MCP661 Datasheet, PDF (23/42 Pages) Microchip Technology – 60 MHz, 6 mA Op Amps

The power de-rating across temperature for an op amp
in a particular package can be easily calculated
(assuming equal power dissipations):
EQUATION 4-5:
Where:
P_OAmax
≤
T_Jmax – T_A
n θ_JA
T_Jmax = absolute max. junction temperature
Several techniques are available to reduce ΔT_JA for a
given P_OAmax:
• Lower θ_JA
- Use another package
- PCB layout (ground plane, etc.)
- Heat sinks and air flow
• Reduce P_OAmax
- Increase R_L
- Limit I_OUT (using R_SER)
- Decrease V_DD
4.3 Distortion
Differential Gain (DG) and Differential Phase (DP)
refer to the non-linear distortion produced by a NTSC
(or PAL) video component. Table 1-2 and Figure 2-34
show the typical performance of the MCP661,
configured as a gain of +2 amplifier (see Figure 4-10),
when driving one back-matched video load (150Ω, for
75Ω cable). Our tests use a sine wave at NTSC’s color
sub-carrier frequency of 3.58 MHz, with a 0.286V_P-P
magnitude. The DC input voltage is changed over a
+0.7V range (positive video) or a -0.7V range (negative
video).
DG is the peak-to-peak change in the AC gain
magnitude (color hue), as the DC level (luminance) is
changed, in units of %. DP is the peak-to-peak change
in the AC gain phase (color saturation), as the DC level
(luminance) is changed, in units of °.
4.4 Improving Stability
4.4.1 CAPACITIVE LOADS
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. A unity gain buffer (G = +1) is the most
sensitive to capacitive loads, though all gains show the
same general behavior.
MCP661/2/3/5
When driving large capacitive loads with these op
amps (e.g., > 20 pF when G = +1), a small series
resistor at the output (R_ISO in Figure 4-6) improves the
feedback loop’s phase margin (stability) by making the
output load resistive at higher frequencies. The
bandwidth will be generally lower than the bandwidth
with no capacitive load.
R_G
R_F
R_ISO
V_OUT
C_L
R_N
MCP66X
FIGURE 4-6:
Output Resistor, R_ISO
stabilizes large capacitive loads.
Figure 4-7 gives recommended R_ISO values for
different capacitive loads and gains. The x-axis is the
normalized load capacitance (C_L/G_N), where G_N is the
circuit’s noise gain. For non-inverting gains, G_N and the
Signal Gain are equal. For inverting gains, G_N is
1+|Signal Gain| (e.g., -1 V/V gives G_N = +2 V/V).
100
10
G_N = +1
G_N ≥ +2
1
1.1E0-p11
11.E00-1p0
1.E1n-09
Normalized Capacitance; C_L/G_N (F)
1.1E0-n08
FIGURE 4-7:
Recommended R_ISO Values
for Capacitive Loads.
After selecting R_ISO for your circuit, double check the
resulting frequency response peaking and step
response overshoot. Modify R_ISO’s value until the
response is reasonable. Bench evaluation and
simulations with the MCP661/2/3/5 SPICE macro
model are helpful.
© 2009 Microchip Technology Inc.
DS22194A-page 23