LTC3561A datasheet(15/20 Pages) LINER

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LTC3561A

3561af

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.

In some applications, a more severe transient can be caused

by switching in loads with large (>1μF) input capacitors.

The discharged input capacitors are effectively put in paral-

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

can deliver enough current to prevent this problem, 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 incorporates cur-

rent limiting, short-circuit protection, and soft-starting.

the losses in LTC3561A circuits: 1) LTC3561A 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 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

losses including diode conduction losses during dead-time

and inductor core losses which generally account for less

than 2% total additional loss.

APPLICATIONS INFORMATION

Figure 5. Power Loss vs Load Currrent

Hot Swap is a trademark of Linear Technology Corporation.

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

LOAD CURRENT (mA)

0.01

0.1

100

10000

1000

3561A F01

0.001

VIN = 3.6V

VOUT = 1.2V TO 1.8V

fO = 1MHz

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