LT3154AVPBF datasheet(17/32 Pages) AD

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LT3154

Rev. 0

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EN/UVLO Pin Comparator and VIN UVLO

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

tain functions of the device, the EN/UVLO pin includes an

accurate internal comparator that allows custom rising

and falling ON/OFF thresholds for VIN to be set with the

addition of external resistor dividers. When the EN/UVLO

pin is driven above a logic threshold (0.7V typical) certain

control circuitry of the IC is enabled, drawing additional

quiescent current. If the EN/UVLO voltage is increased

further, so that it exceeds the accurate comparator rising

threshold (1.2V nominal), all functions of the buck‑boost

converter will be enabled and a startup sequence will

commence, assuming the VIN is above the UVLO level

(see Figure 3). If EN/UVLO is brought below the accurate

comparator falling threshold (1.1V nominal), the buck‑

boost converter will inhibit switching, but certain control

circuitry will remain powered until EN/UVLO is brought

belowthelogicthreshold.Inordertocompletelyshutdown

the IC and reduce the VIN currents to < 1µA (typical), it is

necessary to ensure that EN/UVLO is brought below the

worst case low logic threshold of 0.3V.

In addition to the EN/UVLO pin comparators, the LT3154

incorporates additional under voltage lockout (UVLO)

circuits to control operation. The VIN UVLO comparator

prevents operation below 1.8V over temperature (typical

values are 1.7V rising/1.6V falling).

OPERATION

–

VIN

ENABLE

SWITCHING

ENABLE

INTERNAL CIRCUITS

1.7V

1.2V

0.7V

EN/UVLO

3154 F03

VIN

VIN UVLO

ACCURATE

LOGIC

LEVEL

Figure 3. Accurate and Logic Level EN/UVLO

Pin Comparators and VIN UVLO

Soft-Start

TheLT3154soft‑startcircuitminimizesinputcurrenttran‑

sients and output voltage overshoot on initial power‑up.

An internal soft‑start interval of approximately 2.2ms can

be selected by connecting SS to VIN, eliminating the need

for an external SS capacitor. For applications requiring a

longer or custom soft‑start period, an external capacitor

on SS sets soft‑start period according to the formula:

CSS(nF)= 1.25 • tSS(ms)

The soft‑start circuit slowly ramps the Error Amplifier

output at VC. In doing so, the current command of the

IC is slowly increased, starting from zero. The soft‑start

period is defined as the time it takes the internal or external

SS capacitor to ramp to 0.8V, allowing VC to command

full rated current. In most situations, VOUT comes into

regulation without needing full inductor current, resulting

in power‑up times at a fraction of the soft‑start period.

After initial power‑up, soft‑start can be reset by VIN UVLO

asserting, Thermal Shutdown, or a VOUT short circuit.

Aninternal40kresistorisusedtodischargetheexternalSS

capacitor in shutdown. If SS is connected to VIN,EN/UVLO

should be initially enabled during VIN power‑up to allow

the LT3154 to determine the internal SS pin configuration

and ensure low current shutdown. Otherwise, an external

SS capacitor is recommended for applications where EN/

UVLO is controlled through a microprocessor (see Note 7

in the Electrical Characteristics section).

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