CAT3224 Datasheet, PDF (11/13 Pages) ON Semiconductor – 4 Amp Supercapacitor Flash LED Driver

CAT3224
Figure 19. CAP Output Transient during 4 A Flash
Flash Rate
Between two consecutive flash pulses, the supercapacitor
needs some time to recharge. The supercapacitor time
needed to fully recharge after a flash pulse is a function of
the flash current and duration, and the charging current.
Assuming the driver is in 2x mode, the charging time is
calculated as follows.
T_CHARGE + 2
I_OUT
I_IN
T_FLASH
where I_OUT is the total LED current, T_FLASH is the flash
duration and I_IN is the input current.
For example, a 60 ms 4 A flash pulse with a charge current
of 300 mA corresponds to a recharge time:
4A
T_CHARGE + 2 _{0.3 A} 0.06 s + 1.6 s
Capacitor Selection
The supercapacitor size depends on the flash requirement
including flash duration, LED current and LED forward
voltage. The minimum supercapacitor value is calculated as
follows.
ǒ Ǔ ^{C +}
I_OUT T_FLASH
V_CAP * I_OUT R_CAP−ESR ) R_LEDAB * V_F
where V_CAP is the initial CAP voltage (5.2 V typical), and
V_F is the LED forward voltage. Any interconnection
parasitic resistance is assumed negligible in the calculation.
For example, for a 4 A flash with 0.1 s duration and 3.1 V
LED V_F, the minimum capacitor value is:
C
+
5.2
V
*
4
4A
A^ǒ0.1
W
0.1 s
) 0.1
W^Ǔ
*
3.1
V
^
0.3
F
To support 4 A flash pulses, we recommend using the
0.55 F supercapacitor HS206F from CAP−XX with a
voltage rating of 5.5 V and a low ESR of 85 mΩ.
In addition to the supercapacitor, a small 1 mF ceramic
capacitor is recommended on the CAP output in order to
filter out the charge pump switching noise due to the ESR of
the supercapacitor.
If a single cell supercapacitor is used, it is recommended
to connect a small 1 mF ceramic capacitor between the BAL
pin and GND. This will prevent any oscillation on the BAL
pin and keep the quiescent current low.
Thermal Dissipation
Thermal dissipation occurs in the CAT3224 device due to
the high current flowing in charge mode, as well as in torch
or flash mode. During charge mode, in case the input voltage
is high and the driver operates in 2x charge pump mode, the
power dissipation may increase significantly. In torch and
flash modes, the power dissipation is proportional to the
difference between the CAP and LEDA/B pin voltages. If
the junction temperature exceeds 150°C typical, the device
goes into thermal shutdown mode and resumes normal
operation as soon as the temperature drops by about 20°C.
To improve the thermal performance, the TQFN exposed
pad should be connected to the PCB ground plane
underneath.
Recommended Layout
The ground side of the three current setting resistors, R_C,
R_T, R_F, should be star connected back to the GND of the
PCB. In charge pump mode, the driver switches internally
at a high frequency. Therefore it is recommended to
minimize trace length to all four capacitors. A ground plane
should cover the area under the driver IC as well as the
bypass capacitors. Short connection to ground on capacitors
C_IN and C_OUT can be implemented with the use of multiple
via. A copper area matching the TQFN exposed pad (TAB)
must be connected to the ground plane underneath with a
via.
In order to minimize the IR drop in flash mode, the traces
between the supercapacitor and the CAP pins, and between
LEDA/LEDB pins and the LED(s) should be kept as short
as possible and wide enough to handle the high current
peaks. The supercapacitor negative terminal and the LED
cathodes need to be connected to the ground plane directly.
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