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LTC3633

3633f

APPLICATIONS INFORMATION

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, three main sources usually account for most of

the losses in LTC3633 circuits: 1) I2R losses, 2) switching

losses and quiescent power loss 3) transition losses and

other losses.

1. 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 power MOSFETs. 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)forboththetopandbottomMOSFETscanbe

obtained from the Typical Performance Characteristics

curves. Thus to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

2. The internal LDO supplies the power to the INTVCC rail.

The total power loss here is the sum of the switching

losses and quiescent current losses from the control

circuitry.

Each time a power 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

INTVCC that is typically much larger than the DC control

bias current. In continuous mode, IGATECHG = f(QT + QB),

where QT and QB are the gate charges of the internal

top and bottom power MOSFETs and f is the switching

frequency. For estimation purposes, (QT + QB) on each

LTC3633 regulator channel is approximately 2.3nC.

To calculate the total power loss from the LDO load,

simply add the gate charge current and quiescent cur-

rent and multiply by VIN:

PLDO = (IGATECHG + IQ) • VIN

3. Other “hidden” losses such as transition loss, cop-

per trace resistances, and internal load currents can

account for additional efﬁciency degradations in the

overall power system. Transition loss arises from the

brief amount of time the top power MOSFET spends

in the saturated region during switch node transitions.

The LTC3633 internal power devices switch quickly

enough that these losses are not signiﬁcant compared

to other sources.

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 LTC3633 requires the exposed package backplane

metal (PGND) to be well soldered to the PC board to

provide good thermal contact. This gives the QFN and

TSSOP packages exceptional thermal properties, which

are necessary to prevent excessive self-heating of the part

in normal operation.

In a majority of applications, the LTC3633 does not dis-

sipate much heat due to its high efﬁciency and low thermal

resistance of its exposed-back QFN package. However, in

applications where the LTC3633 is running at high ambi-

ent temperature, high VIN, high switching frequency, and

maximum output current load, 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 until temperature

returns to 140°C.

To prevent the LTC3633 from exceeding the maximum

junction temperature of 125°C, the user will need to do

some thermal analysis. The goal of the thermal analysis

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