LTC3544EUD-TRPBF 查看數據表（PDF） - Linear Technology

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LTC3544EUD-TRPBF

Quad Synchronous Step-Down Regulator: 2.25MHz, 300mA, 200mA, 200mA, 100mA

Linear Technology 

LTC3544

APPLICATIONS INFORMATION

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, simply add RSW to

RL and multiply the result by the square of the average

output current.

Other losses when in switching operation, including CIN

and COUT ESR dissipative losses and inductor core losses,

generally account for less than 2% total additional loss.

Thermal Considerations

The LTC3544 requires the package backplane metal to be

well soldered to the PC board. This gives the QFN package

exceptional thermal properties, making it difficult in normal

operation to exceed the maximum junction temperature

of the part. In most applications the LTC3544 does not

dissipate much heat due to its high efficiency. In appli-

cations where the LTC3544 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 it is not well

thermally grounded. If the junction temperature reaches

approximately 150°C, the power switches will be turned

off and the SW nodes will become high impedance.

To avoid the LTC3544 from exceeding the maximum junc-

tion temperature, the user will need to do some thermal

analysis. The goal of the thermal analysis is to determine

whether the power dissipated exceeds 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.

12

As an example, consider the LTC3544 in dropout at an

input voltage of 2.5V, a total load current (all four regula-

tors) of 800mA and an ambient temperature of 85°C. From

the Typical Performance graphs of switch resistance, the

RDS(ON) of the 300mA P-channel switch at 85°C can be

estimated as 0.67Ω. Therefore, power dissipated by the

300mA channel is:

PD = ILOAD2 • RDS(ON) = 60mW

Similar analysis on the other channels gives a total power

dissipation of 138mW. For the 3mm × 3mm QFN package,

the θJA is 68°C/W. Thus, the junction temperature of the

regulator is:

TJ = 85°C + (0.138)(68) = 94.4°C

which is well below the maximum junction temperature

of 125°C.

Note that at higher supply voltages, the junction tempera-

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

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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