LTC3419
8
3419fa
OPERATION
The LTC3419 uses a constant-frequency, current mode
architecture. The operating frequency is set at 2.25MHz.
Both channels share the same clock and run in-phase.
The output voltage is set by an external resistor divider
returned to the V
FB
pins. An error amplifi er compares the
divided output voltage with a reference voltage of 0.6V and
regulates the peak inductor current accordingly.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V
FB
voltage is below the reference voltage. The
current into the inductor and the load increases until the
peak inductor current (controlled by I
TH
) is reached. The
RS latch turns off the synchronous switch and energy
stored in the inductor is discharged through the bottom
switch (N-channel MOSFET) into the load until the next
clock cycle begins, or until the inductor current begins to
reverse (sensed by the I
RCMP
comparator).
The peak inductor current is controlled by the internally
compensated I
TH
voltage, which is the output of the error
amplifi er. This amplifi er regulates the V
FB
pin to the internal
0.6V reference by adjusting the peak inductor current
accordingly.
Light Load Operation
There are two modes to control the LTC3419 at light load
currents: Burst Mode operation and pulse-skipping mode.
Both automatically transition from continuous operation
to the selected mode when the load current is low.
To optimize effi ciency, Burst Mode operation can be selected
by grounding the MODE pin. When the load is relatively
light, the peak inductor current (as set by I
TH
) remains
fi xed at approximately 60mA and the PMOS switch operates
intermittently based on load demand. By running cycles
periodically, the switching losses are minimized.
The duration of each burst event can range from a few
cycles at light load to almost continuous cycling with
short sleep intervals at moderate loads. During the sleep
intervals, the load current is being supplied solely from
the output capacitor. As the output voltage droops, the
error amplifi er output rises above the sleep threshold,
signaling the burst comparator to trip and turn the top
MOSFET on. This cycle repeats at a rate that is dependent
on load demand.
For applications where low ripple voltage and constant-
frequency operation is a higher priority than light load
effi ciency, pulse-skipping mode can be used by connecting
the MODE pin to V
IN
. In this mode, the peak inductor
current is not fi xed, which allows the LTC3419 to switch
at a constant-frequency down to very low currents, where
it will begin skipping pulses.
Dropout Operation
When the input supply voltage decreases toward the
output voltage the duty cycle increases to 100%, which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
An important design consideration is that the R
DS(ON)
of the P-channel switch increases with decreasing input
supply voltage (see Typical Performance Characteristics).
Therefore, the user should calculate the worst-case power
dissipation when the LTC3419 is used at 100% duty cycle
with low input voltage (see Thermal Considerations in the
Applications Information section).
Soft-Start
In order to minimize the inrush current on the input bypass
capacitor, the LTC3419 slowly ramps up the output voltage
during start-up. Whenever the RUN1 or RUN2 pin is pulled
high, the corresponding output will ramp from zero to
full-scale over a time period of approximately 750μs. This
prevents the LTC3419 from having to quickly charge the
output capacitor and thus supplying an excessive amount
of instantaneous current.
Short-Circuit Protection
When either regulator output is shorted to ground, the
corresponding internal N-channel switch is forced on for
a longer time period for each cycle in order to allow the
inductor to discharge, thus preventing inductor current
runaway. This technique has the effect of decreasing
switching frequency. Once the short is removed, normal
operation resumes and the regulator output will return to
its nominal voltage.
