OPA548F datasheet(11/24 Pages) TI

11 / 24 page

OPA548

SBOS070B

www.ti.com

HEAT SINKING

Most applications require a heat sink to assure that the

maximum operating junction temperature (125

°C) is not

exceeded. In addition, the junction temperature should be

kept as low as possible for increased reliability. Junction

temperature can be determined according to the equation:

TJ = TA + PDθJA

(1)

where,

JA = θJC + θCH + θHA

(2)

TJ = Junction Temperature (°C)

TA = Ambient Temperature (°C)

PD = Power Dissipated (W)

JC = Junction-to-Case Thermal Resistance (°C/W)

CH = Case-to-Heat Sink Thermal Resistance (°C/W)

HA = Heat Sink-to-Ambient Thermal Resistance (°C/W)

JA = Junction-to-Air Thermal Resistance (°C/W)

Figure 7 shows maximum power dissipation versus ambient

temperature with and without the use of a heat sink. Using a

heat sink significantly increases the maximum power dissipa-

tion at a given ambient temperature as shown.

The difficulty in selecting the heat sink required lies in

determining the power dissipated by the OPA548. For dc

output into a purely resistive load, power dissipation is simply

the load current times the voltage developed across the

conducting output transistor, PD = IL(VS–VO). Other loads are

not as simple. Consult Application Bulletin SBOA022 for

further insight on calculating power dissipation. Once power

dissipation for an application is known, the proper heat sink

can be selected.

Combining equations (1) and (2) gives:

TJ = TA + PD(θJC + θCH + θHA)

(3)

TJ, TA, and PD are given. θJC is provided in the specification

table, 2.5

°C/W (dc).

CH can be obtained from the heat sink

manufacturer. Its value depends on heat sink size, area, and

material used. Semiconductor package type, mounting screw

torque, insulating material used (if any), and thermal

joint compound used (if any) also affect

CH. A typical θCH for

a TO-220 mounted package is 1

°C/W. Now we can solve for

HA:

θθ

(

)

°°

°+ °

(

) =°

–

125

135

To maintain junction temperature below 125

°C, the heat sink

selected must have a

HA less than 14°C/W. In other words,

the heat sink temperature rise above ambient must be less

than 67.5

°C (13.5°C/W • 5W). For example, at 5W Thermalloy

model number 6030B has a heat sink temperature rise of

°C above ambient (

HA = 66°C/5W = 13.2°C/W), which is

below the 67.5

°C required in this example. Figure 7 shows

power dissipation versus ambient temperature for a TO-220

package with a 6030B heat sink.

Another variable to consider is natural convection versus

forced convection air flow. Forced-air cooling by a small fan

can lower

CA (θCH + θHA) dramatically. Heat sink manufac-

tures provide thermal data for both of these cases. For

additional information on determining heat sink requirements,

consult Application Bulletin SBOA021.

As mentioned earlier, once a heat sink has been selected,

the complete design should be tested under worst-case load

and signal conditions to ensure proper thermal protection.

ENABLE/STATUS (E/S) PIN

The Enable/Status pin provides two functions: forcing this pin

LOW disables the output stage, or E/S can be monitored to

determine if the OPA548 is in thermal shutdown. One or both

of these functions can be utilized on the same device using

single or dual supplies. For normal operation (output en-

abled), the E/S pin can be left open or pulled HIGH (at least

2.4V above the negative rail). A small value capacitor con-

nected between the E/S pin and V– may be required for noisy

applications.

Output Disable

A unique feature of the OPA548 is its output disable capabil-

ity. This function not only conserves power during idle peri-

ods (quiescent current drops to approximately 6mA), but also

allows multiplexing in low frequency (f < 20kHz), multichan-

nel applications. Signals greater than 20kHz may cause

leakage current to increase in devices that are shutdown.

Figure 18 shows the two OPA548s in a switched amplifier

configuration. The on/off state of the two amplifiers is con-

trolled by the voltage on the E/S pin.

Heat Sink Selection Example

A TO-220 package is dissipating 5W. The maximum ex-

pected ambient temperature is 40

°C. Find the proper heat

sink to keep the junction temperature below 125

°C (150°C

minus 25

°C safety margin).

FIGURE 7. Maximum Power Dissipation vs Ambient

Temperature.

100

125

Ambient Temperature (

°C)

MAXIMUM POWER DISSIPATION

vs AMBIENT TEMPERATURE

TO220 with Thermalloy

6030B Heat Sink

JA = 16.7°C/W

D = (TJ (max) – TA) /

J (max) = 150°C

With infinite heat sink

(

JA = 2.5°C/W),

max P

D = 50W at TA = 25°C.

DDPAK

JA = 26°C/W

(3 in2 one oz

copper mounting pad)

DDPAK or TO-220

JA = 65°C/W (no heat sink)

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