LTC3834-1 Datasheet, PDF(18/28 Page) Linear Technology – 30μA IQ Synchronous Step-Down Controller

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LTC3834-1 Datasheet, PDF (18/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

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LTC3834-1

APPLICATIO S I FOR ATIO

If the external clock frequency is greater than the internal

oscillatorâs frequency, fOSC, then current is sourced con-

tinuously from the phase detector output, pulling up the

PLLLPF pin. When the external clock frequency is less

than fOSC, current is sunk continuously, pulling down the

PLLLPF pin. If the external and internal frequencies are the

same but exhibit a phase difference, the current sources

turn on for an amount of time corresponding to the phase

difference. The voltage on the PLLLPF pin is adjusted until

the phase and frequency of the internal and external

oscillators are identical. At the stable operating point, the

phase detector output is high impedance and the filter

capacitor CLP holds the voltage.

The loop filter components, CLP and RLP, smooth out the

current pulses from the phase detector and provide a

stable input to the voltage-controlled oscillator. The filter

components CLP and RLP determine how fast the loop

acquires lock. Typically RLP = 10k and CLP is 2200pF

to 0.01Î¼F.

Typically, the external clock (on PLLIN/MODE pin) input

high threshold is 1.6V, while the input low thres-

hold is 1.2V.

Table 1 summarizes the different states in which the

PLLLPF pin can be used.

Table 1

PLLLPF PIN

Floating

INTVCC

RC Loop Filter

PLLIN/MODE PIN

DC Voltage

Clock Signal

FREQUENCY

250kHz

400kHz

530kHz

Phase-Locked to External Clock

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the LTC3834-1 is capable of turning on the top

MOSFET. It is determined by internal timing delays and the

gate charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that

tON(MIN)

VOUT

VIN(f)

If the duty cycle falls below what can be accommodated by

the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTC3834-1 is approximately

200ns. However, as the peak sense voltage decreases the

minimum on-time gradually increases up to about 250ns.

This is of particular concern in forced continuous applica-

tions with low ripple current at light loads. If the duty cycle

drops below the minimum on-time limit in this situation,

a significant amount of cycle skipping can occur with

correspondingly larger current and voltage ripple.

38341f

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