XRP6840 Datasheet, PDF (14/17 Pages) Exar Corporation – 4.3A Supercapacitor Flash LED Driver with I2C

XRP6840
4.3A Supercapacitor Flash LED Driver with I²C
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_IN levels. Due to the XRP6840’s low drop out
voltage design, good current regulation can
still be achieved at extremely low V_LED voltage
levels.
When V_LEDX drops below V_DO, and V_OUT is also
below V_{OUT_LIMIT}, the XRP6840 charge pump
will turn on to charge the V_OUT and V_LEDX. If
V_LEDX is still below V_DO after 64 clock cycles or
27µs (at 2.4MHz), the XRP6840 will jump to
the next higher gain. If at any time, V_{OUT_LIMIT}
is reached, then charge pump will turn off.
FAULT PROTECTION
Although most of these modes of operation
have already been previously described, they
are repeated here to emphasize the
robustness of the XRP6840.
The output voltage is directly monitored and
controlled through the V_OUT pin. Should an
open occur on the V_OUT pin, the output is
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 because when the output is
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. Once the temperature
drops below 140°C the part is re-enabled.
OPEN AND SHORT LED PROTECTION
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_OUT to V_LEDX is less than 200mV. Only the
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.
COMPONENT SELECTION
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 input voltage ripple and in-rush
current. Refer to Figures 9 and 10 for
comparison.
The input and output capacitors should be
located as close to the V_IN and V_OUT pins as
possible to obtain best bypassing. Their
returns paths should be connected directly to
the P_GND pin or to the thermal pad ground
located under the XRP6840. The flying
capacitors should be located as close to the
C_1P, C_1N and C_2P, C_2N pins as possible.
All the capacitors should be surface mount
ceramic types for low ESR and for low lead
inductances. These capacitors can also
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_OUT is the target flash current, T_FLASH is the
flash duration, V_{OUT_LIMIT} is the initial Super
Capacitor voltage programmed by STATUS2
register bits [B7 B6 B5], V_F is the LED forward
voltage and V_LED is the maximum LED pin
voltage before dropout (360mV at 125°C).
For example, for a 4.3A flash with 110ms
duration, LED V_F of 3.5V and V_{OUT_LIMIT} voltage
of 5.3V, the minimum capacitance is:
4.8 0.11
5.3 3.5 0.36 4.8 0.05Ω ^0.44
© 2009 Exar Corporation
14/17
Rev. 1.0.0