XRP6840 Exar Corporation, XRP6840 Datasheet - Page 14

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

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