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LTC3521IFE

1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters

Linear Technology 

LTC3521

Operation

The LTC3521 combines dual synchronous buck DC/DC

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

in a 4mm × 4mm QFN package and a 20-pin thermally

enhanced TSSOP 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 converters provide a high ef-

ficiency lower voltage output and support 100% duty cycle

operation to extend battery life. In Burst Mode operation,

the total quiescent current for the LTC3521 is reduced to

30μA. All three converters are synchronized to the same

internal 1.1MHz oscillator.

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 will be determined by the input

voltage less the resistive voltage drop across the main

switch and series resistance of the inductor.

Slope Compensation

Buck Converter Operation

PWM Mode Operation

When the PWM pin is held high, the LTC3521 buck con-

verters use 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-channel 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 synchronous 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 converters operate

with discontinuous inductor current at light loads, which

improves efficiency. At extremely light loads, the minimum

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

converters will begin turning off for multiple cycles in

order to maintain regulation.

Burst Mode Operation

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

automatically transition between Burst Mode operation

at sufficiently light loads (below approximately 15mA)

and PWM mode at heavier loads. Burst Mode entry is

determined by the peak inductor current. 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. In dropout and

near dropout conditions, Burst Mode operation is disabled.

Current mode control requires the use of slope compensa-

tion to prevent subharmonic oscillations in the inductor

current at high duty cycle operation. This is accomplished

internally on the LTC3521 through the addition of a com-

pensating 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 LTC3521 performs current 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 250kHz when the voltage on the buck FB pin falls

below 0.25V. The buck soft-start circuit is reset when the

buck FB pin falls below 0.25V to provide a smooth restart

once the short-circuit condition at the output voltage is

no longer present. Additionally, the PMOS current limit

is decreased from 1050mA to 700mA when the voltage

on the buck FB pin falls below 0.25V.

3521fb

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For more information www.linear.com/LTC3521

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