CAT3661 Datasheet, PDF (9/12 Pages) ON Semiconductor – 1-Channel Low-Power LED Driver

Block Diagram
CAT3661
Figure 24. CAT3661 Functional Block Diagram
Basic Operation
At power−up, the CAT3661 starts operating in 1x mode
where the output will be approximately equal to the input
supply voltage (less any internal voltage losses). If the
output voltage is sufficient to regulate the LED current, the
device remains in 1x operating mode.
If the input voltage is insufficient or falls to a level where
the LED regulated current cannot be maintained, the device
automatically switches into 1.33x mode (after a fixed delay
time of about 400 ms). In 1.33x mode, the output voltage is
approximately equal to 1.33 times the input supply voltage
(less any internal voltage losses).
This sequence repeats in the 1.33x and 1.5x mode until the
driver enters the 2x mode. In 1.5x mode, the output voltage
is approximately equal to 1.5 times the input supply voltage.
While in 2x mode, the output is approximately equal to 2
times the input supply voltage.
If the device detects a sufficient input voltage is present to
drive the LED current in 1x mode, it will change
automatically back to 1x mode. This only applies for
changing back to the 1x mode. The difference between the
input voltage when exiting 1x mode and returning to 1x
mode is called the 1x mode transition hysteresis (V_HYS) and
is about 300 mV.
LED Current Setting
The current flowing out of the RSET pin to ground mirrors
the current in the LED channel with a gain of 300. The LED
current can be adjusted from 0.1 mA to 5 mA. Connecting
a resistor between RSET and GND allows a reference
current to flow due to the voltage on the RSET pin being
regulated to 0.6 V. The internal gain of the current mirror is
300. It is possible to calculate the current in the LED channel
by the following equation:
0.6 V
I_LED + _RSET 300
Adjustable Battery Indicator
The CAT3661 contains an adjustable low battery
indicator that is active when the device is enabled. If the
voltage on the internal resistor divider trip point node is less
than V_ADJ (0.6 V), the nFLTB output is driven low and
remains low for 500 ms after the EN pin is driven high. The
CAT3661 will still function normally below this voltage
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