OPA688M

UNITY GAINSTABLE WIDEBAND

VOLTAGE LIMITING AMPLIFIER

SGLS144B − APRIL 2003 − DECEMBER 2006

20

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

thermal considerations

The OPA688 will not require heat-sinking under most operating conditions. Maximum desired junction

temperature will set a maximum allowed internal power dissipation as described below. In no case should the

maximum junction temperature be allowed to exceed 150

°C.

The total internal power dissipation (PD) is the sum of quiescent power (PDQ) and the additional power

dissipated in the output stage (PDL) while delivering load power. PDQ is simply the specified no-load supply

current times the total supply voltage across the part. PDL depends on the required output signals and loads.

For a grounded resistive load, and equal bipolar supplies, it is at a maximum when the output is at 1/2 either

supply voltage. In this condition, PDL = VS2/(4RL) where RL includes the feedback network loading. Note that

it is the power in the output stage, and not in the load, that comprises PDL.

The operating junction temperature is: TJ = TA + PD ΘJA, where TA is the ambient temperature.

For example, the maximum TJ for a OPA688M with G = +2, RFB = 402 Ω, RL = 100 Ω, and ±VS = ±5 V at the

maximum TA = + 85°C is calculated as:

P

DQ +

(10 V

20 mA)

+ 200 mW

P

DL +

(5V)

2

4

100

W ø 804 W

+ 70 mW

P

D + 200 mW ) 70 mW + 270 mW

TJ = 85°C + 270 mW × (119°C/W) = 117°C

capacitive loads

Capacitive loads, such as the input to ADCs, will decrease the amplifier’s phase margin, which may cause

high-frequency peaking or oscillations. Capacitive loads

× 2 pF should be isolated by connecting a small resistor

in series with the output as shown in Figure 40. Increasing the gain from +2 will improve the capacitive drive

capabilities due to increased phase margin.

OPA688

C

L

R

L

R

S

V

O

R L is optional

Figure 40. Driving Capacitive Loads

In general, capacitive loads should be minimized for optimum high-frequency performance. The capacitance

of coax cable (29 pF/foot for RG-58) will not load the amplifier when the coaxial cable, or transmission line, is

terminated in its characteristic impedance.