ADDC02808PB
RESR = 10 mΩ. The di/dt is 12 A/µs. As can be seen, the peak
deviations for these curves are close to each other and com-
parable to the negative deviation shown in Figure 6 for a simi-
larly sized positive step change in load current.
8.1
8.0
7.9
7.8
7.7
7.6
100
90
V
O
100mV
7.5
7.4
10
0%
–200 –100
0
100 200 300 400 500 600 700 800
100s
TIME – s
Figure 25. Predicted Response for 24 A Step Load Change,
di/dt = 12 A/µs, with Factory Set Internal Compensation
Optimized for CLOAD = 4,000 µF and RESR = 2.5 mΩ
Figure 28. Output Voltage Transient Response to a 25 A
to 1 A Step Change in Load, di/dt/ = 12 A/µs, with
1,000 µF Load Capacitance (RESR = 10 mΩ)
RESPONSE AT END OF PULSE
The previous section describes how the ADDC02808PB con-
verter responds to the positive step change in load current that
occurs at the beginning of a power pulse. This section will
discuss the converter’s response at the end of the power pulse
when the load current is abruptly returned to a small value.
100
90
V
O
100mV
Figures 26-29 show the converter’s measured output voltage as
the load current is stepped from 25 A down to 4 A, 2 A, 1 A, and
0.1 A, respectively. The load capacitance is 1,000 µF with
10
0%
1ms
100
90
100mV
Figure 29. Output Voltage Transient Response to a 25 A
to 0.1 A Step Change in Load, di/dt/ = 12 A/µs, with
1,000 µF Load Capacitance (RESR = 10 mΩ)
V
O
What is different about these curves is the settling time. Once
the converter’s output voltage rises above nominal, the con-
verter cannot help to discharge the load capacitor. It can only
reduce its output current to zero; it cannot draw a negative
current. As such, the time it takes to bring the output voltage
back down to its nominal value depends on the load current
during the low load portion of the cycle. The rate at which the
output voltage falls to its nominal value is the load current
divided by the load capacitance (including the 150 µF capaci-
tance that is inside the converter). The smaller the load current,
the longer it takes to get the output voltage back to its nominal
value.
10
0%
100s
Figure 26. Output Voltage Transient Response to a 25 A
to 4 A Step Change in Load, di/dt/ = 12 A/µs, with 1,000 µF
Load Capacitance (RESR = 10 mΩ)
100
90
100mV
V
O
During the time that the output voltage is too high, the integra-
tor in the converter’s feedback circuitry is continuing to ramp
out of range. As the output voltage then falls below its nominal
value, it must have an undershoot error to bring the integrator
back into range. As can be seen from these figures, the lower
the load current, the longer the output voltage remains too
high, and the longer and the greater the output voltage under-
shoot is.
10
0%
100s
Figure 27. Output Voltage Transient Response to a 25 A
to 2 A Step Change in Load, di/dt/ = 12 A/µs, with 1,000 µF
Load Capacitance (RESR = 10 mΩ)
–10–
REV. A
ADI [ ADI ]
