LTC3546EUFDPBF datasheet(21/28 Pages) LINER

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LTC3546

3546fb

APPLICATIONS INFORMATION

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 VINdropstowardVOUT,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.

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 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% – (P1 + P2 + P3+…)

where P1, P2, 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 LTC3546 circuits: 1) LTC3546 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 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 moves from

VIN to ground. The resulting charge over the switching

period is a current out of VIN that is typically much larger

than the DC bias current. 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 the 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 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 Character-

istics curves. Thus, to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

Where RL is the resistance of the inductor.

4. Other hidden losses such as copper trace and inter-

nal battery resistances can account for additional ef-

ﬁciency 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 generally account for less than 2% total

additional loss.

Thermal Considerations

The LTC3546 requires the backplane metal (Pin 29) to be

well soldered to the PC board. This gives the UFD pack-

age exceptional thermal properties, compared to similar

packages of this size, making it difﬁcult in normal opera-

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