LTC1778EGN-1 データシートの表示（PDF） - Linear Technology

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LTC1778EGN-1

Wide Operating Range, No RSENSE™ Step-Down Controller

Linear Technology 

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OPERATIO

Main Control Loop

The LTC1778 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 PGND and SW pins using

the bottom MOSFET on-resistance . The voltage on the ITH

pin sets the comparator threshold corresponding to in-

ductor valley current. The error amplifier EA adjusts this

voltage by comparing the feedback signal VFB from the

output voltage with an internal 0.8V reference. 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 (0.8V) to

initiate another cycle. Discontinuous mode operation is

disabled by comparator F when the FCB pin is brought

below 0.8V, forcing continuous synchronous operation.

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 ±7.5% window around the regulation point.

LTC1778/LTC1778-1

Furthermore, in an overvoltage condition, M1 is turned off

and M2 is turned on 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 by clamp Q3 to a 1V

level set by Q4 and Q6. This reduces the inductor valley

current level to one sixth of its maximum value as VFB

approaches 0V.

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 VIN. 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 input voltage is low and INTVCC drops below 3.5V,

undervoltage lockout circuitry prevents the power

switches from turning on.

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