LTC3411 datasheet(13/20 Pages) LINER

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LTC3411

sn3411 3411fs

The output voltage settling behavior is related to the

stability of the closed-loop system and will demonstrate

the actual overall supply performance. For a detailed

explanation of optimizing the compensation components,

including a review of control loop theory, refer to Linear

Technology Application Note 76.

Although a buck regulator is capable of providing the full

output current in dropout, it should be noted that as the

input voltage VIN drops toward VOUT, the load step capa-

bility does decrease due to the decreasing voltage across

the inductor. Applications that require large load step

capability near dropout should use a different topology

such as SEPIC, Zeta or single inductor, positive buck/

boost.

In some applications, a more severe transient can be

caused by switching in loads with large (>1uF) input

capacitors. 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 prob-

lem, if the switch connecting the load has low resistance

and is driven quickly. 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 incorpo-

rates current 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 percentage

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC3411 circuits: 1) LTC3411 VIN current,

2) switching losses, 3) I2R losses, 4) other losses.

1) The VIN current is the DC supply current given in the

electrical characteristics which excludes MOSFET driver

and control currents. VIN current results in a small (<0.1%)

loss that increases with VIN, even at no load.

2) The switching 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 VIN to

ground. The resulting dQ/dt is a current out of VIN that is

typically much larger than the DC bias current. In continu-

ous mode, IGATECHG = fO(QT + QB), where QT and QB are

the gate charges of the internal top and bottom MOSFET

switches. The gate charge losses are proportional to VIN

and thus their effects will be more pronounced at higher

supply voltages.

3) I2R Losses are calculated from the DC resistances of the

internal switches, RSW, and external inductor, RL. In

continuous mode, the average output current flowing

through inductor L but is “chopped” between the internal

top and bottom switches. Thus, the series resistance

looking into the SW pin is a function of both top and

bottom MOSFET RDS(ON) and the duty cycle (DC) as

follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs can be

obtained from the Typical Performance Characteristics

curves. Thus, to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

4) Other “hidden” losses such as copper trace and internal

battery resistances can account for additional efficiency

degradations in portable systems. It is very important to

include these “system” level losses in the design of a

system. The internal battery and fuse resistance losses

can be minimized by making sure that CIN has adequate

charge storage and very low ESR at the switching fre-

quency. Other losses including diode conduction losses

during dead-time and inductor core losses generally ac-

count for less than 2% total additional loss.

APPLICATIO S I FOR ATIO

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