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LTC3413

3413fc

APPLICATIONS INFORMATION

The junction temperature, TJ, is given by:

TJ = TA + TR

where TA is the ambient temperature.

As an example, consider the LTC3413 in dropout at an

input voltage of 3.3V, a load current of 3A and an ambi-

ent temperature of 70°C. From the Typical Performance

graph of switch resistance, the RDS(ON) of the P-channel

switch at 70°C is approximately 97mΩ. Therefore, power

dissipated by the part is:

PD = (ILOAD2)(RDS(ON)) = (3A)2(97mΩ) = 0.87W

For the TSSOP package, the

θJA is 38°C/W. Thus the junc-

tion temperature of the regulator is:

TJ = 70°C + (0.87W)(38°C/W) = 103°C

which is below the maximum junction temperature of

125°C.

Notethatathighersupplyvoltages,thejunctiontemperature

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 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

Figure 1a will provide adequate compensation for most

applications.

Output Voltage Tracking of VREF

For applications in which the VREF pin is connected to

the VIN pin, the output voltage will be equal to one-half

of the voltage on the VIN pin. Because the output voltage

will track the input voltage, any disturbance on VIN will

appear on VOUT. For example, a load step transient could

cause the input voltage to drop if there is insufﬁcient bulk

capacitance at the VIN pin. The corresponding drop in the

output voltage during the load step transient is caused by

the VOUT tracking of VIN and should not be confused with

poor load regulation.

Design Example

As a design example, consider using the LTC3413 in an

application with the following speciﬁcations: VIN = 2.5V,

VOUT = 1.25V, IOUT(MAX) = ±3A, f = 1MHz.

First, calculate the timing resistor:

OSC =Ω =

323 10

110

313

.•

•

–

Use a standard value of 309k. Next, calculate the inductor

value for about 40% ripple current:

MHz

= ⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

125

11 2

125

•.

–

μH

Using a 0.47μH inductor results in a maximum ripple

current of:

Δ=

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

MHz

125

10 47

125

•.

–

== 133

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. For this design,

two 100μF ceramic capacitors will be used. CIN should be

sized for a maximum current rating of:

RMS

⎛

⎝⎜

⎞

⎠⎟

125

11 5

–.

Decoupling the PVIN pins with two 100μF capacitors is

adequate for most applications. Connect the VREF pin

directly to SVIN. Connecting the VFB pin directly to VOUT

will set the output voltage equal to one-half of the volt-

age on the VREF pin. The complete circuit for this design

example is illustrated in Figure 3.

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