ADP5135ACPZ-R7 datasheet(22/24 Pages) AD

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ADP5135

Data Sheet

Rev. 0 | Page 22 of 24

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 P-channel MOSFET gate capacitance.

CGATE_N is the N-channel MOSFET gate capacitance.

For the ADP5135, the total of (CGATE_P + CGATE_N) is

approximately 150 pF.

The transition losses occur because the P-channel power

MOSFET cannot be turned on or off instantaneously, and the

SWx node takes some time to slew from near ground to near

VOUT1 (and from VOUT1 to ground). The amount of transition

loss (PTRAN) is calculated by

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

(11)

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

respectively, of the switching node, SWx. For the ADP5135, the

rise and fall times of SWx are in the order of 5 ns.

If Equation 1 to Equation 11 and their associated 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, include a sufficient safety margin in

the estimate.

The total power dissipation in the ADP5135 simplifies to

PD = PDBUCK1 + PDBUCK2 + PDBUCK3

(12)

JUNCTION TEMPERATURE

In cases where the board temperature, TA, is known, the

thermal resistance parameter, θJA, can be used to estimate the

junction temperature rise. TJ is calculated from TA and PD using

the formula

TJ = TA + (PD × θJA)

(13)

Refer to Table 6 for the thermal resistance values of the LFCSP

package. A very important factor to consider is that θJA is based

on a 4-layer, 4 in × 3 in, 2.5 oz copper printed circuit board

(PCB), as per JEDEC standard, and real applications may use

different sizes and layers. It is important to maximize the copper

used to remove the heat from the device. Copper exposed to air

dissipates heat better than copper used in the inner layers.

Solder the exposed pad to the ground plane with several vias.

If the case temperature can be measured, the junction

temperature is calculated by

TJ = TC + (PD × θJC)

(14)

where TC is the case temperature and θJC is the junction to case

thermal resistance provided in Table 6.

When designing an application for a particular ambient tempera-

ture range, calculate the expected ADP5135 power dissipation

(PD) due to the losses of all channels by using Equation 8 to

Equation 12. From this power calculation, the junction tempera-

ture, TJ, can be estimated using Equation 13.

The reliable operation of the converter can be achieved only

if the estimated die junction temperature of the ADP5135

(Equation 14) is less than 125°C. Reliability and mean time

between failures (MTBF) are highly affected by increasing the

junction temperature. Additional information about product

reliability can be found in the ADI Reliability Handbook at

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