LTC1709 datasheet(10/28 Pages) LINER

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LTC1709

OPERATIO

(Refer to Functional Diagram)

Main Control Loop

The LTC1709 uses a constant frequency, current mode

step-down architecture with inherent current sharing.

During normal operation, the top MOSFET is turned on

each cycle when the oscillator sets the RS latch, and

turned off when the main current comparator, I1, resets

the RS latch. The peak inductor current at which I1 resets

the RS latch is controlled by the voltage on the ITH pin,

which is the output of the error amplifier EA. The differen-

tial amplifier, A1, produces a signal equal to the differential

voltage sensed across the output capacitor but re-refer-

ences it to the internal signal ground (SGND) reference.

The EAIN pin receives a portion of this voltage feedback

signal at the DIFFOUT as determined by VID logic input

pins (VID0 to VID4) and is compared to the internal

reference voltage by the EA. When the load current in-

creases, it causes a slight decrease in the EAIN pin voltage

relative to the 0.8V reference, which in turn causes the ITH

voltage to increase until the average inductor current

matches the new load current. After the top MOSFET has

turned off, the bottom MOSFET is turned on for the rest of

the period.

The top MOSFET drivers are biased from floating boot-

strap capacitor CB, which normally is recharged during

each off cycle through an external Schottky diode. When

VIN decreases to a voltage close to VOUT, however, the loop

may enter dropout and attempt to turn on the top MOSFET

continuously. A dropout detector detects this condition

and forces the top MOSFET to turn off for about 400ns

every 10th cycle to recharge the bootstrap capacitor, CB.

The main control loop is shut down by pulling Pin 1 (RUN/

SS) low. Releasing RUN/SS allows an internal 1.2

µA

current source to charge soft-start capacitor CSS. When

CSS reaches 1.5V, the main control loop is enabled with the

ITH voltage clamped at approximately 30% of its maximum

value. As CSS continues to charge, ITH is gradually re-

leased allowing normal operation to resume. When the

RUN/SS pin is low, all LTC1709 functions are shut down.

If VOUT has not reached 70% of its nominal value when CSS

has charged to 4.1V, an overcurrent latchoff can be

invoked as described in the Applications Information

section.

Low Current Operation

The LTC1709 operates in a continuous, PWM control

mode. The resulting operation at low output currents

optimizes transient response at the expense of substantial

negative inductor current during the latter part of the

period. The level of ripple current is determined by the

inductor value, input voltage, output voltage, and fre-

quency of operation.

Frequency Synchronization

The phase-locked loop allows the internal oscillator to be

synchronized to an external source via the PLLIN pin. The

output of the phase detector at the PLLFLTR pin is also the

DC frequency control input of the oscillator that operates

over a 140kHz to 310kHz range corresponding to a DC

voltage input from 0V to 2.4V. When locked, the PLL aligns

the turn on of the top MOSFET to the rising edge of the

synchronizing signal. When PLLIN is left open, the PLLFLTR

pin goes low, forcing the oscillator to minimum frequency.

Input capacitance ESR requirements and efficiency losses

are substantially reduced because the peak current drawn

from the input capacitor is effectively divided by two and

power loss is proportional to the RMS current squared. A

two stage, single output voltage implementation can re-

duce input path power loss by 75% and radically reduce

the required RMS current rating of the input capacitor(s).

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

of the IC circuitry is derived from INTVCC. When the

EXTVCC pin is left open, an internal 5V low dropout

regulator supplies INTVCC power. If the EXTVCC pin is

taken above 4.7V, the 5V regulator is turned off and an

internal switch is turned on connecting EXTVCC to INTVCC.

This allows the INTVCC power to be derived from a high

efficiency external source such as the output of the regu-

lator itself or a secondary winding, as described in the

Applications Information section. An external Schottky

diode can be used to minimize the voltage drop from

EXTVCC to INTVCC in applications requiring greater than

the specified INTVCC current. Voltages up to 7V can be

applied to EXTVCC for additional gate drive capability.

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