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LTC3549

3549f

Thermal Considerations

In most applications the LTC3549 does not dissipate much

heat due to its high efﬁciency. But, in applications where the

LTC3549 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.

To avoid the LTC3549 from exceeding the maximum

junction temperature, the user will need to do a thermal

analysis. The goal of the thermal analysis is to determine

whether the operating conditions exceed the maximum

junction temperature of the part. The temperature rise is

given by:

TR = (PD)(θJA)

where PD is the power dissipated by the regulator and θJA

is the thermal resistance from the junction of the die to

the ambient temperature.

The junction temperature, TJ, is given by:

TJ = TA + TR

where TA is the ambient temperature.

As an example, consider the LTC3549 in dropout at an input

voltage of 1.6V, a load current of 250mA and an ambient

temperature of 75°C. In the Switch Resistance graph

shown in the Typical Performance Characteristics, the

RDS(ON) of the P-channel switch at 75°C is approximately

0.8Ω. Therefore, power dissipated by the part is:

PD = ILOAD

2 • R

DS(ON) = 50mW

For the DCB6 package, the θJA is 64°C/W. Thus, the junc-

tion temperature of the regulator is:

TJ = 75°C + (0.05)(64) = 78.2°C

which is well below the maximum junction temperature

of 125°C.

Note that at higher supply voltages, the junction temperature

is lower due to reduced switch resistance (RDS(ON)).

APPLICATIO S I FOR ATIO

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to (ΔILOAD • ESR), where ESR is the effective series

resistance of COUT. ΔILOAD also begins to charge or dis-

charge COUT, which generates a feedback error signal. The

regulator loop then acts to return VOUT to its steady state

value. During this recovery time VOUT can be monitored

for overshoot or ringing that would indicate a stability

problem. For a detailed explanation of switching control

loop theory, see Application Note 76.

A second, more severe transient is caused by switching

in loads with large (> 1µF) supply bypass capacitors. The

discharged bypass 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

load switch resistance is low and it is driven quickly. The

only solution is to limit the rise time of the switch drive

so that the load rise time is limited to approximately (25

• CLOAD). Thus, a 10µF capacitor charging to 3.3V would

require a 250µs rise time, limiting the charging current

to about 130mA.

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