LTC3731 Datasheet, PDF (11/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

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OPERATIO (Refer to Functional Diagram)
C) Continuous Current Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When forcing con-
tinuous operation and sinking current, this current will be
forced back into the main power supply, potentially
boosting the input supply to dangerous voltage levels—
BEWARE!
LTC3731
Power Good
The PGOOD pin is connected to the drain of an internal
N-channel MOSFET. The MOSFET is turned on once an
internal delay of about 100µs has elapsed and the output
voltage has been away from its nominal value by greater than
10%. If the output returns to normal prior to the delay
timeout, the timer is reset. There is no delay time for the
rising of the PGOOD output once the output voltage is within
the ±10% “window.”
Frequency Synchronization
The phase-locked loop allows the internal oscillator to be
synchronized to an external source using the PLLIN pin.
The output of the phase detector at the PLLFLTR pin is also
the DC frequency control input of the oscillator which
operates over a 250kHz to 600kHz range corresponding to
a voltage input from 0V to 2.4V. When locked, the PLL
aligns the turn on of the top MOSFET to the rising edge of
the synchronizing signal. When no frequency information
is supplied to the PLLIN pin, PLLFLTR goes low, forcing the
oscillator to minimum frequency. A DC source can be
applied to the PLLFLTR pin to externally set the desired
operating frequency. An approximate 20µA discharge cur-
rent will be present at the pin with no PLLIN input signal.
Input capacitance ESR requirements and efficiency losses
are reduced substantially in a multiphase architecture
because the peak current drawn from the input capacitor
is effectively divided by the number of phases used and
power loss is proportional to the RMS current squared. A
3-stage, single output voltage implementation can reduce
input path power loss by 90%.
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both V_OUT+ and V_OUT– benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses. This sensing also
isolates the physical power ground from the physical
signal ground preventing the possibility of troublesome
“ground loops” on the PC layout and prevents voltage
errors caused by board-to-board interconnects, particu-
larly helpful in VRM designs.
Phase Mode
The PHASMD pin determines the phase shift between the
rising edge of the TG1 output and the rising edge of the
CLKOUT signal. Grounding the pin will result in 30 degrees
phase shift and tying the pin to V_CC will result in 60
degrees. These phase shift values enable extension to 6-
and 12-phase systems. The PGOOD function above and
the PHASMD function are tied to a common pin in the UH
package.
Undervoltage Shutdown Adjust
The voltage applied to the UVADJ pin is compared to the
internal 1.2V reference to have an externally program-
mable undervoltage shutdown. The RUN/SS pin is inter-
nally held low until the voltage applied to the UVADJ pin
exceeds the 1.2V threshold.
Short-Circuit Detection
The RUN/SS capacitor is used initially to turn on and limit
the inrush current from the input power source. Once the
controllers have been given time, as determined by the
capacitor on the RUN/SS pin, to charge up the output
capacitors and provide full load current, the RUN/SS
capacitor is then used as a short-circuit timeout circuit. If
the output voltage falls to less than 70% of its nominal
output voltage, the RUN/SS capacitor begins discharg-
ing, assuming that the output is in a severe overcurrent
and/or short-circuit condition. If the condition lasts for a
long enough period, as determined by the size of the
RUN/SS capacitor, the controller will be shut down until
the RUN/SS pin voltage is recycled. This built-in latchoff
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