LTC3124 datasheet(14/36 Pages) AD

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LT3120

Rev. 0

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LT3120 to be synchronized to an external clock signal

applied to the SYNC/MODE pin. The phase-locked loop is

onlyabletoincreasethefrequencyoftheinternaloscillator

to obtain synchronization. Therefore, the resistor RT must

be chosen to program the internal oscillator to a lower fre-

quencythanthefrequencyoftheclockappliedtotheSYNC/

MODE pin. Sufficient margin must be included to account

for the frequency variation of the external synchronization

clock as well as the worst-case variation in frequency of

the internal oscillator. In most cases setting the internal

oscillator frequency 20% lower than the synchronization

clock is sufficient. Whether operating from its internal

oscillator or synchronized to an external clock signal, the

LT3120 is able to operate with a switching frequency from

400kHz to 2MHz, providing the ability to trade-off small

solution size and optimum power conversion efficiency.

EN/UVLO Pin Comparator

In addition to serving as a logic-level input to enable the IC,

the EN/UVLO pin features an accurate internal comparator

allowing it to be used to set custom rising and falling input

undervoltage lockout thresholds with the addition of an

external resistor divider. When the EN/UVLO pin is driven

above its logic threshold (typically 0.8V), the VCC regulator

is enabled which provides power to the internal control cir-

cuitryoftheICandtheaccurateEN/UVLOpincomparatoris

enabled. If the EN/UVLO pin voltage is increased further so

that it exceeds the EN/UVLO comparator threshold (1.205V

nominal), the buck-boost converter will be enabled. If the

EN/UVLO pin is brought below the EN/UVLO comparator

threshold, the buck-boost converter will inhibit switching,

but the VCC regulator and control circuitry will remain

powered unless the EN/UVLO pin is brought below its logic

threshold. Therefore, in order to place the part in shutdown

and reduce the input current to its minimum level (3µA),

it is necessary to ensure that the EN/UVLO pin is brought

below the worst-case logic threshold (0.3V). The EN/UVLO

pin is a high voltage input and can be connected directly to

VINtocontinuouslyenablethepartwhentheinputsupplyis

present. If the EN/UVLO pin is forced above approximately

5V, it will sink a small current as given by Equation 1.

IEN/UVLO =

VEN/UVLO −5V

(

)

(1)

With the addition of an external resistor divider as shown

in Figure 2, the EN/UVLO pin can be used to establish a

custom input undervoltage lockout threshold. The buck-

boost converter is enabled when the EN/UVLO pin reaches

1.205V which allows the rising UVLO threshold to be

set via the resistor divider ratio. Once the EN/UVLO pin

reaches the threshold voltage, the comparator switches

and the buck-boost converter is enabled. When the part

is enabled an internal 0.25µA (typical) current source is

enabled which sources current out of the EN/UVLO pin

raising the EN/UVLO pin voltage away from the threshold.

1.205V

0.8V

VIN

0.25µA

LT3120

ENA

3120 F02

EN/UVLO

ENABLE

SWITCHING

INPUT LOGIC

THRESHOLDS

ENABLE

VCC REGULATOR AND

CONTROL CIRCUITS

–

Inordertodisablethepart,VINmustbereducedsufficiently

to overcome the hysteresis generated by this current as

well as the 90mV hysteresis of the EN/UVLO comparator.

As a result, the amount of hysteresis can be independently

programmed without affecting the rising UVLO threshold

by scaling the values of both resistors.

VCC REGULATOR

An internal low dropout regulator generates the 3.73V

(nominal) VCC rail from VIN. The VCC rail powers the in-

ternal control circuitry and powers device gate drivers of

the LT3120. In addition to powering the internal circuitry

of the LT3120, the VCC regulator can also support an ex-

ternal load of 10mA. The VCC regulator is disabled when

the EN/UVLO pin is below its logic threshold and enabled

when the EN/UVLO pin is above its logic threshold. The

VCCregulatorincludescurrentlimitprotectiontosafeguard

against short-circuiting and overload conditions. For ap-

Figure 2. Accurate EN/UVLO Pin Comparator

OPERATION

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