LTC3602 datasheet(25/36 Pages) LINER

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LTC3613

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APPLICATIONS INFORMATION

In some applications, a more severe transient can be caused

by switching in loads with large (>10μF) input capacitors.

If the switch connecting the load has low resistance and

is driven quickly, then the discharged input capacitors are

effectively put in parallel with COUT, causing a rapid drop in

VOUT. No regulator can deliver enough current to prevent

this problem. The solution is to limit the turn-on speed of

the load switch driver. A Hot Swap™ controller is designed

specifically for this purpose and usually incorporates cur-

rent limiting, short-circuit protection and soft starting.

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 most 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, four main sources account for

most of the losses:

1. I2R losses. These arise from the resistances of the

MOSFETs, inductor and PC board traces and cause

the efficiency to drop at high output currents. In

continuous mode the average output current flows

though the inductor L, but is chopped between the

top and bottom MOSFETs.

2. Transition loss. This loss arises from the brief amount

of time the top MOSFET spends in the saturated region

during switch node transitions. It depends upon the

input voltage, load current, driver strength and MOSFET

capacitance, among other factors. The loss is significant

at input voltages above 20V.

3. INTVCC current. This is the sum of the MOSFET driver

and control currents. The MOSFET driver current re-

sults 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

controller IQ current.

Supplying INTVCC power through EXTVCC could save

several points of efficiency, especially for high VIN ap-

plications. Connecting EXTVCC to an output-derived

source will scale the VIN current required for the driver

and controller circuits by a factor of Duty Cycle/Effi-

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.

4. CIN loss. The input capacitor has the difficult job of

filtering the large RMS input current to the regulator. It

must have a very low ESR to minimize the AC I2R loss

and sufficient capacitance to prevent the RMS current

from causing additional upstream losses in cabling,

fuses or batteries.

Other losses, which include the COUT ESR loss, bottom

MOSFET reverse-recovery loss and inductor core loss

generally account for less than 2% additional loss.

When making adjustments to improve efficiency, the input

current is the best indicator of changes in efficiency. If you

make a change and the input current decreases, then the

efficiency has increased. If there is no change in input

current there is no change in efficiency.

Power losses in the switching regulator will reflect as a

longer than ideal on-time. This efficiency accounted on-

time in continuous mode can be calculated as:

tON(REAL) ≈

tON(IDEAL)

Efficiency

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