P r o d u c t I n n o v a t i o n F r o m

MP400

MP400U

9

SUPPLY CURRENT vs. FREQUENCY

0

5

10

15

20

25

30

1000

100

10

Frequency, (KHz sine wave)

V

A

C

C

R

R

R

R

V

V

I

0

2

4

6

8

10

12

14

16

18

V

A

C

C

R

R

R

R

V

1

9

8

7

6

5

4

3

2

0

OVERDRIVE RECOVERY

-300

-200

-100

0

100

200

300

TIME, µs

-6

-4

-2

0

2

4

6

OUTPUT

INPUT

A

C

C

R

R

R

R

V

-4

12

10

8

6

4

2

0

-2

-6

PULSE RESPONSE

-0.05

0

0.05

0.1

0.15

0.2

TIME,µs

6

0

-1

A

C

R

R

R

V

5

4

3

2

1

SR+/SR- (25%-75%)

0

200

400

600

800

1000

1200

1400

1600

INPUT VOLTAGE, VOLTS PEAK-TO-PEAK

SR+(A

SR-(A

SR+(A

SR-(A

R

R

R

V

C

0

1

2

3

4

5

6

7 8

9 10 11 12 13 14 15

SR+/SR- (25%-75%)

R

R

R

V

C

0

200

400

600

800

1000

1200

PEAK TO PEAK INPUT VOLTAGE

SR+(A

SR-(A

SR+(A

SR-(A

0

1

2

3

4

5

6

7 8

9 10 11 12 13 14 15

General

Please read Application note 1 “General operating considerations” which covers stability, power supplies,

heat sinking, mounting, current limit, SOA interpretation, and specification interpretation. Visit www.cirrus.com for

design tools that help automate tasks such as calculations for stability, internal power dissipation, and current limit.

There you will also find a complete application notes library, technical seminar workbook, and evaluation kits.

theory oF oPeration

The PA78 is designed specifically as a high speed pulse amplifier. In order to achieve high slew rates with low idle

current, the internal design is quite different from traditional voltage feedback amplifiers. Basic op amp behaviors

like high input impedance and high open loop gain still apply. But there are some notable differences, such as signal

dependent supply current, bandwidth and output impedance, among others. The impact of these differences varies

depending on application performance requirements and circumstances. These different behaviors are ideal for

some applications but can make designs more challenging in other circumstances.

sUPPly cUrrent anD ByPass caPacitance

A traditional voltage feedback amplifier relies on fixed current sources in each stage to drive the parasitic capaci-

tances of the next stage. These currents combine to define the idle or quiescent current of the amplifier. By design,

these fixed currents are often the limiting parameter for slew rate and bandwidth of the amplifier. Amplifiers which

are high voltage and have fast slew rates typically have high idle currents and dissipate notable power with no sig-

nal applied to the load. At the heart of the PA78 design is a signal dependent current source which strikes a new

balance between supply current and dynamic performance. With small input signals, the supply current of the PA78

is very low, idling at less than 1 mA. With large transient input signals, the supply currents increase dramatically to

allow the amplifier stages to respond quickly. The Pulse Response plot in the typical performance section of this

datasheet describes the dynamic nature of the supply current with various input transients.

Choosing proper bypass capacitance requires careful consideration of the dynamic supply currents. High frequency

ceramic capacitors of 0.1 µF or more should be placed as close as possible to the amplifier supply pins. The in-

ductance of the routing from the supply pins to these ceramic capacitors will limit the supply of peak current during

transients, thus reducing the slew rate of the PA78. The high frequency capacitance should be supplemented by

additional bypass capacitance not more than a few centimeters from the amplifier. This additional bypass can be

a slower capacitor technology, such as electrolytic, and is necessary to keep the supplies stable during sustained

output currents. Generally, a few microfarad is sufficient.

sMall siGnal PerForMance

The small signal performance plots in the typical performance section of this datasheet describe the behavior when

the dynamic current sources described previously are near the idle state. The selection of compensation capacitor

directly affects the open loop gain and phase performance.

Depending on the configuration of the amplifier, these plots show that the phase margin can diminish to very low

levels when left uncompensated. This is due to the amount of bias current in the input stage when the part is in

standby. An increase in the idle current in the output stage of the amplifier will improve phase margin for small sig-

nals although will increase the overall supply current.