Datasheet

MCP4706/4716/4726
DS22272C-page 38 © 2011-2012 Microchip Technology Inc.
4.4.3 OUTPUT SLEW RATE
Figure 4-5 shows an example of the slew rate of the
V
OUT
pin. The slew rate can be affected by the
characteristics of the circuit connected to the V
OUT
pin.
FIGURE 4-5: V
OUT
pin Slew Rate.
4.4.4 SMALL CAPACITIVE LOAD
With a small capacitive load, the output buffer’s current
is not affected by the capacitive load (C
L
). However, the
V
OUT
pin’s voltage is not a step transition from one
output value (wiper code value) to the next output
value. The change of the V
OUT
voltage is limited by the
output buffer’s characteristics, so the V
OUT
pin voltage
will have a slope from the old voltage to the new
voltage. This slope is fixed for the output buffer, and is
referred to as the buffer slew rate (SR
BUF
).
4.4.5 LARGE CAPACITIVE LOAD
With a larger capacitive load, the slew rate is deter-
mined by two factors:
• The output buffer’s short circuit current (I
SC
)
•The V
OUT
pin’s external load
I
OUT
cannot exceed the output buffer’s short circuit
current (I
SC
) which fixes the output buffer slew rate
(SR
BUF
). The voltage on the capacitive load (C
L
), V
CL
,
changes at a rate proportional to I
OUT
, which fixes a
capacitive load slew rate (SR
CL
).
So the V
CL
voltage slew rate is limited to the slower of
the output buffer’s internally set slew rate (SRBUF) and
the capacitive load slew rate (SR
CL
).
4.4.6 DRIVING RESISTIVE AND
CAPACITIVE LOADS
The V
OUT
pin can drive up to 100 pF of capacitive load
in parallel with a 5 kΩ resistive load (to meet electrical
specifications). Figure 2-94 shows the V
OUT
vs.
Resistive Load.
V
OUT
drops slowly as the load resistance decreases
after about 3.5 kΩ. It is recommended to use a load
with R
L
greater than 5 kΩ.
Driving large capacitive loads can cause stability
problems for voltage feedback op amps. As the load
capacitance increases, the feedback loop’s phase
margin decreases and the closed-loop bandwidth is
reduced. This produces gain peaking in the frequency
response with overshoot and ringing in the step
response. That is, since the V
OUT
pin’s voltage does
not quickly follow the buffer’s input voltage (due to the
large capacitive load), the output buffer will overshoot
the desired target voltage. Once the driver detects this
overshoot, it compensates by forcing it to a voltage
below the target. This causes voltage ringing on the
V
OUT
pin.
So, when driving large capacitive loads with the output
buffer, a small series resistor (R
ISO
) at the output (see
Figure 4-6) improves the output buffer’s stability
(feedback loop’s phase margin) by making the output
load resistive at higher frequencies. The bandwidth will
be generally lower than the bandwidth with no
capacitive load.
FIGURE 4-6: Circuit to Stabilize Output
Buffer for Large Capacitive Loads (C
L
).
The R
ISO
resistor value for your circuit needs to be
selected. The resulting frequency response peaking
and step response overshoot for this R
ISO
resistor
value should be verified on the bench. Modify the
R
ISO
’s resistance value until the output characteristics
meet your requirements.
A method to evaluate the system’s performance is to
inject a step voltage on the V
REF
pin and observe the
V
OUT
pin’s characteristics.
Time
Slew Rate =
Wiper = A
V
OUT
V
OUT(A)
V
OUT(B)
Wiper = B
| V
OUT(B)
- V
OUT(A)
|
Δ T
Note: Additional insight into circuit design for
driving capacitive loads can be found in
AN884 “Driving Capacitive Loads With Op
Amps” (DS00884).
V
OUT
Op
Amp
V
W
C
L
R
ISO
R
L
V
CL