LP5524 datasheet(7/14 Pages) TI1

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LP5524

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SNVS500 – JULY 2007

IDX = 515 / RISET (Amps)

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

LED current is set to default current.

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 the IDX equation 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 turn–off time

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

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

cycle will turn off the LEDs and a 50% duty cycle will result 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 will result in an average

IDX of 7.5mA.

LED HEADROOM VOLTAGE

A single current source is connected internally between VIN and DX outputs (D1A, D2A, D1B and D2B). The

voltage across the current source, (VIN – VDX), is referred to as headroom voltage (VHR). The current source

requires a sufficient amount of headroom voltage to be present across it in order to regulate properly.

FigureOutput Current vs Headroom Voltage shows how output current of the LP5524 varies with respect to

headroom voltage. On the flat part of the graph, the current is regulated properly as there is sufficient headroom

voltage for regulation. On the sloping part of the graph the headroom voltage is too small, the current source is

squeezed, and the current drive capability is limited. Thus, operating the LP5524 with insufficient headroom

voltage across the current source should be avoided.

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.

Recommended External Components

INPUT CAPACITOR, CIN

Although not required for normal operation, a capacitor can be added to VIN to reduce line noise. A surface-

mount multi-layer ceramic capacitor (MLCC) is recommended. MLCCs with a X7R or X5R temperature

characteristic are preferred.

CURRENT SET RESISTOR, RISET

If other than 5 mA current is required, RISET resistor can be used to adjust the current. For 15.9 mA current 32.4

Ω resistor is required. Accuracy of the resistor directly effects to the accuracy of the LED current. 1% or better is

recommended.

LED

Forward voltage of LED must be less than minimum input voltage minus minimum headroom voltage (VHR). For

example with 2.7V input voltage and 20 mA LED current the maximum LED forward voltage is 2.7V - 100 mV =

2.6V.

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