LTC3129 datasheet(17/34 Pages) AD

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LTC3114-1

Rev. D

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OPERATION

Depending on the particular application, either of these

UVLO thresholds could be the limiting factor affecting

the minimum input voltage required for operation. The

LTC3114-1 LDO regulator uses VIN for its power input. If

LDO is not bootstrapped, then there exists a voltage drop

or dropout voltage between VIN and LDO. The dropout

voltage is proportional to the loading on LDO, which is

primarily due to the gate charge and capacitive charging

currents inherent to the internal power switches. The

loading on LDO and the LDO dropout voltage, therefore,

are proportional to VIN and VOUT. For this reason, the min-

imum input voltage required for operation is limited by the

LDO minimum voltage as input voltage (VIN) will always

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

configuration. The Typical Performance Characteristics

section of this data sheet provides guidance on the drop-

out voltage between VIN and LDO over the range of VIN

and VOUT.

In applications where LDO is bootstrapped (powered by

VOUT through a Schottky diode or auxiliary power rail),

the minimum input voltage for operation (after start-up)

will be limited only by the VIN UVLO threshold (2.1V typ-

ical). Please note that if the bootstrap voltage is derived

from the LTC3114-1 VOUT and not an independent power

rail, then the minimum input voltage required for initial

start-up is still limited by the minimum LDO voltage (2.6V

typical).

RUN PIN COMPARATOR

In addition to serving as a logic-level input to enable cer-

tain functions of the IC, the RUN pin includes an accurate

internal comparator that allows it to be used to set custom

rising and falling on/off thresholds with the addition of

an external resistor divider. When RUN is driven above

its logic threshold (0.7V typical), the LDO regulator is

enabled, which provides power to the internal control

circuitry of the IC. If the voltage on RUN is increased

further so that it exceeds the RUN comparator accurate

analog threshold (1.2V nominal), all functions of the buck-

boost converter will be enabled and a startup sequence

will ensue.

If RUN is brought below the accurate comparator thresh-

old, the buck-boost converter will inhibit switching, but

the LDO regulator and control circuitry will remain pow-

ered unless RUN is brought below its logic threshold.

Therefore, in order to completely shut down the IC and

reduce the VIN current to 3µA (typical), it is necessary to

ensure that RUN is brought below its worst-case low-

logic threshold of 0.3V. RUN is a high voltage input and

can be tied directly to VIN to continuously enable the IC

when the input supply is present. Also note that RUN can

be driven above VIN or VOUT as long as it stays within the

operating range of the IC, that is, less than 40V. If RUN

is forced above 5V, it will sink a small current as given by

the following equation:

IRUN ≈

VRUN – 5V

With the addition of an optional resistor divider as shown

in Figure 3, the RUN pin can be used to establish a

user-programmable turn on and turn off threshold.

The buck-boost converter is enabled when the voltage on

RUN reaches 1.205V (nominal). Therefore, the turn-on

voltage threshold on VIN is given by:

VTURNON = 1.205V 1+

⎛

⎝⎜

⎞

⎠⎟

Once the converter is enabled, the RUN comparator

includes a built-in hysteresis of approximately 140mV,

so that the turn-off threshold will be approximately 8.33%

lower than the turn-on threshold. Put another way, the

internal threshold level for the RUN comparator looks like

1.1V after the IC is enabled.

Figure 3. Accurate RUN Pin Comparator

ENABLE SWITCHING

ENABLE LDO AND

CONTROL CIRCUITS

1.2V

ACCURATE THRESHOLD

LTC3114-1

LOGIC THRESHOLD

0.7V

31141 F03

RUN

VIN

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