Datasheet

CAT3649
http://onsemi.com
12
Unused LED Channels
For applications with five LEDs or less, it is required to tie
the unused LED pin(s) directly to VOUT (see Figure 30).
GND
VIN
2.4 V
to
5.5 V
LED1
LED2
LED3
LED4
LED5
LED6
CPWM
ADIM
CAT3649
VIN
VOUT
CIN
47 nF
CPWM
C1−C1+ C2−C2+
1 mF
PWM
COUT
Figure 30. Application with 5 LEDs
1 mF
1 mF
1 mF
Protection Modes
As soon as the output voltage (V
OUT
) exceeds about 6 V,
the driver resets itself and re−evaluates the mode.
The driver supports automatic LED detection for both
Open LED and Short LED conditions. This feature disables
any unused channels (by connecting the LED pins to
VOUT) or during an LED Short condition. The LED
detection is always active, during power−up and in normal
operation.
OPEN LED Detection
When an LED channel becomes open−circuit, the device
will go into charge pump mode and drive the output (VOUT)
above 4.5 V. If that channel is still not working at VOUT
greater than 4.5 V, the channel is locked out from signaling
a charge pump mode change and the device returns to
normal operation like a 5−channel device. If an Open LED
condition is removed, the device will resume normal
operation.
SHORT LED Detection
If the LED forward voltage (V
F
= V
OUT
– LED pin
voltage) is less than 1 V, the channel is disabled and removed
from signaling charge pump mode changes. A 5 mA (typical)
test current is placed in the (shorted) channel. In case the
LED short goes away and V
F
is higher than 1 V, the channel
resumes normal operation.
Thermal Protection
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 1.3 V to
3.8 V may be used. Selecting LEDs with lower V
F
is
recommended in order to keep the driver in 1x mode longer
as the battery voltage decreases. For example, if a white
LED with a 3.3 V V
F
is selected over one with 3.5 V V
F
, the
driver will stay in 1x mode for lower supply voltage of 0.2 V.
This extends battery life.
External Components
The driver requires four external 1 mF 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 1 mF 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 C
IN
and C
OUT
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.