CAT3661 Datasheet, PDF (11/12 Pages) ON Semiconductor – 1-Channel Low-Power LED Driver

CAT3661
Protection Mode
Open LED Protection
An LED is deemed open circuit if the LED current sink is
unable to regulate the LED channel to the programmed
current for greater than 2 ms. The driver will sense this
condition and the nFLTL pin will be driven low. The device
will be placed into a standby−mode until the Open LED
condition is removed or the device is re−enabled (EN goes
low then high again) at which point the Open LED condition
will be evaluated.
Short LED Protection
An LED is deemed to be short circuit if the difference
between VOUT pin and LED pin is less than 1.0 V when the
programmed current is driven in the channel for greater than
2 ms. If this is the case, then the LED sink is turned off and
a 5 mA test current is placed in the channel. The nFLTL pin
is driven low. Once the short condition is removed normal
operation will resume and nFLTL will be floated high.
When the device is shutdown (EN low), the nFLTL pin
will float high to ‘zero’ current state.
Input Current Limiting
The charge pump contains an input current limit circuit
that limits the current through the input pin. The current is
limited to 1000 times (G_{I_MAX}) the current flowing in
RSET. Use the following formula:
0.6 V
I_MAX + _RSET 1000
The input current limit insures the battery is never loaded
with more than 3.3 times the LED current during a short
circuit condition, Charge Pump startup condition or charge
pump mode change. The device will only ever use a
maximum of 2 times the programmed LED current plus
quiescent operating current when in normal 2x mode of
operation.
Lithium coin cell batteries have high internal resistances
so a robust current limit is a very important feature of the
device to prevent large voltage droops from triggering
device resets during operation of the CAT3661.
Over Voltage, Over Temperature Protection
As soon as VOUT is pumped above 4.5 V, the driver will
stop advancing modes if the LED sink is not in regulation.
This indicates a possible Open LED condition and stops the
device from seeing excessive voltages on the output pin
greater than the absolute maximum ratings for VOUT. An
additional fail safe over−voltage detector prevents the
VOUT output from ever exceeding 6.5 volts.
If the die temperature exceeds +150°C, the driver will
enter a thermal protection shutdown mode and the LED will
be turned off. The nFLTL pin will be driven low. Once the
device temperature drops by about 20°C, the device will
resume normal operation and nFLTL will be floated high.
External Components
The driver requires four external 1 mF ceramic capacitors
for decoupling input, output, and for the charge pump “fly”
capacitors. Capacitors type X5R and X7R are recommended
for the LED driver application. In all charge pump modes,
the input current ripple is kept very low by design and an
input bypass capacitor of 1 mF is sufficient. In 1x mode, the
device operates in linear mode and does not introduce
switching noise back onto the supply.
LED Selection
LEDs with forward voltages (V_F) ranging from 1.3 V to
4.2 V may be used. Selecting LEDs with lower V_F is
recommended in order to improve the efficiency by keeping
the driver in 1x mode longer as the battery voltage decreases.
For example, if a white LED with a V_F of 3.3 V is selected
over one with V_F of 3.5 V, the driver will stay in 1x mode to
a lower supply voltage of 0.2 V. This helps improve the
efficiency and extends battery life.
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