ADP2105ACPZ-3.3-R7 datasheet(22/32 Pages) AD

22 / 32 page

ADP2105/ADP2106/ADP2107

Rev. 0 | Page 22 of 32

The rise in temperature of the package is directly proportional

to the power dissipation in the package. The proportionality

constant for this relationship is defined as the thermal

resistance from the junction of the die to the ambient

temperature, as shown in the following equation:

TR = θJA × PD

where:

TR is the rise in temperature of the package.

PD is the power dissipation in the package.

θJA is the thermal resistance from the junction of the die to the

ambient temperature of the package.

For example, consider an application where the ADP2107-1.8

is used with an input voltage of 3.6 V and a load current of 2 A.

Also, assume that the maximum ambient temperature is 85°C.

At a load current of 2 A, the most significant contributor of

power dissipation in the dc-to-dc converter package is the

conduction loss of the power switches. Using the graph of

switch resistance vs. temperature (see Figure 27), as well as the

equation of power loss given in the Power Switch Conduction

Losses section, the power dissipation in the package can be

calculated by

PSW − COND = [RDS(ON) − P × D + RDS(ON) − N × (1 − D)] × IOUT2 =

[109 mΩ × 0.5 + 90 mΩ × 0.5] × (2 A)2 ~ 400 mW

The θJA for the LFCSP_VQ package is 40°C/W, as shown in

Table 3. Thus, the rise in temperature of the package due to

power dissipation is

TR = θJA × PD = 40°C/W × 0.40 W = 16°C

The junction temperature of the converter is

TJ = TA + TR = 85°C + 16°C = 101°C

which is below the maximum junction temperature of 125°C.

Thus, this application operates reliably from a thermal point

of view.

DESIGN EXAMPLE

Consider an application with the following specifications:

Input Voltage = 3.6 V to 4.2 V.

Output Voltage = 2 V.

Typical Output Current = 600 mA.

Maximum Output Current = 1.2 A.

Soft Start Time = 2 ms.

Overshoot ≤ 100 mV under all load transient conditions.

Choose the dc-to-dc converter that satisfies the maximum

output current requirement. Because the maximum output

current for this application is 1.2 A, the ADP2106 with a

maximum output current of 1.5 A is ideal for this

application.

See whether the output voltage desired is available as a

fixed output voltage option. Because 2 V is not one of the

fixed output voltage options available, choose the adjustable

version of ADP2106.

The first step in external component selection for an

adjustable version converter is to calculate the resistance of

the resistive voltage divider that sets the output voltage.

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Calculate the minimum inductor value as follows:

For the ADP2106:

L > (0.83 μH/V) × VOUT

L > 0.83 μH/V × 2 V

L > 1.66 μH

Next, calculate the ideal inductor value that sets the

inductor peak-to-peak current ripple, ΔIL, to1/3 of the

maximum load current at the maximum input voltage.

−

μH

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(

)

(MAX

LOAD

OUT

IDEAL

μH

2.18

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−

The closest standard inductor value is 2.2 μH. The

maximum rms current of the inductor should be greater

than 1.2 A, and the saturation current of the inductor

should be greater than 2 A. One inductor that meets these

criteria is the LPS4012-2.2 μH from Coilcraft.

Choose the output capacitor based on the transient

response requirements. The worst-case load transient is

1.2 A, for which the overshoot must be less than 100 mV,

which is 5% of the output voltage. Therefore, for a 1 A load

transient, the overshoot must be less than 4% of the output

voltage. For these conditions, Figure 37 gives

Output Capacitor × Output Voltage = 60 μC

μF

μC

≈

⇒

Capacitor

Output

Next, taking into account the loss of capacitance due to dc

bias, as shown in Figure 38, two 22 μF X5R MLCC capacitors

from Murata (GRM21BR60J226M) are sufficient for this

application.

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