LTC3405 datasheet(11/16 Pages) LINER

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LTC3405

3405fa

ambient temperature of 70°C. From the typical perfor-

mance graph of switch resistance, the RDS(ON) of the

P-channel switch at 70°C is approximately 0.94Ω. There-

fore, power dissipated by the part is:

PD = ILOAD2 • RDS(ON) = 84.6mW

For the SOT-23 package, the θJA is 250°C/ W. Thus, the

junction temperature of the regulator is:

TJ = 70°C + (0.0846)(250) = 91.15°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 (∆ILOAD • ESR), where ESR is the effective series

resistance of COUT. ∆ILOAD also begins to charge or

discharge 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 moni-

tored 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 parallel

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.

APPLICATIO S I FOR ATIO

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 Charateristics

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 including CIN and COUT ESR dissipative

losses and inductor core losses generally account for less

than 2% total additional loss.

Thermal Considerations

In most applications the LTC3405 does not dissipate

much heat due to its high efficiency. But, in applications

where the LTC3405 is running at high ambient tempera-

ture with low supply voltage and high duty cycles, such

as in dropout, the heat dissipated may exceed the maxi-

mum 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 LTC3405 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 LTC3405 in dropout at an

input voltage of 2.7V, a load current of 300mA and an

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