LT3955 datasheet(14/30 Pages) LINER

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LT3955

3955fb

For more information www.linear.com/LT3955

applicaTions inFormaTion

LT3955

VOUT

100k

3955 F03

LED

ARRAY

RSEN(EXT)

COUT

–

Figure 3. Feedback Resistor Connection for

Buck Mode or Buck-Boost Mode LED Driver

feature minimizes recovery time when the PWM signal

goes high. To further improve the recovery time, a dis-

connect switch may be used in the LED current path to

prevent the ISP node from discharging during the PWM

signal low phase.

The minimum PWM on or off time is affected by choice

of operating frequency and external component selection.

The best overall combination of PWM and analog dimming

capability is available if the minimum PWM pulse is at least

six switching cycles.

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

timesifitwereallowedtointerruptthesoft-startsequence.

Therefore, once start-up is initiated by PWM > 1.3V, it will

ignore a logical disable by the external PWM input signal.

The device will continue to soft-start with switching and

PWMOUT enabled until either the voltage at SS reaches

the 1V level, or the output current reaches one-tenth of

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

following the dimming control as designated by PWM.

PWM Dimming Signal Generator

The LT3955 features a PWM dimming signal generator

withprogrammabledutycycle.Thefrequencyofthesquare

wave signal at PWMOUT is set by a capacitor CPWM from

the PWM pin to GND according to the equation:

fPWM = 14kHz • nF/CPWM

The duty cycle of the signal at PWMOUT is set by a µA

scale current into the DIM/SS pin (see Figure 4).

Figure 4. PWMOUT Duty Ratio vs DIM Voltage for RDIM = 124k

ISP/ISN Short-Circuit Protection Feature

The ISP/ISN pins have a protection feature independent

of their LED current sense feature. The purpose of this

feature is to prevent the development of excessive cur-

rents that could damage the power components or the

load. The action threshold (VISP-ISN > 600mV, typical) is

above the default LED current sense threshold, so that no

interference will occur with current regulation. Exceeding

the threshold activates pull-downs on the DIM/SS and

PWM pins and causes the power switch to be turned off,

and the PWMOUT pin to be driven low for at least 4µs.

If an overcurrent condition is sensed at ISP/ISN and the

PWM pin is configured either to make an internal dimming

signal, or for always-on operation as shown in the appli-

cation titled Boost LED Driver with Output Short-Circuit

Protection, then the LT3955 will enter a hiccup mode of

operation. In this mode, after the initial response to the

fault, the PWMOUT pin re-enables the output switch at an

interval set by the capacitor on the PWM pin. If the fault

is still present, the PWMOUT pin will go low after a short

delay (typically 7µs) and turn off the output switch. This

fault-retry sequence continues until the fault is no longer

present in the output.

PWM Dimming Control

There are two methods to control the current source for

dimming using the LT3955. One method uses the CTRL

pin to adjust the current regulated in the LEDs. A second

method uses the PWM pin to modulate the current source

between zero and full current to achieve a precisely pro-

grammed average current. To make PWM dimming more

accurate, the switch demand current is stored on the VC

node during the quiescent phase when PWM is low. This

DIM VOLTAGE (V)

100

3955 F04

CPWM = 47nF

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