LTC3783 Datasheet, PDF (9/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

U
OPERATIO
Main Control Loop
The LTC3783 is a constant frequency, current mode
controller for PWM LED as well as DC/DC boost, SEPIC
and flyback converter applications. In constant current
LED applications, the LTC3783 provides an especially
wide PWM dimming range due to its unique switching
scheme, which allows PWM pulse widths as short as
several converter switching periods.
For voltage feedback circuit operation (defined by V_FBP ≤
1.23V), please refer to the Block Diagram of the IC and the
Typical Application on the first page of this data sheet. In
normal operation with PWMIN high, the power MOSFET is
turned on (GATE goes high) when the oscillator sets the
PWM latch, and is turned off when the ITRIP current
comparator resets the latch. Based on the error voltage
represented by (V_FBP – V_FBN), the error amplifier output
signal at the I_TH pin sets the ITRIP current comparator
input threshold. When the load current increases, a fall in
the FBN voltage relative to the reference voltage at FBP
causes the I_TH pin to rise, causing the ITRIP current com-
parator to trip at a higher peak inductor current value. The
average inductor current will therefore rise until it equals
the load current, thereby maintaining output regulation.
When PWMIN goes low, PWMOUT goes low, the I_TH
switch opens and GATE switching is disabled. Lowering
PWMOUT and disabling GATE causes the output capacitor
C_OUT to hold the output voltage constant in the absence of
load current. Opening the I_TH switch stores the correct
load current value on the I_TH capacitor C_ITH. As a result,
when PWMIN goes high again, both I_TH and V_OUT are
instantly at the appropriate levels.
In voltage feedback operation, an overvoltage compara-
tor, OV, senses when the OV/FB pin exceeds the reference
voltage by 7% and provides a reset pulse to the main RS
latch. Because this RS latch is reset-dominant, the power
MOSFET is actively held off for the duration of an output
overvoltage condition.
LTC3783
For constant current/constant voltage regulation opera-
tion (defined by V_FBP > 2.5V), please refer to the Block
Diagram of the IC and Figure 11. Loop operation is similar
to the voltage feedback, except FBP and FBN now sense
the voltage across sense resistor R_L in series with the load.
The I_TH pin now represents the error from the desired
differential set voltage, from 10mV to 100mV, for I_LIM
values of 0.123V to 1.23V. That is, with V_ILIM = 1.23V, the
loop will regulate such that V_FBP – V_FBN = 100mV; lower
values of I_LIM attenuate the difference proportionally.
PWMIN is still functional as above, but will only work
properly if load current can be disconnected by the
PWMOUT signal.
In constant current/constant voltage operation, the OV/FB
pin becomes a voltage feedback pin, which causes the
loop to regulate such that V_OV/FB = 1.23V, provided the
above current-sense voltage is not reached. In this way,
the loop regulates either voltage or current, whichever
parameter hits its preset limit first.
The nominal operating frequency of the LTC3783 is pro-
grammed using a resistor from the FREQ pin to ground
and can be controlled over a 20kHz to 1MHz range. In
addition, the internal oscillator can be synchronized to an
external clock applied to the SYNC pin and can be locked
to a frequency between 100% and 130% of its nominal
value. When the SYNC pin is left open, it is pulled low by
an internal 100k resistor. With no load, or an extremely
light one, the controller will skip pulses in order to main-
tain regulation and prevent excessive output ripple.
The RUN pin controls whether the IC is enabled or is in a
low current shutdown state. A micropower 1.248V refer-
ence and RUN comparator allow the user to program the
supply voltage at which the IC turns on and off (the RUN
comparator has 100mV of hysteresis for noise immunity).
With the RUN pin below 1.248V, the chip is off and the
input supply current is typically only 20µA.
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