LT3519 LINER [Linear Technology], LT3519 Datasheet - Page 8

• Manufacturer Part Number: LT3519
• Description: LED Driver with Integrated Schottky Diode
• Manufacturer: LINER [Linear Technology]

OPERATION
LT3519
The LT3519 are constant frequency, current mode regula-
tors 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 amplifi er 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 V
either a constant-current or a constant-voltage mode.
The LT3519 are designed to transition cleanly between
these two modes of operation. The current sense ampli-
fi er senses the voltage across R
×4 pre-gain to amplifi er A1. The output of A1 is simply
an amplifi ed version of the difference between the volt-
age across R
manner, the error amplifi er sets the correct peak switch
current level to regulate the current through R
the error amplifi er’s output increases, more current is
delivered to the output; if it decreases, less current is
delivered. The current regulated in R
by changing the input voltage V
is implemented by the amplifi er A2. When the voltage loop
dominates, The V
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
Dimming Control
There are two methods to control the current source for
dimming using the LT3519. The fi rst method, PWM Dim-
ming, uses the PWM pin to modulate the current source
between zero and full current to achieve a precisely pro-
grammed average current. To make this method of cur-
rent control more accurate, the switch demand current is
stored on the internal V
APPLICATIONS INFORMATION
8
C
SENSE
node voltage, and forces the converter into
C
and the lower of V
node voltage is set by the amplifi ed
C
node during the quiescent phase
CTRL
SENSE
SENSE
. The FB voltage loop
CTRL
and provides an
can be adjusted
or 1.1V. In this
SENSE
. If
the reference voltage, the switch demand current will de-
crease. 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 V
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 amplifi er is turned off
so that it does not drive the V
on the V
the V
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
C
when PWM is low. This feature minimizes recovery time
when the PWM signal goes high. To obtain best PWM
dimming performance, it is necessary to use an external
disconnect switch in the LED current path to prevent
the output capacitor from discharging during the PWM
signal low phase. For best product of analog and PWM
dimming, the minimum PWM low or high time should be
at least six switching cycles. Maximum PWM period is
OUT
C
from discharging.
node will be saved on the internal compensation
C
node are disabled so that the charge state of
REF
.
C
node. Also, all internal loads
3519f