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CAT3637
Unused LED Channels
For applications with 5 LEDs or less, unused LEDs can be
disabled by connecting the LED pin directly to VOUT, as
shown on Figure 26. If LED pin voltage is within 1 V of
VOUT, then the channel is switched off and a 200 m A test
current is placed in the channel to sense when the channel
moves below VOUT – 1 V.
1 m F
1 m F
C IN
1 m F
LED2
GND
V IN
2.5 V
to
5.5 V
1 ? Wire
Programming
C1 ? C1+ C2 ? C2+
VIN VOUT
LED1
CAT3637
LED3
LED4
EN/SET LED5
LED6
V OUT
C OUT
1 m F
Figure 26. Five LED Application
Protection Mode
If an LED is disconnected, the output voltage VOUT
automatically limits at about 5.5 V. 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 1.3 V to
5.0 V may be used with the CAT3637. 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 CAT3637 will stay in 1x mode
for lower supply voltage of 0.2 V. This helps improve the
efficiency and extends battery life.
External Components
The driver requires two external 1 m F 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 m F is sufficient.
In 1x mode, the device operates in linear mode and does
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. Recommended Layout
not introduce switching noise back onto the supply.
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