OPA547

10

SBOS056A

www.ti.com

THERMAL PROTECTION

The OPA547 has thermal shutdown that protects the ampli-

fier from damage. Activation of the thermal shutdown cir-

cuit during normal operation is an indication of excessive

power dissipation or an inadequate heat sink. Depending on

load and signal conditions, the thermal protection circuit

may cycle on and off. This limits the dissipation of the

amplifier but may have an undesirable effect on the load.

The thermal protection activates at a junction temperature of

approximately 160

°C. However, for reliable operation junc-

tion temperature should be limited to 150

°C. To estimate the

margin of safety in a complete design (including heat sink),

increase the ambient temperature until the thermal protection

is activated. Use worst-case load and signal conditions. For

good reliability, the thermal protection should trigger more

than 35

°C above the maximum expected ambient condition

of your application. This produces a junction temperature of

125

°C at the maximum expected ambient condition.

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 OPA547 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

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

+2.4V above the negative rail).

Output Disable

A unique feature of the OPA547 is its output disable capa-

bility. This function not only conserves power during idle

periods (quiescent current drops to approximately 4mA) but

also allows multiplexing in low frequency (f<10kHz), mul-

tichannel applications. Signals that are greater than 10kHz

may cause leakage current to increase in devices that are

shutdown. Figure 15 shows the two OPA547s in a switched

amplifier configuration. The on/off state of the two amplifi-

ers is controlled by the voltage on the E/S pin.

To disable the output, the E/S pin is pulled low, no greater

than 0.8V above the negative rail. Typically the output is

shutdown in 1

µs. Figure 5 provides an example of how to

implement this function using a single supply. Figure 6 gives

a circuit for dual supply applications. To return the output to

an enabled state, the E/S pin should be disconnected (open) or

pulled to at least (V–) + 2.4V. It should be noted that pulling

the E/S pin high (output enabled) does not disable internal

thermal shutdown.

OPA547

V+

E/S

V–

NOTE: (1) Optional—may be required to limit leakage

current of optocoupler at high temperatures.

(1)

6

1

1

4N38

Optocoupler

5

4

HCT or TTL In

5V

FIGURE 6. Output Disable with Dual Supplies.

Thermal Shutdown Status

Internal thermal shutdown circuitry shuts down the output

when the die temperature reaches approximately 160

°C, reset-

ting when the die has cooled to 140

°C. The E/S pin can be

monitored to determine if shutdown has occurred. During

normal operation the voltage on the E/S pin is typically 3.5V

above the negative rail. Once shutdown has occurred this

voltage drops to approximately 350mV above the negative rail.

Figure 7 gives an example of monitoring shutdown in a

single supply application. Figure 8 provides a circuit for dual

supplies. External logic circuitry or an LED could be used to

indicate if the output has been thermally shutdown, see

Figure 13.

FIGURE 7. Thermal Shutdown Status with a Single Supply.

FIGURE 8. Thermal Shutdown Status with Dual Supplies.

FIGURE 5. Output Disable with a Single Supply.

OPA547

V+

E/S

HCT

OR

TTL

2.49k

Ω

Zetex

ZVN3310

5V

V–

OPA547

V+

E/S

V–

1k

Ω

5V

22k

Ω

470

Ω

2N3906

Zetex

ZVN3310

OPA547

V+

E/S

V–

CMOS or TTL