LT3519 查看數據表（PDF） - Linear Technology

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LT3519

LED Driver with Integrated Schottky Diode

Linear Technology 

LT3519/LT3519-1/LT3519-2

OPERATION

The LT3519/LT3519-1/LT3519-2 are constant frequency,

current mode regulators with an internal power switch and

Schottky. Operation can be best understood by referring

to the Block Diagram. At the start of each oscillator cycle,

the SR latch is set, which turns on the Q1 power switch.

A voltage proportional to the switch current is added to

a stabilizing ramp and the resulting sum is fed into the

positive terminal of the PWM comparator, A4. When this

voltage exceeds the level at the negative input of A4, the

SR latch is reset, turning off the power switch. The level

at the negative input of A4 is set by the error amplifier A3.

A3 has two inputs, one from the voltage feedback loop and

the other one from the current loop. Whichever feedback

input is lower takes precedence to set the VC node voltage,

and forces the converter into either a constant-current or

a constant-voltage mode.

The LT3519/LT3519-1/LT3519-2 are designed to transi-

tion cleanly between these two modes of operation. The

current sense amplifier senses the voltage across RSENSE

and provides an ×4 pre-gain to amplifier A1. The output

of A1 is simply an amplified version of the difference be-

tween the voltage across RSENSE and the lower of VCTRL

or 1.1V. In this manner, the error amplifier sets the correct

peak switch current level to regulate the current through

RSENSE. If the error amplifier’s output increases, more

current is delivered to the output; if it decreases, less

current is delivered. The current regulated in RSENSE can

be adjusted by changing the input voltage VCTRL. The FB

voltage loop is implemented by the amplifier A2. When

the voltage loop dominates, the VC node voltage is set by

the amplified difference of the internal reference of 1.22V

and the FB pin. If FB voltage is lower than the reference

voltage, the switch current will increase; if FB voltage is

higher than the reference voltage, the switch demand

current will decrease. The LED current sense feedback

interacts with the FB voltage feedback so that FB will not

exceed the internal reference and the voltage between ISP

and ISN will not exceed the threshold set by the CTRL pin.

For accurate current or voltage regulation, it is necessary

to be sure that under normal operating conditions the

appropriate loop is dominant. To deactivate the voltage

loop entirely, FB can be connected to GND. To deactivate

the LED current loop entirely, the ISP and ISN should be

tied together and the CTRL input tied to VREF.

When the FB input exceeds a voltage about 60mV lower

than the FB regulation voltage, the pull-down driver on

the OPENLED pin is activated. This function provides a

status indicator that the load may be disconnected and

the constant-voltage feedback loop is taking control of

the switching regulator.

Dimming of the LED array is accomplished by pulsing the

current using the PWM pin. When the PWM pin is low,

switching is disabled and the error amplifier is turned off

so that it does not drive the VC node. Also, all internal loads

on the VC node are disabled so that the charge state of

the VC node will be saved on the internal compensation

capacitor. This feature reduces transient recovery time.

When the PWM input again transitions high, the demand

current for the switch returns to the value just before PWM

last transitioned low. To further reduce transient recovery

time, an external MOSFET should be used to disconnect

the LED array current loop when PWM is low, stopping

COUT from discharging.

3519fa

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