LTC3417 Datasheet, PDF (8/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

LTC3417
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OPERATIO
The LTC3417 uses a constant frequency, current mode
architecture. Both channels share the same clock fre-
quency. 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 MODE pin allows the user to
trade off noise for efficiency.
The output voltages are set by external dividers returned
to the V_FB1 and V_FB2 pins. An error amplifier compares the
divided output voltage with a reference voltage of 0.8V and
adjusts the peak inductor current accordingly. Undervolt-
age 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 reference
voltage. The current into the inductor and the load in-
creases until the current limit is reached. The switch turns
off and energy stored in the inductor flows 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 amplifier. This
amplifier 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
error amplifier 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 auto-
matically switch from continuous operation to the se-
lected mode when the load current is low.
To optimize efficiency, 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 compara-
tor trips when I_TH is below 0.24V, shutting off the switch
and reducing the power. The output capacitor and the
inductor 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 fixed output voltage ripple at
all output current levels. This feature is desirable in tele-
communications since the noise is a constant frequency
and is thus easy to filter 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.
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