LTC3709 Datasheet, PDF (10/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

LTC3709
U
OPERATIO (Refer to Functional Diagram)
MAIN CONTROL LOOP
The LTC3709 is a constant on-time, current mode step-
down controller with two channels operating 180 degrees
out of phase. In normal operation, each top MOSFET is
turned on for a fixed interval determined by its own one-
shot timer OST. When the top MOSFET is turned off, the
bottom MOSFET is turned on until the current comparator
I_CMP trips, restarting the one-shot timer and repeating the
cycle. The trip level of the current comparator is set by the
I_TH voltage, which is the output of error amplifier EA.
Inductor current is determined by sensing the voltage
between the SENSE^– and SENSE⁺ pins using either the
bottom MOSFET on-resistance or a separate sense resis-
tor. At light load, the inductor current can drop to zero and
become negative. This is detected by current reversal
comparator I_REV, which then shuts off the bottom MOSFET,
resulting in discontinuous operation. Both switches will
remain off with the output capacitor supplying the load
current until the I_TH voltage rises above the zero current
level (0.8V) to initiate another cycle. Discontinuous mode
operation is disabled when the FCB pin is tied to ground,
forcing continuous synchronous operation.
The main control loop is shut down by pulling the RUN/SS
pin low, turning off both top MOSFET and bottom MOSFET.
Releasing the pin allows an internal 1.2µA current source
to charge an external soft-start capacitor C_SS. When this
voltage reaches 1.4V, the LTC3709 turns on and begins
operating with a clamp on the noninverting input of the
error amplifier. This input is also the reference input of the
error amplifier. As the voltage on RUN/SS continues to
rise, the voltage on the reference input also rises at the
same rate, effectively controlling output voltage slew rate.
Operating Frequency
The operating frequency is determined implicitly by the
top MOSFET on time and the duty cycle required to
maintain regulation. The one-shot timer generates an on-
time that is proportional to the ideal duty cycle, thus
holding the frequency approximately constant with changes
in V_IN. The nominal frequency can be adjusted with an
external resistor R_ON.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>10%) as well as other more serious condi-
tions that may overvoltage the output. In this condition,
the top MOSFET is turned off and the bottom MOSFET is
turned on and held on until the condition is cleared.
Power Good (PGOOD) Pin
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ±10% window around the regulation point. In
addition, the output feedback voltage must be out of this
window for a continuous duration of at least 100µs before
the PGOOD is pulled low. This is to prevent any glitch on
the feedback voltage from creating a false power bad
signal. The PGOOD will indicate a good power immediately
when the feedback voltage is in regulation.
Short-Circuit Detection and Protection
After the controller has been started and been given
adequate time to charge the output capacitor, the RUN/SS
capacitor is used in a short-circuit time-out circuit. If the
output voltage falls to less than 67% of its nominal output
voltage, the RUN/SS capacitor begins discharging on the
assumption that the output is in an 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 latch off can
be overridden by providing a >5µA pull-up at a compli-
ance of 5V to the RUN/SS pin. This current shortens the
soft-start period but also prevents net discharge of the
RUN/SS capacitor during an overcurrent and/or short-
circuit condition.
3709f
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