LT3956
3956f
operaTion
The LT3956 is a constant-frequency, current mode
converter with a low side N-channel MOSFET switch.
The switch and PWMOUT pin drivers, and other chip
loads, are powered from INTV
CC
, which is an internally
regulated supply. In the discussion that follows, it will be
helpful to refer to the Block Diagram of the IC. In normal
operation, with the PWM pin low, the power switch is
turned off and the PWMOUT pin is driven to GND, the
V
C
pin is high impedance to store the previous switching
state on the external compensation capacitor, and the ISP
and ISN pin bias currents are reduced to leakage levels.
When the PWM pin transitions high, the PWMOUT pin
transitions high after a short delay. At the same time, the
internal oscillator wakes up and generates a pulse to set
the PWM latch, turning on the internal power MOSFET
switch. A voltage input proportional to the switch current,
sensed by an internal current sense resistor, is added to
a stabilizing slope compensation ramp and the resulting
switch-current sense signal is fed into the positive termi-
nal of the PWM comparator. The current in the external
inductor increases steadily during the time the switch is
on. When the switch-current sense voltage exceeds the
output of the error amplifier, labeled V
C
, the latch is reset
and the switch is turned off. During the switch off phase,
the inductor current decreases. At the completion of each
oscillator cycle, internal signals such as slope compensa-
tion return to their starting points and a new cycle begins
with the set pulse from the oscillator.
Through this repetitive action, the PWM control algorithm
establishes a switch duty cycle to regulate a current or
voltage in the load. The V
C
signal is integrated over many
switching cycles and is an amplified version of the differ-
ence between the LED current sense voltage, measured
between ISP and ISN, and the target difference voltage
set by the CTRL pin. In this manner, the error amplifier
sets the correct peak switch-current level to keep the
LED current in regulation. If the error amplifier output
increases, more current is demanded in the switch; if it
decreases, less current is demanded. The switch current
is monitored during the on-phase and is not allowed to
exceed the current limit threshold of 3.9A (typical). If the
SW pin exceeds the current limit threshold, the SR latch is
reset regardless of the output state of the PWM compara-
tor. Likewise, at an ISP/ISN common mode voltage less
than 3V, the difference between ISP and ISN is monitored
to determine if the output is in a short-circuit condition. If
the difference between ISP and ISN is greater than 335mV
(typical), the SR latch will be reset regardless of the PWM
comparator. These functions are intended to protect the
power switch, as well as various external components in
the power path of the DC/DC converter.
In voltage feedback mode, the operation is similar to that
described above, except the voltage at the V
C
pin is set by
the amplified difference of the internal reference of 1.25V
(nominal) and the FB pin. If FB is lower than the reference
voltage, the switch current will increase; if FB 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 V
REF
.
Two LED specific functions featured on the LT3956 are
controlled by the voltage feedback pin. First, when the
FB pin exceeds a voltage 50mV lower (–4%) than the FB
regulation voltage, the pull-down driver on the
VMODE
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.
When the FB pin exceeds the FB regulation voltage by 60mV
(5% typical), the PWMOUT pin is driven low, ignoring the
state of the PWM input. In the case where the PWMOUT
pin drives a disconnect NFET, this action isolates the
LED load from GND, preventing excessive current from
damaging the LEDs. If the FB input exceeds the overvolt-
age threshold (1.31V typical), then an externally driven
overvoltage event may have caused the FB pin to be too
high and the
VMODE
pull-down will be deactivated until
the FB pin drops below the overvoltage threshold.