CAT3636_07 Datasheet, PDF (13/16 Pages) Catalyst Semiconductor – 6-Channel Quad-ModeTM Fractional LED Driver in TQFN3x3

Unused LED Channels
For applications with only four or two LEDs, unused
LED banks can be disabled via the enable register
internally and left to float.
For applications with 5 LEDs or less, unused LEDs
can also be disabled by connecting the LED pin
directly to VOUT, as shown on Figure 4. If LED pin
voltage is within 1V of VOUT, then the channel is
switched off and a 200μA test current is placed in the
channel to sense when the channel moves below
VOUT – 1V.
Figure 4. Five LED Application
CAT3636
External Components
The driver requires two external 1μF ceramic
capacitors for decoupling input, output, and for the
charge pump. Both 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
1μF is sufficient.
In 1x mode, the device operates in linear mode and
does not introduce switching noise back onto the
supply.
Recommended Layout
In charge pump mode, the driver switches internally at
a high frequency. It is recommended to minimize trace
length to all four capacitors. A ground plane should
cover the area under the driver IC as well as the
bypass capacitors. Short connection to ground on
capacitors CIN and COUT can be implemented with
the use of multiple via. A copper area matching the
TQFN exposed pad (TAB) must be connected to the
ground plane underneath. The use of multiple via
improves the package heat dissipation.
Protection Mode
If an LED is disconnected, the output voltage VOUT
automatically limits at about 5.5V. This is to prevent
the output pin from exceeding its absolute maximum
rating.
If the die temperature exceeds +150°C the driver will
enter a thermal protection shutdown mode. When the
device temperature drops by about 20°C the device
will resume normal operation.
LED Selection
LEDs with forward voltages (V_F) ranging from 1.3V to
5.0V may be used with the CAT3636. 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 3.3V is
selected over one with V_F of 3.5V, the CAT3636 will
stay in 1x mode for lower supply voltage of 0.2V. This
helps improve the efficiency and extends battery life.
Figure 5. Recommended Layout
© 2007 Catalyst Semiconductor, Inc.
13
Characteristics subject to change without notice
Doc. No. 5020 Rev. C