
4
4
2
2
or
0
0
C
C