LTC3548AIDDPBF datasheet(13/20 Pages) LINER

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LTC3548A

3548af

APPLICATIONS INFORMATION

For approximately a 1ms ramp time, use RSS = 4.7M and

CSS = 680pF at VIN = 3.3V.

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, 4 main sources usually account for most of the

losses in LTC3548A circuits: 1. VIN quiescent current, 2.

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

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

the Electrical Characteristics section 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 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 continuous 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 ﬂows

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 (D) as

follows:

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

The RDS(ON)forboththetopandbottomMOSFETscanbe

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

account for less than 2% total additional loss.

Thermal Considerations

In a majority of applications, the LTC3548A does not

dissipate much heat due to its high efﬁciency. However,

in applications where the LTC3548A is running at high

ambient temperature with low supply voltage and high

duty cycles, such as in dropout, the heat dissipated may

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 150°C,

both power switches will be turned off and the SW node

will become high impedance.

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