CAT3643
Unused LED Channels
External Components
For applications not requiring all the channels, it is
recommended the unused LED pins be tied directly to
VOUT (see Figure 4).
The driver requires four external 1μF ceramic capaci–
tors 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.
1µF
1µF
C1+
IN
C1- C2+
C2-
V
IN
V
V
OUT
C
IN
In 1x mode, the device operates in linear mode and
does not introduce switching noise back onto the
supply.
C
OUT
1µF
CAT3643
1µF
ENABLE
DIMMING
EN/DIM
RSET
LED1
LED2
LED3
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.
GND
R
SET
Figure 4. Application with 2 LEDs
Protection Mode
If an LED is disconnected, the driver senses that and
automatically ignores that channel. When all LEDs are
disconnected, the driver goes to 1x mode where the
output is equal to the input voltage.
C2
Pin1
As soon as the output exceeds about 6V, the driver
resets itself and reevaluate the mode.
RSET
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.
EN/DIM
C1
LED Selection
C
OUT
C
IN
GND
VIN
LEDs with forward voltages (VF) ranging from 1.3V to
4.3V may be used. Selecting LEDs with lower VF is
recommended in order to improve the efficiency by
keeping the driver in 1x mode longer as the battery
voltage decreases.
GND
VOUT
Figure 5. TDFN-12 Recommended Layout
For example, if a white LED with a VF of 3.3V is
selected over one with VF of 3.5V, the driver will stay
in 1x mode for lower supply voltage of 0.2V. This
helps improve the efficiency and extends battery life.
Doc. No. MD-5022 Rev. E
12
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
CATALYST [ CATALYST SEMICONDUCTOR ]
