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LP5524

SNVS500C – JULY 2007 – REVISED NOVEMBER 2016

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Copyright © 2007–2016, Texas Instruments Incorporated

Feature Description (continued)

7.3.3 LED Outputs

If more than 25 mA of output current is required LED outputs can be connected parallel. Connecting LED outputs

of different group parallel generates a simply two stage brightness control. With IDX set to 25 mA, enabling one

group sets the LED current to 25 mA. Enabling second bank increases the LED current to 50 mA. Unused LED

outputs can be left floating or tied to VIN.

7.3.4 PWM Brightness Control

The brightness of LEDs can be linearly varied from zero up to the maximum programmed current level by

applying a pulse–width–modulated signal to the ENx pin of the LP5524. The following procedures illustrate how

to program the LED drive current and adjust the output current level using a PWM signal.

1. Determine the maximum desired LED current. Use Equation 1 to calculate RISET.

2. Brightness control can be implemented by pulsing a signal at the ENx pin. LED brightness is proportional to

the duty cycle (D) of the PWM signal.

For linear brightness control over the full duty cycle adjustment range, the LP5524 uses a special turnoff time

delay to compensate the turn–on time of the device.

If the PWM frequency is much less than 100 Hz, flicker may be seen in the LEDs. For the LP5524, zero duty

cycle turns off the LEDs and a 50% duty cycle results in an average IDX being half of the programmed LED

current. For example, if RISET is set to program LED current to 15 mA, a 50% duty cycle results in an average IDX

of 7.5mA.

7.4 Device Functional Modes

7.4.1 Enable Mode

The LP5524 has four constant current LED outputs, which are split into two independently controlled banks.

Each bank has its own enable input. ENA is used to control bank A and ENB is used to control bank B. Both

enables are active high and have internal pulldown resistors. When both enables are low part is in low power

standby mode. Driving either enable high activates the device and corresponding LED outputs.

7.4.2 ISET Pin

An external resistor (RISET) connected to ISET pin sets the output current of all the LEDs. The internal current

mirror sets the LEDs output current with a 416:1 ratio to the current through RISET. Equation 1 approximates the

LED current:

IDX = 515 / RISET (Amps)

(1)

The use of RISET is optional. If RISET is not used the ISET pin can be left floating or connected to VIN. In these

cases LED current is set to default current.