CAT3604V
http://onsemi.com
11
device runs diagnostics and goes to 1x mode where the
output is basically equal to the input voltage.
As soon as the output exceeds about 6 V, the driver resets
itself and reevaluate the mode.
If the die temperature reaches +150°C, the device enters
a thermal protection shutdown mode. When the device
temperature drops to about +130°C, the device resumes
normal operation.
LED Selection
LEDs with forward voltages (V
F
) ranging from 1.3 V to
4.3 V may be used. Selecting LEDs with lower V
F
is
recommended in order to extend battery life and keep the
driver in 1x mode longer as the battery voltage decreases.
For example, if a 3.3 V V
F
LED is selected instead of a
3.5 V V
F
LED, the driver will stay in 1x mode for a lower
supply voltage of 0.2 V.
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.
Figure 27. PCB Layout
PWM Dimming
The EN pin is used to provide total Shutdown of the device
as well as High Resolution PWM dimming control on the
LED Channels.
Shutdown of the device occurs after the EN pin has been
held low for 1.5 ms. During the “soft−start” power−up
sequence from the shutdown mode, the LED current
typically settles within 40 ms (for 1X mode operation). This
LED current settling time becomes 400 ms if 1.33X
operation is needed (i.e low battery voltage).
For High Resolution PWM dimming control (typically
frequencies at 2 kHz or above), the device will remain
powered and only the LED channels output will be switched
on and off during the PWM (the rest of the device will
remain powered−up). This allows the output channels to
have “instant−on” response, where the LED current settles
within 1 ms of the applied PWM dimming signals. This
“instant−on” modes makes the device suitable for extremely
high frequency PWM dimming schemes.
Figure 28 shows the output current for PWM frequencies
up to 100 kHz and with duty cycles of 30% and 70%.
Figure 28. Output Current vs. PWM Frequency
10
20
30
40
50
60
70
80
90
100
0.1 1 10 100
PWM FREQUENCY (kHz)
OUTPUT CURRENT (%)
70% DC
30% DC
For best performance, the duty cycle off−time T
OFF
should meet the following timing limits:
• for slow frequency ≤ 600 Hz, T
OFF
≥ 800 ms
• for fast frequency ≥ 1 kHz, T
OFF
≤ 400 ms
