LM27952

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SNVS364B – MAY 2005 – REVISED MAY 2013

ADJUSTING LED BRIGHTNESS (PWM control)

Perceived LED brightness can be adjusted using a PWM control signal on the LM27952 PWM logic input pin,

turning the current sources ON and OFF at a rate faster than perceptible by the eye. When this is done, the total

brightness perceived is proportional to the duty cycle (D) of the PWM signal (D = the percentage of time that the

LED is on in every PWM cycle). A simple example: if the LEDs are driven at 15mA each with a PWM signal that

has a 50% duty cycle, perceived LED brightness will be about half as bright as compared to when the LEDs are

driven continuously with 15mA.

The minimum recommended PWM frequency is 100Hz. Frequencies below this may be visibly noticeable as

flicker or blinking. The maximum recommended PWM frequency is 1kHz. Frequencies above this may cause

interference with internal current driver circuitry and/or noise in the audible range. Due to the regulation control

loop, the maximum frequency and minimum duty cycle applied to the PWM pin should be chosen such that the

minimum ON time is no less than 30µs in duration. If a PWM signal is applied to the EN pin instead, the

maximum frequency and minimum duty cycle should be chosen to accommodate both the LM27952 startup time

(330µs typ.) and the 30µs control loop delay.

The preferred method to adjust brightness is to keep the master EN voltage ON continuously and apply a PWM

signal to the dedicated PWM input pin. The benefit of this type of connection can be best understood with a

contrary example. When a PWM signal is connected to the master enable (EN) pin, the charge pump repeatedly

turns on and off. Every time the charge pump turns on, there is an inrush of current as the capacitances, both

internal and external, are recharged. This inrush current results in a current spike and a voltage dip at the input

of the part. By only applying the PWM signal to PWM logic input pin, the charge pump continuously stays on,

resulting in much lower input noise.

In cases where a PWM signal must be connected to the EN pin, measures can be taken to reduce the

magnitude of the charge-pump turn-on transient response. More input capacitance, series resistors and/or ferrite

beads may provide benefits. If the current spikes and voltage dips can be tolerated, connecting the PWM signal

to the EN pin does provide a benefit of lower supply current consumption. When the PWM signal to the EN pin is

low, the LM27952 will be shutdown and input current will only be a few micro-amps. This results in a lower time-

averaged input current than the prior suggestion, where EN is kept on continuously.

MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE

The LM27952 can drive 4 LEDs at 30mA each from an input voltage as low as 3.0V, so long as the LEDs have a

forward voltage of 3.5V or less (room temperature).

The statement above is a simple example of the LED drive capabilities of the LM27952. The statement contains

The equation below can be used to estimate the total output current capability of the LM27952:

(3)

(4)

current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the

LM27952 is typically 3.3

(eq. 2)

(5)

proper regulation. This minimum voltage is proportional to the programmed LED current, so the constant has

(eq. 3)

(6)

voltage. Output current capability can be increased by raising the minimum input voltage of the application, or by

selecting LEDs with a lower forward voltage. Excessive power dissipation may also limit output current capability

of an application.

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