Power-Supply Bypassing and Layout
APPLICATION NOTES
Driving Capacitive Loads
The SGM852X family operates from either a single +2.5V to
+5.5V supply or dual ±1.25V to ±2.75V supplies. For
single-supply operation, bypass the power supply VDD with a
0.1µF ceramic capacitor which should be placed close to the
VDD pin. For dual-supply operation, both the VDD and the VSS
supplies should be bypassed to ground with separate 0.1µF
The SGM852X can directly drive 250pF in unity-gain without
oscillation. The unity-gain follower (buffer) is the most sensitive
configuration to capacitive loading. Direct capacitive loading
reduces the phase margin of amplifiers and this results in
ringing or even oscillation. Applications that require greater
capacitive drive capability should use an isolation resistor
between the output and the capacitive load like the circuit in
Figure 1. The isolation resistor RISO and the load capacitor CL
form a zero to increase stability. The bigger the RISO resistor
value, the more stable VOUT will be. Note that this method
results in a loss of gain accuracy because RISO forms a voltage
ceramic capacitors. 2.2µF tantalum capacitor can be added for
better performance.
VDD
10µF
VDD
10µF
0.1µF
divider with the RLOAD
.
0.1µF
Vn
Vp
VOUT
RISO
Vn
Vp
SGM8521
VOUT
SGM8521
VOUT
SGM8521
VIN
CL
10µF
0.1µF
VSS(GND)
Figure 1. Indirectly Driving Heavy Capacitive Load
An improvement circuit is shown in Figure 2, It provides DC
accuracy as well as AC stability. RF provides the DC accuracy
by connecting the inverting signal with the output, CF and RIso
serve to counteract the loss of phase margin by feeding the
high frequency component of the output signal back to the
amplifier’s inverting input, thereby preserving phase margin in
the overall feedback loop.
VSS
Figure 3. Amplifier with Bypass Capacitors
CF
RF
RISO
SGM8521
VOUT
VIN
CL
RL
Figure 2. Indirectly Driving Heavy Capacitive Load with DC
Accuracy
For no-buffer configuration, there are two others ways to
increase the phase margin: (a) by increasing the amplifier’s
gain or (b) by placing a capacitor in parallel with the feedback
resistor to counteract the parasitic capacitance associated with
inverting node.
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SGM8521/8522/8524
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
