LTC3565IDDPBF datasheet(16/22 Pages) LINER

16 / 22 page

LTC3565

3565fa

Although a buck regulator is capable of providing the full

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

inputvoltageVINdropstowardVOUT,theloadstepcapability

does decrease due to the decreasing voltage across the

inductor. Applications that require large load step capabil-

ity near dropout should use a different topology such as

SEPIC, Zeta or single inductor, positive buck/boost.

Insomeapplications,amoreseveretransientcanbecaused

byswitchinginloadswithlarge(>1µF)inputcapacitors.The

discharged input capacitors are effectively put in parallel

with COUT, causing a rapid drop in VOUT. No regulator can

deliverenoughcurrenttopreventthisproblem,iftheswitch

connectingtheloadhaslowresistanceandisdrivenquickly.

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 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 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 LTC3565 circuits: 1) LTC3565 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

andcontrolcurrents.VINcurrentresultsinasmall(<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

fromswitchingthegatecapacitanceofthepowerMOSFETs.

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 is “chopped” between the internal top

and bottom switches. Thus, the series resistance look-

ing 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 frequency. Other

applications inForMation

Figure 3. Power Loss vs Load Currrent

LOAD CURRENT (mA)

VOUT = 1.2V

VOUT = 1.5V

VOUT = 1.8V

0.0001

0.001

0.1

100

1000

10000

3565 F05

0.01

0.1

VIN = 3.6V

fO = 1MHz

VOUT = 1.2V - 1.8V

LTC3565IDDPBF Datasheet(PDF) 16 Page - Linear Technology

LTC3565IDDPBF Datasheet(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL