LTC3522 datasheet(9/20 Pages) LINER

9 / 20 page

LTC3522

3522f

OPERATION

The LTC3522 combines a synchronous buck DC/DC

converter and a 4-switch buck-boost DC/DC converter

in a single 3mm

× 3mm QFN package. The buck-boost

converter utilizes a proprietary switching algorithm which

allows its output voltage to be regulated above, below or

equal to the input voltage. The buck converter provides a

high efﬁciency lower voltage output and supports 100%

duty cycle operation to extend battery life. In Burst Mode

operation, the combined quiescent current for both con-

verters is reduced to 25μA. Both converters operate from

the same internal 1MHz oscillator.

BUCK CONVERTER OPERATION

PWM Mode Operation

When the PWM pin is held high, the LTC3522 buck con-

verter uses a constant frequency, current mode control

architecture. Both the main (P-channel MOSFET) and

synchronous rectiﬁer (N-channel MOSFET) switches are

internal. At the start of each oscillator cycle, the P-chan-

nel switch is turned on and remains on until the current

waveform with superimposed slope compensation ramp

exceeds the error ampliﬁer output. At this point, the syn-

chronous rectiﬁer is turned on and remains on until the

inductor current falls to zero or a new switching cycle is

initiated. As a result, the buck converter operates with

discontinuous inductor current at light loads which im-

proves efﬁciency. At extremely light loads, the minimum

on-time of the main switch will be reached and the buck

converter will begin turning off for multiple cycles in order

to maintain regulation.

Burst Mode Operation

When the PWM pin is forced low, the buck converter will

automatically transition between Burst Mode operation

at sufﬁciently light loads (below approximately 10mA)

and PWM mode at heavier loads. Burst Mode entry is

determined by the peak inductor current and therefore

the load current at which Burst Mode operation will be

entered depends on the input voltage, the output voltage

and the inductor value. Typical curves for Burst Mode

entry threshold are provided in the Typical Performance

Characteristics section of this data sheet. Under dropout

and near dropout conditions, Burst Mode operation will

not be entered.

Dropout Operation

As the input voltage decreases to a value approaching the

output regulation voltage, the duty cycle increases toward

the maximum on-time. Further reduction of the supply

voltage will force the main switch to remain on for more

than one cycle until 100% duty cycle operation is reached

where the main switch remains on continuously. In this

dropout state, the output voltage will be determined by

the input voltage less the resistive voltage drop across the

main switch and series resistance of the inductor.

Slope Compensation

Current mode control requires the use of slope compensa-

tion to prevent sub-harmonic oscillations in the inductor

current waveform at high duty cycle operation. This is ac-

complished internally on the LTC3522 through the addition

of a compensating ramp to the current sense signal. In

some current mode ICs, current limiting is performed by

clamping the error ampliﬁer voltage to a ﬁxed maximum.

This leads to a reduced output current capability at low

step-down ratios. In contrast, the LTC3522 performs cur-

rent limiting prior to addition of the slope compensation

ramp and therefore achieves a peak inductor current limit

that is independent of duty cycle.

Short-Circuit Protection

When the output is shorted to ground, the error ampliﬁer

will saturate high and the P-channel MOSFET switch will

turn on at the start of each cycle and remain on until the

current limit trips. During this minimum on-time, the in-

ductor current will increase rapidly and will decrease very

slowly during the remainder of the period due to the very

small reverse voltage produced by a hard output short.

To eliminate the possibility of inductor current runaway

in this situation, the buck converter switching frequency

is reduced to approximately 250kHz when the voltage on

FB2 falls below 0.3V.

LTC3522 データシート(PDF) 9 Page - Linear Technology

LTC3522 データシート(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

データシートダウンロード