OPA452FASLASH500G3 datasheet(12/21 Pages) TI1

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OPA452, 453

SBOS127C

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INCREASING OUTPUT CURRENT

In those applications where the 50mA of output current is not

sufficient to drive the desired load, output current can be

increased by connecting two or more OPA452s or OPA453s

in parallel, as shown in Figure 7. Amplifier A1 is the master

amplifier and may be configured in virtually any op amp

circuit. Amplifier A2, the slave, is configured as a unity gain

buffer. Alternatively, external output transistors can be used

to boost output current. The circuit in Figure 8 is capable of

supplying output currents up to 1A. Alternatively, the OPA547,

OPA548, and OPA549 series power op amps should be

considered for high output current drive, along with program-

mable current limit and output disable capability.

FIGURE 7. Parallel Amplifiers Increase Output Current Ca-

pability.

INPUT PROTECTION

The OPA452 and OPA453 feature internal clamp diodes to

protect the inputs when voltages beyond the supply rails are

encountered. However, input current should be limited to

5mA. In some cases, an external series resistor may be

required. Many input signals are inherently current-limited,

therefore, a limiting resistor may not be required. Please

consider that a large series resistor, in conjunction with the

input capacitance, can affect stability.

USING THE OPA453 IN LOW GAINS

The OPA453 is intended for applications with signal gains of

5 or greater, but it is possible to take advantage of its high

slew rate in lower gains using an external compensation

technique in an inverting configuration. This technique main-

tains low noise characteristics of the OPA453 architecture at

low frequencies. Depending on the application, a small in-

crease in high-frequency noise may result. This technique

shapes the loop gain for good stability while giving an easily

controlled 2nd-order low-pass frequency response.

Considering only the noise gain (noninverting signal gain) for

the circuit of Figure 9, the low-frequency noise gain (NG1) will

be set by the resistor ratios, whereas the high-frequency

noise gain (NG2) will be set by the capacitor ratios. The

capacitor values set both the transition frequencies and the

high-frequency noise gain. If this noise gain, determined by

NG2 = 1 + CS/CF, is set to a value greater than the recom-

mended minimum stable gain for the op amp and the noise

gain pole, set by 1/RFCF, is placed correctly, a very well

controlled, 2nd-order low-pass frequency response will result.

To choose the values for both CS and CF, two parameters

and only three equations need to be solved. First, the target

for the high-frequency noise gain (NG2) should be greater

than the minimum stable gain for the OPA453. In the circuit

in Figure 9, a target NG2 of 10 is used. Second, the signal

gain of –1 in Figure 10 sets the low-frequency noise gain to

NG1 = 1 + RF/RG (= 2 in this example). Using these two gains,

knowing the Gain Bandwidth Product (GBP) for the OPA453

(7.5MHz), and targeting a maximally flat 2nd-order, low-pass

Butterworth frequency response (Q = 0.707), the key fre-

quency in the compensation can be found.

For the values in Figure 9, the f–3dB will be approximately

180kHz. This is less than that predicted by simply dividing the

GBP by NG1. The compensation network controls the band-

width to a lower value while providing good slew rate at the

output and an exceptional distortion performance due to

increased loop gain at frequencies below NG1 • Z0. The

capacitor values in Figure 10 are calculated for NG1 = 2 and

NG2 = 10 with no adjustment for parasitics.

Actual circuit values can be optimized by checking the small-

signal step response with actual load conditions. See Figure 9

for the small-signal step response of this OPA453, G = –1

circuit with a 1000pF load. It is well-behaved with no tendency

to oscillate. If CS and CF were removed, the circuit would be

unstable.

FIGURE 8. External Output Transistors Boost Output Cur-

rent Up to 1 Amp.

OPA452

“SLAVE”

“MASTER”

(1)

Ω

(1)

Ω

NOTE: (1) R

S resistors minimize the circulating

current that can flow between the two devices

due to V

OS errors.

OPA452

TIP30C

TIP29C

+40V

–40V

(1)

100

Ω

NOTE: (1) R

3 provides current limit and allows the amplifier to

drive the load when the output is between 0.7V and –0.7V.

0.2

Ω

0.2

Ω

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