CAT3224HV3-GT2 Datasheet, PDF (10/14 Pages) ON Semiconductor – 4 Amp Supercapacitor Flash LED Driver

CAT3224
Torch Mode
The torch mode allows the LEDs to run for extended
time duration but at a lower current than in the flash
mode. When the TORCH input is set high, the driver
is in torch mode and the LED channel current is set
according to the external resistor connected
between the RT pin and ground. The torsh mode
LED current per channel follows the equation:
I_TORCH
≈ 120 × I_RT
= 120 × ^VRT
R_T
= 120 × ^0.6V
R_T
How long the LED current is regulated depends on
the initial CAP voltage, capacitor value, the charge
current, LED forward voltage and the LED torch
current setting. In order to maintain regulation in 2x
mode, the torch output current should be less than
half the charging current. If the requested torch
current is greater than half the input current, the
LEDs will dim progressively according to the input
current.
Flash Mode
When the FLASH input is set high, the driver is in
flash mode and the LED channel current is set
according to the external resistor connected
between the RF pin and ground. The flash mode
LED channel current can be calculated by the
following equation (approximation).
I_FLASH
≈ 900 × I_RF
= 900 ×
V_RF
R_F
= 900 ×
0.6V
R_F
Table 1 shows some standard resistor values for R_F
and the corresponding LED channel current.
Table 1. RSET Resistor Settings
LED current
per Channel [A]
1
1.5
2
R_F[Ω]
549
360
261
The maximum flash duration where the LED current
is regulated depends on the initial CAP voltage,
capacitor value, LED forward voltage and the LED
flash current setting. The flash pulse duration can be
calculated as follows.
T_FLASH
=
C×
ΔV_CAP
I_FLASH
where C is the total supercapacitor value, ΔV_CAP is
the drop in the CAP voltage during the flash. See the
Capacitor Selection section for more details.
The RF pin has a current limit of 3.5mA typical. If the
RF pin is shorted to ground, the maximum flash LED
current is 1000 x 3.5mA or 3.5A.
During flash mode, the LEDs stay in regulation as
long as their forward voltage does not exceed a
maximum voltage calculated as follows:
( ) V_Fmax = V_CAP − I_OUT × R_CAP−ESR + R_LEDAB
where I_OUT is the CAP total output current, R_CAP-ESR
is the supercapacitor ESR (equivalent series
resistance), and R_LEDAB is the LEDA/B combined
dropout resistance of the CAT3224.
The transient waveform in Figure 3 shows the CAP
output voltage during a 4A flash pulse (2A per LED
channel) with CHRG low (not in charge mode). The
initial drop on the CAP voltage (Vesr) is due to the
supercapacitor ESR. In this example, it is calculated
as follows.
Vesr = 2 x I_LED x R_CAP-ESR = 2x 2A x 0.1Ω = 0.4V
Figure 3. CAP output transient during 4A flash
Flash Rate
Between two consecutive flash pulses, the superca-
pacitor needs some time to recharge. The superca-
pacitor 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 × ^IOUT
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 60ms 4A flash pulse with a charge
current of 300mA corresponds to a recharge time:
T_CHARGE
=
2×
4A
0.3A
×
0.06s
=
1.6s
Doc. No. MD-5043, Rev. A.
10
© 2009 SCILLC. All rights reserved.
Characteristics subject to change without notice