MCP651-ESLASHMF datasheet(24/44 Pages) MICROCHIP

24 / 44 page

MCP651/2/5

DS22146A-page 24

Figure 4-7 shows a capacitive load (CL), which is

driven by a sine wave with DC offset. The capacitive

load causes the op amp to output higher currents at

higher frequencies. Because the output rectifies IOUT,

the op amp’s dissipated power increases (even though

the capacitor does not dissipate power).

FIGURE 4-8:

Diagram for Capacitive Load

Power Calculations.

The output voltage is assumed to be:

EQUATION 4-4:

The op amp’s currents are:

EQUATION 4-5:

The op amp’s instantaneous power, average power

and peak power are:

EQUATION 4-6:

The power dissipated in a package depends on the

powers dissipated by each op amp in that package:

EQUATION 4-7:

The maximum ambient to junction temperature rise

(

ΔTJA) and junction temperature (TJ) can be calculated

using the maximum expected package power (PPKG),

ambient temperature (TA) and the package thermal

resistance (

θJA) found in Table 1-4:

EQUATION 4-8:

The worst case power de-rating for the op amps in a

particular package can be easily calculated:

EQUATION 4-9:

Several techniques are available to reduce

ΔTJA for a

given package:

• Reduce

θJA

- Use another package

- Improve the PCB layout (ground plane, etc.)

- Add heat sinks and air flow

• Reduce max(PPKG)

- Increase RL

- Decrease CL

- Limit IOUT using RISO (see Figure 4-9)

- Decrease VDD

MCP65X

VDD

VOUT

IDD

ISS

IOUT

VSS

VOUT

VDC VAC

ωt

()

sin

Where:

VDC =DC offset (V)

VAC = Peak output swing (VPK)

ω = Radian frequency (2π f) (rad/s)

IOUT

dVOUT

-----------------

⋅

VACωCL

ωt

()

cos

IDD IQ max 0 IOUT

()

≈

ISS

– Q min 0 IOUT

()

≈

Where:

IQ = Quiescent supply current for one

op amp (mA/amplifier)

POA

IDD VDD VOUT

–

() I

SS VSS

VOUT

–

()

ave POA

()

VDD VSS

–

() I

4VAC fCL

------------------------

⎝⎠

⎛⎞

max POA

()

VDD VSS

–

() I

2VAC fCL

()

PPKG

POA

∑

Where:

n = Number of op amps in package (1 or 2)

ΔT

PPKGθJA

TA ΔTJA

PPKG

TJmax TA

–

--------------------------

≤

Where:

TJmax = Absolute maximum junction

temperature (°C)

TA = Ambient temperature (°C)

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