ADDC02808PB

REV. A

–8–

PULSEWIDTH – ms

250

200

0

0

150

50

100

150

100

50

MAXIMUM PEAK

POWER LIMIT

MAXIMUM CONTINUOUS

POWER LIMIT

100W ave

50W ave

10W ave

25

75

125

Figure 17. Largest On-State Power vs. Pulsewidth that

TRANSIENT RESPONSE

The standard ADDC02808PB is designed to deliver large

changes, or pulses, in load current with minimum output volt-

age deviation and an ultrafast return to the nominal output

voltage. The compensation of the feedback loop is optimized,

and output stability is insured, for a broad range of external load

capacitance extending from 500

µF (R

Ω) to 4,000 µF

Ω). The variables that impact pulse performance

(the maximum output voltage deviation and the settling time)

are:

1. Size of step change in the output current.

2. Amount of external load capacitance.

3. Internal compensation of the feedback loop (factory set).

4. Connection from converter output to load.

Extensive modeling of the converter with ADI proprietary soft-

ware permits analysis and prediction of the impact each of these

parameters has on the pulse response. The analyses in this data

sheet are based on the load capacitance being comprised of

100

µF, 100 mΩ tantalum load capacitors such as the CSR21

style. Figure 18 is the prediction of the standard converter’s

response to a 24 A step change in load current (from 1 A to

25 A) with a load capacitance of 1,000

µF (R

Ω).

This is very close to the measured pulse response under the

same conditions shown in Figure 6.

8.1

7.4

–200

800

–100

0

100

200

300

400

500

600

700

8

7.9

7.8

7.7

7.6

7.5

TIME –

s

Figure 18. Predicted Response to 24 A Step Change in

Load Current, di/dt = 12 A/

µs, for C

8.1

7.4

–200

800

–100

0

100

200

300

400

500

600

700

8

7.9

7.8

7.7

7.6

7.5

TIME –

s

12A STEP CHANGE

24A STEP CHANGE

Figure 19. Predicted Response to 12 A and 24 A Step

Change in Load Current, di/dt = 12 A/

µs, for C

1000

µF and R

Step Change

If the step change is less than 24 A, the pulse response will

improve. For instance, with a 12 A step change, Figure 19

shows a comparison of the response for a 24 A step change and

a 12 A step change in load.

Load Capacitance

µF (R

Ω) and C

LOAD

= 4,000

µF (R

Ω) results in the predicted waveforms

shown in Figures 20, 21, and 22. As can be seen, the larger the

capacitor, the smaller the deviation, but the longer the settling

time. Table I lists the maximum output voltage deviations and

mentioned above. Note that these are based on the standard

compensation for the feedback loop.

Table I. Output Response to a 24 A (1 A–25 A) Step in Load

Current (Standard Compensation)

Typical

Settling Time

See

Deviation

(Within 1%)

Figure

500

µF

20 m

Ω

–7%

150

µs20

1,000

µF

10 m

Ω

–6%

175

µs18

2,000

µF5 mΩ

–5%

200

µs21

4,000

µF

2.5 m

Ω

–4%

250

µs22

8.1

7.4

–200

800

–100

0

100

200

300

400

500

600

700

8

7.9

7.8

7.7

7.6

7.5

TIME –

s

Figure 20. Predicted Response for 24 A Step Load

Change in Load Current, di/dt = 12 A/

µs, for C

LOAD

= 500

µF and R