LTC3783
9
3783fb
OPERATION
Main Control Loop
The LTC3783 is a constant frequency, current mode con-
troller 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 comparator
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.
For constant current/constant voltage regulation operation
(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 dif-
ferential 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 maintain
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.
