LTC3417
8
3417fd
The LTC3417 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 de-
termined 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 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 PGOOD output low when either
output voltage is 6% below its targeted value.
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 LTC3417 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
selected. When the load is relatively light, the LTC3417
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
skipping mode can be used. In this mode, the LTC3417
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 LTC3417 is set using the MODE
pin. The MODE pin sets the mode for both the 800mA and
the 1.4A step-down DC/DC converters.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases to 100%. In this dropout
condition, 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 inductor.
Low Supply Operation
The LTC3417 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.07V to prevent unstable operation.
OPERATION
