LT3797
12
3797fa
For more information www.linear.com/LT3797
operaTion
The LT3797 uses a fixed frequency, current mode control
scheme to provide excellent line and load regulation. It
contains three independent switching regulators. Operation
can be best understood by referring to the Block Diagram
in Figure 1. The oscillator, internal power supply etc., are
shared among the three converters. The LED current control
circuitry, gate drivers etc., are replicated for each of the
three converters. For simplicity, Figure 1 shows the shared
circuits and the channel specific circuits for converter 1.
The LED current regulation can be understood by following
the operation of converter 1. The start of each oscillator
cycle sets the SR latch SR1 and turns on the external
power MOSFET switch M1 through gate driver G2 (the
three converters share the same oscillator, which means
if all the three channels are enabled the GATE pins of all
the three channels transition high at the same instant). The
switch current flows through the external current sensing
resistor R
SW_SEN1
and generates a voltage proportional to
the switch current. This current sense voltage (amplified
by A14) is added to a stabilizing slope compensation ramp
and the resulting sum V
ISENSE1
is fed into the negative
terminal of the PWM comparator A12. The current in the
external inductor L1 increases steadily during the time
the switch is on. When V
ISENSE1
exceeds the level at the
negative input of A12 (VC1), SR1 is reset, turning off the
power switch. During the switch-off phase, L1 current
decreases.
Through this repetitive action, the PWM control algorithm
establishes a switch duty cycle to regulate a current in the
LED string. The VC1 voltage is set by the error amplifier
A8 and is an amplified version of the difference between
the LED current sense voltage, measured between ISP1
and ISN1, and the target difference voltage set by the
CTRL1 pin. In this manner, the error amplifier sets the
correct switch peak current level to keep the LED current
in regulation.
The LT3797 has a switch current limit function. The switch
current sense signal is input to the current limit comparator
A13. If the current sense voltage is higher than the sense
current limit threshold, V
SENSE(MAX)
(typical 110mV), A13
will reset SR1 and turn off M1 immediately.
The LT3797 provides the constant voltage regulation mode
to allow the users to accurately program the output regula-
tion voltage in an open-LED event. In voltage regulation
mode, the operation
is similar to that described above,
except the VC1 voltage is set by A9 and is an amplified
version of the difference between the internal reference of
1.25V (typical) and the output feedback voltage, V
FB1
, which
is measured between ISP1 and FBH1 (the absolute value):
V
FB1
= |ISP1-FBH1|
The LED current sense feedback interacts with the FBH1
voltage feedback so that the sense voltage between ISP1
and ISN1 does not exceed the threshold set by the CTRL1
pin, and V
FB1
does not exceed 1.25V (typical).
For accurate current or voltage regulation, it is necessary
to be sure that under normal operating conditions, the ap
-
propriate loop is dominant. To deactivate the voltage loop
entirely, FBH1 can be connected to ISP1. To deactivate the
LED current loop entirely, the ISP1 and ISN1 should be
tied together and the CTRL1 input tied to V
REF
.
It requires ISP to be no less than 4.5V to maintain an
accurate V
FB1
voltage sense. If ISP falls below 4.5V, the
voltage regulation is deactivated and the current regulation
dominates regardless of the |ISP1-FBH1| value.
Two LED driver specific functions featured on the LT3797
are controlled by the voltage feedback pin FBH1. First,
when the V
FB1
exceeds a voltage 50mV lower (–4%) than
the V
FB1
regulation voltage (typical 1.25V), it indicates that
the LED may be disconnected and the constant-voltage
feedback loop is taking control of the switching regulator.
F LT 1 is pulled low to report a fault condition. Second, when
V
FB1
exceeds the V
FB1
regulation voltage by 60mV (5%
typical), it indicates an output overvoltage fault. In this
condition, TG1 pin is driven high by G3 and G4, turning off
the external PMOS M2. This action disconnects the LED
load from the power path, preventing excessive current
from damaging the LEDs. F LT 1 is kept low to report the
fault condition.
