EC3210 datasheet(10/16 Pages) E-CMOS

10 / 16 page

EC3210

1.5MHz, 800mA, Synchronous

Step-Down Regulator Dropout

E-CMOS Corp. (www.ecmos.com.tw)

Page 10 of 16

3I06N-Rev.F003

Function Description

the EA

amplifier’s output rises above the sleep threshold

Main Control Loop

The EC3210 uses a constant frequency, 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 reversalcomparator IRCMP, or the

beginning of the next clock cycle.

Burst Mode Operation

The EC3210 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 30uA. In

this sleep state, the load current is being supplied solely

from the output capacitor. As the output voltage droops,

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 EC3210 is used at 100% duty cycle

with low input voltage (See Thermal Considerations in

the Applications Information section).

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