CAT3224HV3-GT2 Datasheet, PDF (11/14 Pages) ON Semiconductor – 4 Amp Supercapacitor Flash LED Driver

Capacitor Selection
The supercapacitor size depends on the flash
requirement including flash duration, LED current
and LED forward voltage. The minimum supercapa-
citor value is calculated as follows.
( ) ^{C =}
I_OUT × T_FLASH
V_CAP − I_OUT R_CAP−ESR + R_LEDAB − V_F
where V_CAP is the initial CAP voltage (5.2V typical), and
V_F is the LED forward voltage. Any interconnection
parasitic resistance is assumed negligible in the
calculation.
For example, for a 4A flash with 0.1s duration and
3.1V LED V_F, the minimum capacitor value is:
C
=
5.2V
−
4A × 0.1s
4A(0.1Ω + 0.1Ω) −
3.1V
≅
0.3F
To support 4A flash pulses, we recommend using
the 0.55F supercapacitor HS206F from CAP-XX with
a voltage rating of 5.5V and a low ESR of 85mΩ.
In addition to the supercapacitor, a small 1μF
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 1μF ceramic
capacitor between the BAL pin and GND. This will
prevent any oscillation on the BAL pin and keep the
quiescent current low.
CAT3224
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 150ºC
typical, the device goes into thermal shutdown mode
and resumes normal operation as soon as the
temperature drops by about 20ºC. 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.
Therfore 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)
shout 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.
© 2009 SCILLC. All rights reserved.
11
Characteristics subject to change without notice
Doc. No. MD-5043, Rev. A.