LTC3717-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3717-1
Linear Technology
'LTC3717-1' PDF : 20 Pages View PDF
LTC3717-1
U
OPERATIO
Main Control Loop
The LTC3717-1 is a current mode controller for DC/DC
step-down converters. In normal 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 com-
parator ICMP trips, restarting the one-shot timer and initi-
ating the next cycle. Inductor current is determined by
sensing the voltage between the SENSE+ and SENSE– pins
using the bottom MOSFET on-resistance . The voltage on
the ITH pin sets the comparator threshold corresponding
to inductor valley current. The error amplifier EA adjusts
this ITH voltage by comparing 2/3 of the feedback signal
VFB from the output voltage with a reference equal to 1/3
of the VREF 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. As a result in normal DDR
operation VOUT is equal to 1/2 of the VREF voltage.
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 VIN. The nominal frequency can be adjusted with an
external resistor RON.
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.
Furthermore, in an overvoltage condition, M1 is turned off
and M2 is turned on and held on until the overvoltage
condition clears.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2µA current source to charge
up an external soft-start capacitor CSS. When this voltage
reaches 1.5V, the controller turns on and begins switch-
ing, but with the ITH voltage clamped at approximately
0.6V below the RUN/SS voltage. As CSS continues to
charge, the soft-start current limit is removed.
INTVCC/EXTVCC 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
floating bootstrap capacitor CB. This capacitor is re-
charged from INTVCC through an external Schottky diode
DB when the top MOSFET is turned off. When the EXTVCC
pin is grounded, an internal 5V low dropout regulator
supplies the INTVCC power from VCC. If EXTVCC rises
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTVCC to INTVCC. This allows
a high efficiency source connected to EXTVCC, such as an
external 5V supply or a secondary output from the
converter, to provide the INTVCC power. Voltages up to
7V can be applied to EXTVCC for additional gate drive. If
the VCC voltage is low and INTVCC drops below 3.4V,
undervoltage lockout circuitry prevents the power
switches from turning on.
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sn37171 37171fs
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