LT3518
OPERATION
The LT3518 is a constant frequency, current mode regula-
tor with an internal power switch. Operation can be best
understoodbyreferringtotheBlockDiagraminFigure 1.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.Whicheverfeedbackinputislowertakesprecedence,
and forces the converter into either constant-current or
constant-voltage mode. The LT3518 is designed to transi-
tion cleanly between these two modes of operation. The
through R
. If the error amplifier’s output increases,
SENSE
more current is delivered to the output; if it decreases,
less current is delivered. The current regulated in R
can be adjusted by changing the input voltage V
The current sense amplifier provides rail-to-rail current
sense operation. The FB voltage loop is implemented by
the amplifier A2. When the voltage loop dominates, the
error amplifier and the amplifier A2 regulate the FB pin to
1.01V (constant-voltage mode).
SENSE
.
CTRL
Dimming of the LED array is accomplished by pulsing the
LED 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 V pin. Also, all internal
C
loads on the V pin are disabled so that the charge state
C
of the V pin will be saved on the external compensation
C
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 PMOS is used to disconnect
the LED array current loop when PWM is low, stopping
current sense amplifier senses the voltage across R
SENSE
and provides a pre-gain to amplifier A1. The output of A1
is simply an amplified version of the difference between
the voltage across R
and the lower of V
/10
CTRL
SENSE
or 100mV. In this manner, the error amplifier sets the
correct peak switch current level to regulate the current
C
from discharging.
FILT
3518fb
9
Linear [ Linear ]
