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LTM8005

8005f

For more information www.linear.com/LTM8005

APPLICATIONS INFORMATION

PWM Dimming Control for Brightness

There are two methods to control the LED current for

dimming using the LTM8005. One method uses the CTRL

pins to adjust the current regulated in the LEDs. A sec-

ond method uses the PWM pin to modulate the LED cur-

rent between zero and full current to achieve a precisely

programmed average current, without the possibility of

color shift that occurs at low current in LEDs. To make

PWM dimming more accurate, the switch demand cur-

rent is stored on the VC node during the quiescent phase

when PWM is low. This feature minimizes recovery time

when the PWM signal goes high. To further improve the

recovery time, a disconnect MOSFET switch has been

implemented to open the LED current path to prevent the

output capacitor from discharging during the PWM signal

low phase. The minimum PWM on or off time depends

on the choice of operating frequency set by the RT input.

For best current accuracy, the minimum recommended

PWM high time should be at least three switching cycles

(3µs for fSW = 1MHz).

A low duty cycle PWM signal can cause excessive start-up

times if it is allowed to interrupt the soft-start sequence.

Therefore, once start-up is initiated by a PWM signal, the

LTM8005 will ignore a logical disable by the external PWM

input signal. The device will continue to soft-start with

switching and TG enabled until either the voltage at SS

reaches about 1V or the output current reaches one-fourth

of the full-scale current. At this point the device will begin

following the dimming control as designated by PWM. If

at any time an output overcurrent is detected, the internal

MOSFETs will be disabled even as SS continues to charge.

Programming the Switching Frequency

The RT frequency adjust pin allows the user to program

the switching frequency from 100kHz to 1MHz to opti-

mize efficiency/performance or external component size.

Higher frequency operation yields smaller component

size but increases switching losses and gate driving cur-

rent, and may not allow sufficiently high or low duty cycle

operation. Lower frequency operation gives better perfor-

mance at the cost of larger external component size. For

an appropriate RT resistor value see Table 1. An external

resistor from the RT pin to GND is required—do not leave

this pin open.

Table1. Typical Switching Frequency vs RT Value (1% Resistor)

fOSC(kHz)

RT(k)

1000

6.65

900

7.50

800

8.87

700

10.2

600

12.4

500

15.4

400

19.6

300

26.1

200

39.2

100

82.5

Spread Spectrum Frequency Modulation

Switching regulators can be particularly troublesome

for applications where electromagnetic interference

(EMI) is a concern. To improve the EMI performance,

the LTM8005 includes a spread spectrum frequency fea-

ture. If there is a capacitor (CRAMP) at the RAMP pin, a

triangle wave sweeping between about 1V and 2V is gen-

erated. This signal is then fed into the internal oscillator

to modulate the switching frequency between about 70%

of the base frequency and the base frequency, which is

set by the RT resistor. The modulation frequency is set

by 12µA/(2 • CRAMP). The results of EMI measurements

are sensitive to the RAMP frequency selected with the

capacitor. 1kHz is a good starting point to optimize peak

measurements, but some fine tuning of this selection

may be necessary to get the best overall EMI results in a

particular system. Consult factory applications for more

detailed information about EMI reduction. The Typical

Performance Characteristics section contains plots that

show the LTM8005 conducted and radiated emissions

with and without Spread Spectrum enabled.

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