LTC3417A-1
8
3417a1fa
The LTC3417A-1 uses a constant frequency, current
mode architecture. Both channels share the same clock
frequency. The PHASE pin sets whether the channels are
running in-phase or out of phase. The operating frequency
is determined by connecting the FREQ pin to V
IN
for
1.5MHz operation or by connecting a resistor from FREQ
to ground for a frequency from 0.6MHz to 4MHz. To suit
a variety of applications, the SYNC/MODE pin allows the
user to trade off noise for effi ciency.
The output voltages are set by external dividers returned
to the V
FB1
and V
FB2
pins. An error amplifi er compares the
divided output voltage with a reference voltage of 0.8V and
adjusts the peak inductor current accordingly. Undervoltage
comparators will pull the POR output low when either output
voltage is 8% below its targeted value. The POR output
will go high after 212,992 cycles (when FREQ is high) or
294,912 cycles (when FREQ is tied to ground through an
external resistor), or about 150ms, after both regulators
are above -6% of the target output voltage.
Main Control Loop
For each regulator, during normal operation, the P-chan-
nel MOSFET power switch is turned on at the beginning
of a clock cycle when the V
FB
voltage is below the refer-
ence voltage. The current into the inductor and the load
increases until the current limit is reached. The switch
turns off and energy stored in the inductor fl ows through
the bottom N-channel MOSFET switch into the load until
the next clock cycle.
The peak inductor current is controlled by the voltage
on the I
TH
pin, which is the output of the error amplifi er.
This amplifi er compares the V
FB
pin to the 0.8V reference.
When the load current increases the V
FB
voltage decreases
slightly below the reference. This decrease causes the er-
ror amplifi er to increase the I
TH
voltage until the average
inductor current matches the new load current.
The main control loop is shut down by pulling the RUN pin
to ground. A digital soft-start is enabled after shutdown,
which will slowly ramp the peak inductor current up over
1024 clock cycles.
Low Current Operation
Three modes are available to control the operation of the
LTC3417A-1 at low currents. Each of the three modes
automatically switch from continuous operation to the
selected mode when the load current is low.
To optimize effi ciency, Burst Mode operation can be se-
lected. When the load is relatively light, the LTC3417A-1
automatically switches into Burst Mode operation in which
the PMOS switches operate intermittently based on load
demand. By running cycles periodically, the switching
losses, which are dominated by the gate charge losses
of the power MOSFETs, are minimized. The main control
loop is interrupted when the output voltage reaches the
desired regulated value. The hysteresis voltage comparator
trips when I
TH
is below 0.24V, shutting off the switch and
reducing the power. The output capacitor and the induc-
tor supply the power to the load until I
TH
exceeds 0.31V,
turning on the switch and the main control loop which
starts another cycle.
For lower output voltage ripple at low currents, pulse skip-
ping mode can be used. In this mode, the LTC3417A-1
continues to switch at constant frequency down to very
low currents, where it will begin skipping pulses used to
control the power MOSFETs.
Finally, in forced continuous mode, the inductor current is
constantly cycled creating a fi xed output voltage ripple at all
output current levels. This feature is desirable in telecom-
munications since the noise is a constant frequency and is
thus easy to fi lter out. Another advantage of this mode is
that the regulator is capable of both sourcing current into
a load and sinking some current from the output.
The mode selection for the LTC3417A-1 is set using the
SYNC/MODE pin. The SYNC/MODE pin sets the mode for
both the1A and the 1.5A step-down DC/DC converters.
OPERATION
