CAT3224 Datasheet, PDF (10/13 Pages) ON Semiconductor – 4 Amp Supercapacitor Flash LED Driver

CAT3224
Basic Operation
The CAT3224 integrates in a single device two main
functions: a dual cell supercapacitor charger and an LED
driver. Two LED channels provide accurately regulated and
matched current up to 2 A per channel. The charging mode
is activated when the CHRG control input is pulled high and
can remain active even during torch or flash mode. This
allows continuous torch mode operation. The two modes,
torch and flash, are activated using separate control inputs
repectively TORCH and FLASH.
Charge Mode
When the CHRG input is set high, the driver is in charge
mode and the input supply current cannot exceed the current
limit set by an external resistor connected between the RC
pin and ground. The charging current limit is calculated by
the following equation (approximation).
I_IN [ 400
I_RC + 400
V_RC
R_C
+
400
0.6 V
R_C
If the CAP output voltage is lower than the charge
threshold, the charging cycle starts. The driver charge pump
initially starts in 1x mode and remains there as long as the
supply voltage VIN is high enough to drive the CAP output
voltage directly. In 1x mode, the output current charging the
supercapacitor is approximately equal to the input current.
The driver enters the 2x charge pump mode when the CAP
pin voltage approaches VIN (V_CAP ≈ VIN – 0.3 V). In 2x
mode, the output current is approximately half of the input
current. The charge cycle stops when either the CHRG input
is pulled low or when the CAP output reaches the “CAP
charge off voltage” threshold. As soon as the CAP output
reaches the “CAP voltage FLAG pulled low” threshold, the
FLAG output is pulled low. There is an hysteresis on the
FLAG output which is illustrated in the timing diagram on
Figure 2.
The charge time is a function of the input voltage, input
current setting, supercapacitor value, final CAP voltage.
The RC pin has a current limit of 3.5 mA typical. If the RC
pin is shorted to ground, the maximum charge current is 400
x 3.5 mA or 1.4 A.
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 torch
mode LED current per channel follows the equation:
I_TORCH [ 120
I_RT + 120
V_RT
R_T
+
120
0.6 V
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
0.6 V
R_F
Table 7 shows some standard resistor values for R_F and the
corresponding LED channel current.
Table 7. RSET Resistor Settings
LED Current per Channel [A]
1
1.5
2
R_F [W]
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
DV_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 3.5 mA typical. If the RF
pin is shorted to ground, the maximum flash LED current is
1000 x 3.5 mA or 3.5 A.
During flash mode, the LEDs stay in regulation as long as
their forward voltage does not exceed a maximum voltage
calculated as follows:
ǒ Ǔ _{VFmax + VCAP * IOUT RCAP−ESR ) RLEDAB}
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 19 shows the CAP
output voltage during a 4 A flash pulse (2 A 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 I_LED R_CAP−ESR + 2x 2A 0.1 W + 0.4 V
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