LTC3129 datasheet(13/30 Pages) LINER

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LTC3129

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operaTion

functions of the IC, significantly reducing the quiescent

current of the LTC3129 to just 1.3µA typical. This greatly

improves overall power conversion efficiency when the

output load is light. Since the converter is not operating

in sleep, the output voltage will slowly decay at a rate

determined by the output load resistance and the output

capacitor value. When the output voltage has decayed by

a small amount, typically 1%, the LTC3129 will wake and

resume normal PWM switching operation until the volt-

age on VOUT is restored to the previous level. If the load

is very light, the LTC3129 may only need to switch for a

few cycles to restore VOUT and may sleep for extended

periods of time, significantly improving efficiency. If the

load is suddenly increased above the burst transition

threshold, the part will automatically resume continuous

PWM operation until the load is once again reduced.

A feedforward capacitor on the feedback divider can be

used to reduce Burst Mode VOUT ripple. This is discussed

in more detail in the Applications Information section of

this data sheet.

Note that Burst Mode operation is inhibited until soft-start

is done, the MPPC pin is greater than 1.175V and VOUT

has reached regulation.

Soft-Start

The LTC3129 soft-start circuit minimizes input current

transientsandoutputvoltageovershootoninitialpowerup.

The required timing components for soft-start are internal

to the LTC3129 and produce a nominal soft-start dura-

tion of approximately 3ms. The internal soft-start circuit

slowly ramps the error amplifier output, VC. In doing so,

the current command of the IC is also slowly increased,

starting from zero. It is unaffected by output loading or

output capacitor value. Soft-start is reset by the UVLO on

both VIN and VCC, the RUN pin and thermal shutdown.

VCC Regulator

An internal low dropout regulator (LDO) generates a nomi-

nal 4.1V VCC rail from VIN. The VCC rail powers the internal

control circuitry and the gate drivers of the LTC3129. The

VCC regulator is disabled in shutdown to reduce quiescent

current and is enabled by raising the RUN pin above its

logic threshold. The VCC regulator includes current-limit

protectiontosafeguardagainstaccidentalshort-circuiting

of the VCC rail.

Undervoltage Lockout (UVLO)

There are two undervoltage lockout (UVLO) circuits within

the LTC3129 that inhibit switching; one that monitors VIN

and another that monitors VCC. Either UVLO will disable

operation of the internal power switches and keep other

IC functions in a reset state if either VIN or VCC are below

their respective UVLO thresholds.

The VIN UVLO comparator has a falling voltage threshold

of 1.8V (typical). If VIN falls below this level, IC operation

is disabled until VIN rises above 1.9V (typical), as long as

the VCC voltage is above its UVLO threshold.

The VCC UVLO has a falling voltage threshold of 2.19V

(typical). If the VCC voltage falls below this threshold, IC

operation is disabled until VCC rises above 2.25V (typical)

as long as VIN is above its nominal UVLO threshold level.

Depending on the particular application, either of these

UVLO thresholds could be the limiting factor affecting the

minimuminputvoltagerequiredforoperation.Becausethe

VCC regulator uses VIN for its power input, the minimum

input voltage required for operation is determined by the

VCC minimum voltage, as input voltage (VIN) will always

be higher than VCC in the normal (non-bootstrapped)

configuration. Therefore, the minimum VIN for the part

to startup is 2.25V (typical).

In applications where VCC is bootstrapped (powered

through a Schottky diode by either VOUT or an auxiliary

power rail), the minimum input voltage for operation will

be limited only by the VIN UVLO threshold (1.8V typical).

Please note that if the bootstrap voltage is derived from

the LTC3129 VOUTandnotanindependentpowerrail,then

the minimum input voltage required for initial startup is

still 2.25V (typical).

Note that if either VIN or VCC are below their UVLO thresh-

olds, or if RUN is below its accurate threshold of 1.22V

(typical), then the LTC3129 will remain in a soft shutdown

state, where the VIN quiescent current will be only 1.9µA

typical.

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