LTC3879_15 Datasheet, PDF (10/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

LTC3879
OPERATION
Main Control Loop
The LTC3879 is a valley current mode controller IC for
use in DC/DC step-down converters. In normal continu-
ous operation, the top MOSFET is turned on for a fixed
interval determined by a 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 initiating the next cycle. Inductor
valley current is measured by sensing the voltage between
the PGND and SW pins using the bottom MOSFET on-
resistance. The voltage on the I_TH pin sets the compara-
tor threshold corresponding to inductor valley current.
The error amplifier EA adjusts this voltage by comparing
the feedback signal V_FB from the output voltage to the
feedback reference voltage V_FBREF. Increasing the load
current causes a drop in the feedback voltage relative
to the reference. The EA senses the feedback voltage
drop and adjusts the I_TH voltage higher until the average
inductor current matches the load current.
With DC current loads less than 1/2 of the peak-to-peak
ripple the inductor current can drop to zero or become
negative. In discontinuous operation, negative inductor
current is detected and prevented by the current reversal
comparator I_REV, which shuts off MB. Both switches remain
off with the output capacitor supplying the load current
until the EA moves the I_TH voltage above the zero current
level (0.8V) to initiate another switching cycle. When the
MODE pin is below the internal threshold reference, V_MODE,
the regulator is forced to operate in continuous mode by
disabling reversal comparator, I_REV, thereby allowing the
inductor current to become negative.
The continuous mode operating frequency can be deter-
mined by dividing the calculated duty cycle, V_OUT/V_IN,
by the fixed on-time. The OST generates an on-time
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.
Foldback current limiting is provided to protect against
low impedance shorts. If the controller is in current limit
and V_OUT drops to less 50% of regulation, the current limit
set-point “folds back” to progressively lower values. To
recover from foldback current limit, the excessive load or
low impedance short needs to be removed.
When RUN is less than 0.7V, the LTC3879 is in the low
power shutdown state with a nominal bias current of
18μA. When RUN is greater than 0.7V and less than 1.5V,
INTV_CC and all internal circuitry are enabled while TG and
BG are forced low. When RUN is taken greater than 1.5V,
switching begins and the TRACK/SS pin is released from
ground. The output voltage follows the TRACK/SS input
multiplied by feedback factor when TRACK/SS is less than
0.6V. Soft-start is complete when TRACK/SS exceeds the
internal 0.6V reference voltage and regulates normally.
3879f
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