LTC3708_15 Datasheet, PDF (10/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3708 uses a constant on-time, current mode step-
down architecture with two control channels operating at
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 each 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 resistor. At low load currents, the inductor current
can drop to zero and become negative. This is detected by
current reversal comparator I_REV, which then shuts off M2
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 brought below
1.9V, forcing continuous synchronous operation.
The main control loop is shut down by pulling the RUN/SS
pin low, turning off both M1 and M2. Releasing the pin al-
lows an internal 1.2μA current source to charge an external
soft-start capacitor, C_SS. When this voltage reaches 1.3V,
the controller turns on and begins switching, but with the
effective reference voltage clamped at 0V. As C_SS continues
to charge, the effective reference ramps up at the same
rate and controls the rise rate of the output voltage.
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 frequency
approximately constant with changes in V_IN. The nominal
frequency can be adjusted with an external resistor R_ON.
When the LTC3708 is synchronized to an external clock,
the operating frequency will then be solely determined by
the external clock.
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,
M1 is turned off and M2 is turned on and held on until
the condition is cleared.
Short-Circuit Detection and Protection
After the controller has been started and given adequate
time to charge the output capacitors, the RUN/SS capacitor
is used as the short-circuit time-out capacitor. If either one
of the output voltages falls to less than 70% 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, both controllers will be shut down until
the RUN/SS pin voltage is recycled. This built-in latchoff
can be overridden by providing >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.
Power Good (PGOOD) Pin
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback volt-
age exceeds 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 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 high immediately when
the feedback voltage is in regulation.
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