LTC3834 Datasheet, PDF(20/28 Page) Linear Technology – 30μA IQ Synchronous Step-Down Controller

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LTC3834 Datasheet, PDF (20/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

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LTC3834

APPLICATIONS INFORMATION

Table 2 summarizes the different states in which the

PLLLPF pin can be used.

Table 2

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 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 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

applications with low ripple current at light loads. If the

duty cycle drops below the minimum on-time limit in this

situation, a signiï¬cant amount of cycle skipping can occur

with correspondingly larger current and voltage ripple.

Efï¬ciency Considerations

The percent efï¬ciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efï¬ciency and which change would

produce the most improvement. Percent efï¬ciency can

be expressed as:

%Efï¬ciency = 100% â (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC3834 circuits: 1) IC VIN current, 2) INTVCC

regulator current, 3) I2R losses, 4) Topside MOSFET

transition losses.

1. The VIN current has two components: the ï¬rst is the

DC supply current given in the Electrical Characteristics

table, which excludes MOSFET driver and control cur-

rents; the second is the current drawn from the 3.3V

linear regulator output. VIN current typically results in

a small (< 0.1%) loss.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results

from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge dQ moves

from INTVCC to ground. The resulting dQ/dt is a cur-

rent out of INTVCC that is typically much larger than the

control circuit current. In continuous mode, IGATECHG

= f(QT + QB), where QT and QB are the gate charges of

the topside and bottom side MOSFETs.

Supplying INTVCC power through the EXTVCC switch

input from an output-derived source will scale the VIN

current required for the driver and control circuits by

a factor of (Duty Cycle)/(Efï¬ciency). For example, in a

20V to 5V application, 10mA of INTVCC current results

in approximately 2.5mA of VIN current. This reduces

the mid-current loss from 10% or more (if the driver

was powered directly from VIN) to only a few percent.

3. I2R losses are predicted from the DC resistances of the

fuse (if used), MOSFET, inductor, current sense resis-

tor, and input and output capacitor ESR. In continuous

mode the average output current ï¬ows through L and

RSENSE, but is âchoppedâ between the topside MOSFET

and the synchronous MOSFET. If the two MOSFETs have

approximately the same RDS(ON), then the resistance

of one MOSFET can simply be summed with the resis-

tances of L, RSENSE and ESR to obtain I2R losses. For

output capacitance losses), then the total resistance

13% as the output current increases from 1A to 5A for

3834fb

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