OPA890IDBVT datasheet(22/34 Pages) NSC

22 / 34 page

750W

150mVOutputAdjustment

= 2

Power-supplydecoupling

notshown.

5kW

226W

0.1 F

324W

20kW

10kW

0.1 F

+5V

OPA890

+5V

200kW

2MW

80kW

Control

- S

DIS

OPA890

SBOS369B – MAY 2007 – REVISED DECEMBER 2009

www.ti.com

current out of Q1, turning the amplifier off. The supply

current in the disable mode is only that required to

operate the circuit of Figure 54. Additional circuitry

ensures that turn-on time occurs faster than turn-off

time (make-before-break).

When disabled, the output and input nodes go to a

high-impedance state. If the OPA890 is operating at a

gain of +1V/V, it shows a very high impedance at the

output and exceptional signal isolation. If operating at

a gain greater than +1V/V, the total feedback network

resistance (RF + RG) appears as the impedance

looking back into the output, but the circuit still shows

very-high forward and reverse isolation. If configured

as an inverting amplifier, the input and output are

connected through the feedback network resistance

(RF + RG) and the isolation is very poor, as a result.

Figure 53. DC-Coupled, Inverting Gain of -2V/V,

with Offset Adjustment

THERMAL ANALYSIS

Maximum desired junction temperature sets the

DISABLE OPERATION

maximum allowed internal power dissipation, as

described below. In no case should the maximum

The OPA890 provides an optional disable feature that

junction temperature be allowed to exceed +150°C.

may be used either to reduce system power or to

implement a simple channel multiplexing operation. If

Operating junction temperature (TJ) is given by TA +

the DIS control pin is left unconnected, the OPA890

PD × θJA. The total internal power dissipation (PD) is

operates normally. To disable the OPA890, the

the sum of quiescent power (PDQ) and additional

control pin must be asserted low. Figure 54 shows a

power dissipated in the output stage (PDL) to deliver

simplified internal circuit for the disable control

load power. Quiescent power is simply the specified

feature.

no-load supply current times the total supply voltage

across the part. PDL depends on the required output

signal and load, but for a grounded resistive load is at

a maximum when the output is fixed at a voltage

equal to 1/2 of either supply voltage (for equal bipolar

supplies). Under this condition, PDL = VS

2/(4 × R

where RL includes feedback network loading.

Note that it is the power in the output stage and not

into

the

load

that

determines

internal

power

dissipation.

As a worst-case example, compute the maximum TJ

using an OPA890IDBV (SOT23-6 package) in the

circuit of Figure 46 operating at the maximum

specified ambient temperature of +85°C and driving a

grounded 100

Ω load.

PD = 10V × 1.25mA + 5

2/(4 × (100Ω || 1.5kΩ)) =

Figure 54. Simplified Disable Control Circuit

79mW

Maximum TJ = +85°C + (79W × 150°C/W) = +97°C.

In normal operation, base current to Q1 is provided

through the 2M

Ω resistor, while the emitter current

Although this result is still well below the specified

through the 80k

Ω resistor sets up a voltage drop that

maximum junction temperature, system reliability

is inadequate to turn on the two diodes in the Q1

considerations may require lower operating junction

emitter. As V DIS is pulled low, additional current is

temperatures.

The

highest

possible

internal

pulled through the 80k

Ω resistor, eventually turning

dissipation occurs if the load requires current to be

on those two diodes ( 15

μA). At this point, any

forced into the output for positive output voltages, or

further current pulled out of V DIS goes through those

sourced from the output for negative output voltages.

diodes, holding the emitter-base voltage of Q1 at

This configuration puts a high current through a large

approximately 0V. This process shuts off the collector

internal voltage drop in the output transistors.

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