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LTC3784

3784fb

For more information www.linear.com/LTC3784

applicaTions inForMaTion

Table 2 summarizes the different states in which the FREQ

pin can be used.

Table 2.

FREQ PIN

PLLIN/MODE PIN

FREQUENCY

DC Voltage

350kHz

INTVCC

DC Voltage

535kHz

Resistor

DC Voltage

50kHz to 900kHz

Any of the Above

External Clock

Phase Locked to

External Clock

Minimum On-Time Considerations

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

that the LTC3784 is capable of turning on the bottom

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.

In forced continuous mode, if the duty cycle falls below

what can be accommodated by the minimum on-time,

the controller will begin to skip cycles but the output will

continuetoberegulated.Morecycleswillbeskippedwhen

VIN increases. Once VIN rises above VOUT, the loop keeps

the top MOSFET continuously on. The minimum on-time

for the LTC3784 is approximately 110ns.

Efficiency Considerations

The percent efficiency 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 efficiency and which change would

produce the greatest improvement. Percent efficiency

can be expressed as:

%Efficiency = 100% – (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, five main sources usually account for most of the

losses in LTC3784 circuits: 1) IC VBIAS current, 2) INTVCC

regulatorcurrent,3)I2Rlosses,4)bottomMOSFETtransi-

tion losses, 5) body diode conduction losses.

1. The VBIAS current is the DC supply current given in the

ElectricalCharacteristicstable,whichexcludesMOSFET

driver and control currents. VBIAS 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 current

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.

3. DC I2R losses. These arise from the resistances of the

MOSFETs,sensingresistor,inductorandPCboardtraces

and cause the efficiency to drop at high output currents.

4. Transition losses apply only to the bottom MOSFET(s),

and become significant only when operating at low

input voltages. Transition losses can be estimated from:

Transition Loss

=(1.7)

VOUT3

VIN

•

IOUT(MAX)

•CRSS • f

5. Body diode conduction losses are more significant at

higher switching frequency. During the dead time, the loss

in the top MOSFETs is IOUT • VDS, where VDS is around

0.7V. At higher switching frequency, the dead time be-

comes a good percentage of switching cycle and causes

the efficiency to drop.

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