FTD6821 datasheet(11/17 Pages) FS

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Function Description

Main Control Loop

The F TD6821 uses a constantfrequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET)

switches are internal. During normal operation, the

internal top power MOSFET is turned on each cycle

when the oscillator sets the RS latch, and turned off

when the current comparator, ICOMP, resets the RS

latch. The peak inductor current at which ICOMP resets

the RS latch, is controlled by

the output of error amplifier EA. When the load current

increases, it causes a slight decrease in the feedback

voltage, FB, relative to the 0.6V reference, which in turn,

causes the EA amplifier’s output voltage to increase until

the average inductor current matches the new load

current. While the top MOSFET is off, the bottom

MOSFET is turned on until either the inductor current

starts to reverse, as indicated by the current reversal

comparator IRCMP, or the beginning of the next clock

cycle.

Burst Mode Operation

The FTD6821 is capable of Burst Mode operation in

which the internal power MOSFETs operate

intermittently based on load demand.

In Burst Mode operation, the peak current of the inductor

is set to approximately 200mA regardless of the output

load. Each burst event can last from a few cycles at light

loads to almost continuously cycling with short sleep

intervals at moderate loads. In between these burst

events, the power MOSFETs and any unneeded

circuitry are turned off, reducing the quiescent current to

20mA. In this sleep state, the load current is being

supplied solely from the output capacitor. As the output

voltage droops,

the EA amplifier’s output rises above the sleep threshold

signaling the BURST comparator to trip and turn the top

MOSFET on. This process repeats at a rate that is

dependent on the load demand.

Short Circuit Protection

When the output is shorted to ground, the frequency of

the oscillator is reduced to about 400kHz, 1/4 the

nominal frequency. This frequency foldback ensures that

the inductor current has more time to decay, thereby

preventing runaway. The oscillator’s frequency will

progressively increase to 1.5MHz when VFB or VOUT

rises above 0V.

Dropout Operation

As the input supply voltage decreases to a value

approaching the output voltage, the duty cycle increases

toward the maximum on-time. Further reduction of the

supply voltage forces the main switch to remain on for

more than one cycle until it reaches 100% duty cycle.

The output voltage will then be determined by the input

voltage minus the voltage drop across the P-channel

MOSFET and the inductor.

An important detail to remember is that at low input

supply voltages, the RDS(ON) of the P-channel switch

increases (see Typical Performance Characteristics).

Therefore, the user should calculate the power

dissipation when the FTD6821 is used at 100% duty cycle

with low input voltage (See Thermal Considerations in

the Applications Information

section).

2013.01. 28

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SEMICONDUCTOR

TECHNICAL D ATA

FTD6821

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FTD6821 Datasheet(PDF) 11 Page - First Silicon Co., Ltd

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