LTC3783
10
3783fb
OPERATION
The SS pin provides a soft-start current to charge an
external capacitor. Enabled by RUN, the soft-start current
is 50µA, which creates a positive voltage ramp on V
SS
to which the internal I
TH
is limited, avoiding high peak
currents on start-up. Once V
SS
reaches 1.23V, the full I
TH
range is established.
The LTC3783 can be used either by sensing the voltage
drop across the power MOSFET or by connecting the
SENSE pin to a conventional shunt resistor in the source
of the power MOSFET, as shown in the Typical Application
on the first page of this data sheet. Sensing the voltage
across the power MOSFET maximizes converter efficiency
and minimizes the component count, but limits the out-
put voltage to the maximum rating for this pin (36V). By
connecting the SENSE pin to a resistor in the source of
the power MOSFET, the user is able to program output
voltages significantly greater than 36V, limited only by
other components’ breakdown voltages.
Externally Synchronized Operation
When an external clock signal drives the SYNC pin at a
rate faster than the chip’s internal oscillator, the oscillator
will synchronize to it. When the oscillator’s internal logic
circuitry detects a synchronizing signal on the SYNC pin,
the internal oscillator ramp is terminated early and the
slope compensation is increased by approximately 25%.
As a result, in applications requiring synchronization, it
is recommended that the nominal operating frequency of
the IC be programmed to be about 80% of the external
clock frequency. Attempting to synchronize to too high
an external frequency (above 1.3f
OSC
) can result in inad-
equate slope compensation and possible subharmonic
oscillation (or jitter).
The external clock signal must exceed 2V for at least 25ns,
and should have a maximum duty cycle of 80%, as shown
in Figure 1. The MOSFET turn-on will synchronize to the
rising edge of the external clock signal.
Programming the Operating Frequency
The choice of operating frequency and inductor value is
a tradeoff between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET and diode switching losses. However, lower
frequency operation requires more inductance for a given
amount of load current.
The LTC3783 uses a constant frequency architecture that
can be programmed over a 20kHz to 1MHz range with
a single external resistor from the FREQ pin to ground,
as shown in the application on the first page of this data
sheet. The nominal voltage on the FREQ pin is 0.615V,
and the current that flows out of the FREQ pin is used to
charge and discharge an internal oscillator capacitor. The
oscillator frequency is trimmed to 300kHz with R
T
= 20k.
A graph for selecting the value of R
T
for a given operating
frequency is shown in Figure 2.
3783 F01
2V TO 7V
MODE/
SYNC
GATE
I
L
t
MIN
= 25ns
0.8T
D = 40%
T T = 1/f
O
Figure 1. MODE/SYNC Clock Input and Switching Waveforms
for Synchronized Operation
FREQUENCY (kHz)
10
R
T
(kΩ)
100
1 100 1000 10000
3783 G09
1
10
1000
Figure 2. Timing Resistor (R
T
) Value
