ADP2105ACPZ-1.8-R7 datasheet(21/32 Pages) AD

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ADP2105/ADP2106/ADP2107

Rev. 0 | Page 21 of 32

EFFICIENCY CONSIDERATIONS

Efficiency is defined as the ratio of output power to input power.

The high efficiency of the ADP2105/ADP2106/ADP2107 has

two distinct advantages. First, only a small amount of power is

lost in the dc-to-dc converter package that reduces thermal

constraints. In addition, high efficiency delivers the maximum

output power for the given input power, extending battery life

in portable applications.

There are four major sources of power loss in dc-to-dc

converters like the ADP2105/ADP2106/ADP2107.

•

Power switch conduction losses

•

Inductor losses

•

Switching losses

•

Transition losses

Power Switch Conduction Losses

Power switch conduction losses are caused by the flow of output

current through the P-channel power switch and the N-channel

synchronous rectifier, which have internal resistances (RDS(ON))

associated with them. The amount of power loss can be approxi-

mated by

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

where D = VOUT/VIN.

The internal resistance of the power switches increases with

temperature but decreases with higher input voltage. Figure 19

in the Typical Performance Characteristics section shows the

change in RDS(ON) vs. input voltage, while Figure 27 in the

Typical Performance Characteristics section shows the change

in RDS(ON) vs. temperature for both power devices.

Inductor Losses

Inductor conduction losses are caused by the flow of current

through the inductor, which has an internal resistance (DCR)

associated with it. Larger sized inductors have smaller DCR,

which can improve inductor conduction losses.

Inductor core losses are related to the magnetic permeability of

the core material. Because the ADP2105/ADP2106/ADP2107

are high switching frequency dc-to-dc converters, shielded ferrite

core material is recommended for its low core losses and low EMI.

The total amount of inductor power loss can be calculated by

PL = DCR × IOUT2 + Core Losses

Switching Losses

Switching losses are associated with the current drawn by the

driver to turn on and turn off the power devices at the

switching frequency. Each time a power device gate is turned on

and turned off, the driver transfers a charge ΔQ from the input

supply to the gate and then from the gate to ground.

The amount of power loss can by calculated by

PSW = (CGATE − P + CGATE − N) × VIN2 × fSW

where:

(CGATE − P + CGATE − N) ~ 600 pF.

fSW = 1.2 MHz, the switching frequency.

Transition Losses

Transition losses occur because the P-channel MOSFET power

switch cannot turn on or turn off instantaneously. At the middle of

a LX node transition, the power switch is providing all the inductor

current, while the source to drain voltage of the power switch is

half the input voltage, resulting in power loss. Transition losses

increase with load current and input voltage and occur twice for

each switching cycle.

The amount of power loss can be calculated by

OUT

TRAN

OFF

)

(

where tON and tOFF are the rise time and fall time of the LX node,

which are approximately 3 ns.

THERMAL CONSIDERATIONS

In most applications, the ADP2105/ADP2106/ADP2107 do not

dissipate a lot of heat due to their high efficiency. However, in

applications with high ambient temperature, low supply voltage,

and high duty cycle, the heat dissipated in the package is large

enough that it can cause the junction temperature of the die to

exceed the maximum junction temperature of 125°C. Once the

junction temperature exceeds 140°C, the converter goes into

thermal shutdown. It recovers only after the junction temperature

has decreased below 100°C to prevent any permanent damage.

Therefore, thermal analysis for the chosen application solution

is very important to guarantee reliable performance over all

conditions.

The junction temperature of the die is the sum of the ambient

temperature of the environment and the temperature rise of the

package due to the power dissipation, as shown in the following

equation:

TJ = TA + TR

where:

TJ is the junction temperature.

TA is the ambient temperature.

TR is the rise in temperature of the package due to power

dissipation in it.

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