CAT3224

http://onsemi.com

10

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).

+ 400

0.6 V

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

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:

+ 120

0.6 V

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).

+ 900

0.6 V

corresponding LED channel current.

Table 7. RSET Resistor Settings

LED Current per Channel [A]

1

549

1.5

360

2

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.

where C is the total supercapacitor value,

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:

supercapacitor ESR (equivalent series resistance), and

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