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LTC3602

3602fb

APPLICATIONS INFORMATION

Thermal Considerations

In most applications, the LTC3602 does not dissipate much

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

LTC3602 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 prevent the LTC3602 from exceeding the maximum

junction 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.

As an example, consider the LTC3602 in dropout at an

input voltage of 8V, a load current of 2.5A and an ambi-

ent temperature of 70°C. From the Typical Performance

graph of Switch Resistance, the RDS(ON) of the top switch

at 70°C is approximately 120mΩ. Therefore, power dis-

sipated by the part is:

PD = (ILOAD2)(RDS(ON)) = (2.5A)2(120mΩ) = 0.75W

For the TSSOP package, the

θJA is 38°C/W. Thus the junc-

tion temperature of the regulator is:

TJ = 70°C + (0.75W)(38°C/W) = 98.5°C

which is below the maximum junction temperature of

125°C.

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, generating a feedback error signal used by the

regulator 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. The ITH

pin external components and output capacitor shown in the

front page application will provide adequate compensation

for most applications.

Design Example

As a design example, consider using the LTC3602 in

an application with the following speciﬁcations: VIN =

8.4V, VOUT = 3.3V, IOUT(MAX) = 2.5A, IOUT(MIN) = 100mA,

f= 1MHz. Because efﬁciency is important at both high and

low load current, Burst Mode operation will be utilized.

First, calculate the timing resistor:

MHz

OSC ==

115 10

105

.•

–

Next, calculate the inductor value for about 40% ripple

current at maximum VIN:

3.3V

1MHz

() 1A

()

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟•1–

3.3V

8.4V

⎛

⎝

⎜

⎞

⎠

⎟=2µH

Using a 2.2μH inductor results in a maximum ripple cur-

rent of:

ΔI

L =

3.3V

1MHz

() 2.2µH

()

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟•1–

3.3V

8.4V

⎛

⎝

⎜

⎞

⎠

⎟ = 0.91A

COUT will be selected based on the ESR that is required

to satisfy the output voltage ripple requirement and the

bulk capacitance needed for loop stability. In this applica-

tion, a tantalum capacitor will be used to provide the bulk

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