參數(shù)資料
型號(hào)： LTC3823IGN#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 4 A SWITCHING CONTROLLER, PDSO28
封裝： 0.150 INCH, LEAD FREE, PLASTIC, SSOP-28
文件頁(yè)數(shù)： 2/26頁(yè)
文件大?。?/td> 302K
代理商： LTC3823IGN#TRPBF
LTC3823
10
3823fd
OPERATION
Main Control Loop
The LTC3823 is a current mode controller for DC/DC
step-down converters. In normal operation, the top
MOSFET is turned on for a xed 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 ICMP trips, restarting the one-shot timer and
initiating the next cycle. Inductor current is determined
by sensing the voltage between the SENSE– and SENSE+
pins using a sense resistor or the bottom MOSFET on-
resistance . The voltage on the ITH pin sets the comparator
threshold corresponding to inductor valley current. The
error amplier EA adjusts this voltage by comparing the
feedback signal, VFB, to an internal reference voltage. If
the load current increases, it causes a drop in the feedback
voltage relative to the reference. The ITH voltage then rises
until the average inductor current again matches the load
current.
At low load currents, the inductor current can drop to zero
and become negative. This is detected by current reversal
comparator IREV which then shuts off M2, resulting in
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the ITH voltage rises above the zero current level (0.75V)
to initiate another cycle. Discontinuous mode operation
is disabled by comparator F when the FCB pin is brought
below 0.6V, forcing continuous synchronous operation.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to main-
tain 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 VIN.
The nominal frequency can be adjusted with an external
resistor, RON.
For applications with stringent constant frequency re-
quirements, the LTC3823 can be synchronized with an
external clock. By programming the nominal frequency
of the LTC3823 the same as the external clock frequency,
the LTC3823 behaves as a constant frequency part against
the load and supply variations.
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 after the
internal 20μs power bad mask timer expires. Furthermore,
in an overvoltage condition, M1 is turned off and M2 is
turned on immediately and held on until the overvoltage
condition clears.
Foldback current limiting is provided if the output is shorted
to ground. As VFB drops, the buffered current threshold
voltage, ITHB, is pulled down and clamped to 0.9V. This
reduces the inductor valley current level to one-tenth of its
maximum value as VFB approaches 0V. Foldback current
limiting is disabled at start-up.
Pulling the RUN pin low forces the controller into its
shutdown state, turning off both M1 and M2. Forcing a
voltage above 1.5V will turn on the device.
INTVCC Power
Power for the top and bottom MOSFET drivers and most of
the internal controller circuitry is derived from the INTVCC
pin. The top MOSFET driver is powered from a oating
bootstrap capacitor, CB. This capacitor is recharged from
INTVCC through an external Schottky diode, DB, when the
top MOSFET is turned off. If the input voltage is low and
INTVCC drops below 3V, undervoltage lockout circuitry
prevents the power switches from turning on.
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