ADA4661-2ARMZ-R7 datasheet(23/32 Pages) AD

23 / 32 page

Data Sheet

ADA4661-2

Rev. 0 | Page 23 of 32

system voltages with proprietary protection circuitry. This regu-

lation circuitry protects the input devices from the high supply

voltages at which the amplifier can operate.

The input devices are also protected from large differential

input voltages by clamp diodes (D1 and D2). These diodes are

buffered from the inputs with two 120 Ω resistors (R1 and R2).

The diodes conduct significant current whenever the differen-

tial voltage exceeds approximately 600 mV; in this condition,

the differential input resistance falls to 240 Ω. It is possible for a

significant amount of current to flow through these protection

diodes. The user must ensure that current flowing into the input

pins is limited to the absolute maximum of 10 mA.

GAIN STAGE

The second stage of the amplifier is composed of an NPN

differential pair (Q1, Q2) and folded-cascode transistors (M13

to M20). The amplifier features nested Miller compensation

(C1 to C3).

OUTPUT STAGE

The ADA4661-2 features a complementary output stage consisting

of the M21 and M22 transistors. These transistors are configured

in a Class AB topology and are biased by the voltage source, V1.

This topology allows the output voltage to go within millivolts

of the supply rails, achieving a rail-to-rail output swing. The

output voltage is limited by the output impedance of the transis-

tors, which are low RON MOS devices. The output voltage swing

is a function of the load current and can be estimated using the

output voltage to supply rail vs. load current graphs (see Figure 20,

Figure 23, Figure 24, and Figure 27). The high voltage and high

current capability of the ADA4661-2 output stage requires the

user to ensure that it operates within the thermal safe operating

area (see the Maximum Power Dissipation section).

MAXIMUM POWER DISSIPATION

The ADA4661-2 is capable of driving an output current up

to 220 mA. However, the usable output load current drive is

limited to the maximum power dissipation allowed by the

device package. The absolute maximum junction temperature

for the ADA4661-2 is 150°C (see Table 5). The junction

temperature can be estimated as follows:

TJ = PD × θJA + TA

The power dissipated in the package (PD) is the sum of the

quiescent power dissipation and the power dissipated by the

output stage transistor. It can be calculated as follows:

PD = (VSY × ISY) + (VSY − VOUT) × ILOAD

where:

VSY is the power supply rail.

ISY is the quiescent current.

VOUT is the output of the amplifier.

ILOAD is the output load.

Do not exceed the maximum junction temperature for the

device, 150°C. Exceeding the junction temperature limit can

cause degradation in the parametric performance or even

destroy the device. To ensure proper operation, it is necessary to

observe the maximum power derating curves. Figure 71 shows

the maximum safe power dissipation in the package vs. the

ambient temperature on a standard 4-layer JEDEC board. The

exposed pad of the LFCSP package is soldered to the board.

Figure 71. Maximum Power Dissipation vs. Ambient Temperature

Refer to Technical Article MS-2251, Data Sheet Intricacies—

Absolute Maximum Ratings and Thermal Resistances, for more

information.

RAIL-TO-RAIL INPUT AND OUTPUT

The ADA4661-2 features rail-to-rail input and output with a

supply voltage from 3 V to 18 V. Figure 72 shows the input and

output waveforms of the ADA4661-2 configured as a unity-gain

buffer with a supply voltage of ±9 V. With an input voltage of

±9 V, the ADA4661-2 allows the output to swing very close to

both rails. Additionally, it does not exhibit phase reversal.

Figure 72. Rail-to-Rail Input and Output

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

100

125

150

AMBIENT TEMPERATURE (°C)

8-LEAD LFCSP

θJA =83.5°C/W

8-LEAD MSOP

θJA =142°C/W

TJMAX =150°C

TIME (200µs/DIV)

–10

–8

–6

–4

–2

VIN

VOUT

VSY = ±9V

VIN = ±9V

AV = 1

RL = 10kΩ

CL = 10pF

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