LT3517
8
3517fh
For more information www.linear.com/LT3517
operation
The LT3517 is a constant frequency, current mode regula-
tor with an internal power switch. Operation can be best
understood by referring to the Block Diagram in Figure
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. Whichever feedback input is lower takes precedence,
and forces the converter into either constant-current or
constant-voltage mode. The LT3517 is designed to transi
-
tion cleanly between these two modes of operation. The
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
SENSE
and the lower of V
CTRL
/10
or 100mV. In this manner, the error amplifier sets the
correct peak switch current level to regulate the current
through R
SENSE
. 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 R
SENSE
can be adjusted by changing the input voltage V
CTRL
.
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).
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 VC pin. Also, all internal
loads on the VC pin are disabled so that the charge state
of the VC pin will be saved on the external 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 PMOS is used to disconnect
the LED array current loop when PWM is low, stopping
C
FILT
from discharging.
applications inForMation
Dimming Control
There are two methods to control the current source for
dimming using the LT3517. The first method uses the
PWM pin to modulate the current source between zero
and full current to achieve a precisely programmed aver-
age current. To make this method of current control more
accurate, the switch demand current is stored on the VC
node during the quiescent phase. This feature minimizes
recovery time when the PWM signal goes high. To further
improve the recovery time, a disconnect switch is used in
the LED current path to prevent the output capacitor from
discharging in the PWM signal low phase. The minimum
PWM on or off time will depend on the choice of operating
frequency through RT input pin or SYNC pin. When us-
ing the SYNC function, the SYNC and PWM signals must
have the aligned rising edges to achieve the optimized
high PWM dimming ratio. For best current accuracy, the
minimum PWM low or high time should be at least four
switching cycles (2µs for f
SW
= 2MHz). Maximum PWM
period is determined by the system and is unlikely to be
longer than 12ms. The maximum PWM dimming ratio
(PWM
RATIO
) can be calculated from the maximum PWM
period (t
MAX
) and the minimum PWM pulse width (t
MIN
)
as follows:
PWM
RATIO
=
MAX
t
(1)
Example:
t
MAX
= 10ms, t
MIN
= 2µs (f
SW
= 2MHz)
PWM
RATIO
= 10ms/2µs = 5000:1
The second method of dimming control uses the CTRL
pin to linearly adjust the current sense threshold during
