MCP621 datasheet(24/44 Pages) MICROCHIP

24 / 44 page

MCP621/2/5

DS22188A-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 Section “Temperature

Specifications”:

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

VDD

VOUT

IDD

ISS

IOUT

VSS

MCP62X

OUT

ωt

()

sin

Where:

VDC =DC offset (V)

VAC = Peak output swing (VPK)

ω = Radian frequency (2π f) (rad/s)

OUT

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

⋅

ACωCL

ωt

()

cos

max 0 I

OUT

()

≈

–

min 0 I

OUT

()

≈

Where:

IQ = Quiescent supply current for one op

amp (mA/amplifier)

DD VDD

OUT

–

() I

SS VSS

OUT

–

()

ave P

()

–

() I

AC fCL

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

⎝⎠

⎛⎞

max P

()

–

() I

AC fCL

()

PKG

∑

Where:

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

ΔT

PKGθJA

ΔT

PKG

Jmax

–

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

≤

Where:

TJmax = Absolute maximum junction

temperature (°C)

TA = Ambient temperature (°C)

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