LTC3521 datasheet(12/20 Pages) LINER

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LTC3521

3521f

Burst Mode Operation

With the PWM pin held low, the buck-boost converter

operatesutilizingavariablefrequencyswitchingalgorithm

designed to improve efficiency at light load and reduce

the standby current at zero load. In Burst Mode operation,

the inductor is charged with fixed peak amplitude current

pulses. These current pulses are repeated as often as

necessary to maintain the output regulation voltage. The

maximum output current which can be supplied in Burst

Mode operation is dependent upon the input and output

voltage as given by the following formula:

OUT MAX BURST

OUT

(

. •

( )

0 1

In BurstModeoperation,the erroramplifierisnotused but

is instead placed in a low current standby mode to reduce

supply current and improve light load efficiency.

Soft-Start

The buck-boost converter has an internal voltage mode

soft-start circuit with a nominal duration of 500μs. The

converter remains in regulation during soft-start and will

therefore respond to output load transients that occur

during this time. In addition, the output voltage rise time

has minimal dependency on the size of the output capaci-

tor or load. During soft-start, the buck-boost converter is

forced into PWM operation regardless of the state of the

PWM pin.

PGOOD Comparator

The PGOOD1 pin is an open-drain output which indicates

the status of the buck-boost converter. In Burst Mode

operation (PWM = Low), the PGOOD1 open-drain output

will pull low when the feedback voltage falls 9% below the

regulationvoltage.Thereisapproximately3%hysteresisin

this threshold when the output voltage is returning good.

In addition, there is a 60μs typical deglitching delay to

prevent false trips due to short duration voltage transients

in response to load steps.

In PWM mode, operation of the PGOOD1 comparator is

complicated by the fact that the feedback pin voltage is

driven to the reference voltage independent of the output

operaTion

voltage through the action of the voltage mode error am-

plifier. Since the soft-start is voltage mode, the feedback

voltage will track the output voltage correctly during

soft-start, and the PGOOD1 output will correctly indicate

the point at which the buck-boost attains regulation at the

end of soft-start. Therefore, the PGOOD1 output can be

utilized for sequencing purposes. Once in regulation, the

feedback voltage will no longer track the output voltage,

and the PGOOD1 pin will not directly respond to a loss

of regulation in the output. However, the only means by

which a loss of regulation can occur is if the current limit

has been reached, thereby preventing the buck-boost

converter from delivering the required output current.

In such cases, the occurrence of current limit will cause

the PGOOD1 flag to fall indicating a fault state. There can

be cases, however, when the buck-boost converter is

continuously in current limit, causing the PGOOD1 output

to pull low, while the output voltage still remains slightly

above the PGOOD1 comparator trip point.

ThePGOOD1outputalsopullslowduringovertemperature

shutdown, undervoltage lockout or if the SHDN1 pin is

pulled low.

COMMON FUNCTIONS

Thermal Shutdown

If the die temperature exceeds 150°C, all three converters

will be disabled. All power devices will be turned off and

all switch nodes will be high impedance. The soft-start

circuits for all three converters are reset during thermal

shutdown to provide a smooth recovery once the over-

temperature condition is eliminated. All three converters

will restart (if enabled) when the die temperature drops

to approximately 140°C.

Undervoltage Lockout

If the supply voltage decreases below 1.7V (typical) then

all three converters will be disabled and all power devices

will be turned off. The soft-start circuits for all three con-

verters are reset during undervoltage lockout to provide

a smooth restart once the input voltage rises above the

undervoltage lockout threshold.

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