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| 型号： | EDGE4707B |
| 英文描述： | Edge4707B PPMU Settling Time & Stability (224k) |
| 中文描述： | Edge4707B PPMU建立时间 |
| 文件页数： | 13/13页 |
| 文件大小： | 224K |
| 代理商： | EDGE4707B |
9
TEST AND MEASUREMENT PRODUCTS
www .semtech.com
PMU-A2
Edge4707B PPMU
Settling Time and Stability
Revision 1 / November 8, 2002
overshoot. Figures 2 through 5 (above) show qualitatively
how the amount of overshoot is reduced as the
compensation capacitors are increased for each range.
Table 4 (below) lists the peak overshoot on each range for
a 10V to 0V step, a force load capacitance of 100pF and
a variety of compensation capacitors. Note that under
these conditions, Range B always has the largest amount
of overshoot. As will be shown later, this is not always the
case with larger amounts of capacitance on the FORCE
output node. But, in designs which have small amounts
of Force capacitance <~200pF), the overshoot on Range
B can often be ignored since the compliance current on
this range is normally only about 20
A, which is insufficient
to cause any stress on the vast majority of test devices.
Table 4. Overshoot vs. Compensation Capacitor Values
for a 10V-0V Step with 100pF load capacitance.
Figure 7 shows the increasing overshoot on Range B as
the amplitude of the voltage step is increased. This change
in overshoot happens when the circuit is underdamped
and inclined towards overshoot.
If the circuit is
overdamped so that the output goes to the final voltage
without any overshoot at low voltages, there will normally
be no change to an overshoot characteristic with increasing
voltage step size.
So if guaranteeing no excessive
overshoot is needed, then relatively large compensation
capacitors should be selected so no overshoot is seen on
any range at low voltages. This will ensure no overshoot
even with large step sizes.
Overshoot is also a function of output capacitance.
Increasing output capacitance can either increase or
decrease the amount of overshoot depending on other
conditions.
Increasing load capacitance increases the
R-C time constant of the output node which slows down
the response time at the output, which tends to reduce
the amplitude of any transients, including overshoot. On
the other hand, this reduced response at the output also
means there is less signal feedback from the sense line.
Less external feedback requires more feedback from the
compensation network in order to have the same amount
of stability. Since the amount of compensation feedback
is fixed with fixed feedback capacitors, this means that
circuit characteristic can go from having little or no
overshoot to having significant amounts of overshoot as
the circuit shifts from being overdamped to being
underdamped.
(Note: some overshoot can occur in the overdamped
situation when in Force voltage mode due to capacitive
coupling to the FORCE output though the COMP4 capacitor.
This occurs primarily on Ranges A and B. It cannot be
entirely eliminated, but is small enough to be ignored for
most applications).
For COMP1-2 capacitance of 22pF and COMP4
capacitance of 47pF, the overshoot on ranges A and B
decrease with increasing load capacitance. (See the graph
in Figure 8). This indicates that these ranges already have
overshoot and ringing due to being underdamped with
these compensation capacitors, even with the smallest
output capacitance. Therefore, the dominating factor on
these ranges is the reduction in overshoot due to the R-C
time constant.
The overshoot on ranges C and D is initially zero with small
load capacitance, indicating that the circuit is overdamped
on these ranges under these conditions. (See Figure 8).
As the load capacitance increases, overshoot occurs first
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