ADA4700-1ARDZ-R7 datasheet(21/28 Pages) AD

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Data Sheet

ADA4700-1

THEORY OF OPERATION

Figure 58. Simplified Schematic of the ADA4700-1

The ADA4700-1 is a high voltage operational amplifier featuring

a slew enhanced bipolar input stage that provides all of the voltage

gain. Single stage amplifiers are noted for their excellent stability

but poor open-loop gain; however, the advanced ADA4700-1

design provides gain comparable to multistage amplifiers and,

therefore, combines the advantages of both.

Referring to Figure 58, the input stage is formed by Q5 to Q8

loaded by the current mirrors, Q9 to Q14. The output stage is

of the complementary Darlington type formed by Q15 to Q18.

Like other bipolar amplifiers, the input stage is internally clamped

to prevent degradation with large differential inputs; however,

the addition of Q1 and Q2 in conjunction with the high voltage

diodes, D1 and D2, maintain high differential input impedance

even when the voltage between the inputs is equal to the supply

voltage. This configuration makes the ADA4700-1 suitable for

applications with unavoidable large differential voltages, such as

rectifiers, peak detectors, and comparators.

The ADA4700-1 uses a single-pole compensation set by C3 and

C4. The internal snubber networks, R1/C1 and R2/C2, further

enhance the stability. This design enables large capacitive loads

to be driven without the risk of oscillation.

The Q19 and Q20 transistors in conjunction with the R3 and R4

resistors provide output short-circuit protection. Additionally, a

thermal regulating circuit (not shown in Figure 58) limits the

die temperature to 145°C or greater to protect against excessive

power dissipation.

With approximately equal split supplies up to ±50 V, the output can

be shorted to ground unconditionally; however, operating this

way is not recommended.

If the voltage between the output and either supply is more than

60 V, avoid a short circuit to the supply. Transient dissipation in

the output transistors can exceed their safe operating area and

cause subsequent destruction.

THERMAL REGULATION

The circuitry for thermal regulation of the ADA4700-1 is

dependent on the ambient temperature and time duration of

the current drive. When thermal regulation of the ADA4700-1

is active, the supply current, ISY, reduces from 1.7 mA to 300 µA.

The output stage remains biased during thermal regulation due

to the parasitics of the output devices in conjunction with the

elevated die temperature. For example, with a current drive, IOUT,

of 30 mA for 180 seconds and with an ambient temperature of

85°C, the thermal regulation is triggered at a junction temperature

of 145°C with an output current level of 22 mA. For additional

information, refer to the Thermal Management section and the

Safe Operating Area section.

Ω

Q20

Q19

D10

–IN

Q17

Q15

Q18

Q10

+IN

OUT

V+

V–

Q11

Q13

Q14

Q12

Q16

Rev. 0 | Page 21 of 28

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