ADP5040ACPZ-1-R7 datasheet(32/40 Pages) AD

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ADP5040

Data Sheet

Rev. 0 | Page 32 of 40

to derive the power lost in the inductor, and then calculate the

power dissipation in the buck converter using Equation 3. Add

the power dissipated in the buck and in the LDOs to find the

total dissipated power.

Note that the buck efficiency curves are typical values and may

not be provided for all possible combinations of VIN, VOUT, and

IOUT. To account for these variations, it is necessary to include a

safety margin when calculating the power dissipated in the buck.

A third way to estimate the power dissipation is analytical and

involves modeling the losses in the buck circuit provided by

Equation 8 to Equation 11 and the losses in the LDOs provided

by Equation 12.

Buck Regulator Power Dissipation

The power loss of the buck regulator is approximated by

PLOSS = PDBUCK + PL

(3)

where:

PDBUCK is the power dissipation on the ADP5040 buck regulator.

PL is the inductor power losses.

The inductor losses are external to the device and they do not

have any effect on the die temperature.

The inductor losses are estimated (without core losses) by

RMS

OUT1

DCR

≅

)

(

(4)

where:

DCRL is the inductor series resistance.

IOUT1(RMS) is the rms load current of the buck regulator.

/12

)

(

OUT1

RMS

OUT1

(5)

where r is the normalized inductor ripple current.

R ≈ VOUT1 × (1 − D)/(IOUT1 × L × fSW)

(6)

where:

L is inductance.

FSW is switching frequency.

D is duty cycle.

D = VOUT1/VIN1

(7)

The ADP5040 buck regulator power dissipation, PDBUCK,

includes the power switch conductive losses, the switch losses,

and the transition losses of each channel. There are other

sources of loss, but these are generally less significant at high

output load currents, where the thermal limit of the application

is. Equation 8 shows the calculation made to estimate the power

dissipation in the buck regulator.

PDBUCK = PCOND + PSW + PTRAN

(8)

The power switch conductive losses are due to the output current,

IOUT1, flowing through the PMOSFET and the NMOSFET power

switches that have internal resistance, RDSON-P and RDSON-N. The

amount of conductive power loss is found by:

PCOND = [RDSON-P × D + RDSON-N × (1 – D)] × IOUT12

(9)

For the ADP5040, at 125°C junction temperature and VIN1 =

3.6 V, RDSON-P is approximately 0.2 Ω, and RDSON-N is approximately

0.16 Ω. At VIN1 = 2.3 V, these values change to 0.31 Ω and 0.21 Ω,

respectively, and at VIN1 = 5.5 V, the values are 0.16 Ω and

0.14 Ω, respectively.

Switching losses are associated with the current drawn by the

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

frequency. The amount of switching power loss is given by:

PSW = (CGATE-P + CGATE-N) × VIN12 × fSW

(10)

where:

CGATE-P is the PMOSFET gate capacitance.

CGATE-N is the NMOSFET gate capacitance.

For the ADP5040, the total of (CGATE-P + CGATE-N) is approximately

150 pF.

The transition losses occur because the PMOSFET cannot be

turned on or off instantaneously, and the SW node takes some

time to slew from near ground to near VOUT1 (and from VOUT1 to

ground). The amount of transition loss is calculated by:

PTRAN = VIN1 × IOUT1 × (tRISE + tFALL) × fSW

(11)

where tRISE and tFALL are the rise time and the fall time of the

switching node, SW. For the ADP5040, the rise and fall times of

SW are in the order of 5 ns.

If the preceding equations and parameters are used for

estimating the converter efficiency, note that the equations do

not describe all of the converter losses, and the parameter

values given are typical numbers. The converter performance

also depends on the choice of passive components and board

layout; therefore, a sufficient safety margin should be included

in the estimate.

LDO Regulator Power Dissipation

The power loss of a LDO regulator is given by:

PDLDO = [(VIN – VOUT) × ILOAD] + (VIN × IGND)

(12)

where:

ILOAD is the load current of the LDO regulator.

VIN and VOUT are input and output voltages of the LDO,

respectively.

IGND is the ground current of the LDO regulator.

Power dissipation due to the ground current is small and it

can be ignored.

The total power dissipation in the ADP5040 simplifies to:

PD = {[PDBUCK + PDLDO1 + PDLDO2]}

(13)

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