Power-Supply Bypassing and Layout
APPLICATION NOTES
Driving Capacitive Loads
The SGM854X 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 SGM854X 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
ceramic capacitors. 2.2µF tantalum capacitor can be added for
better performance.
VDD
10µF
accuracy because RISO forms a voltage divider with the RLOAD
.
VDD
10µF
0.1µF
0.1µF
RISO
Vn
Vp
VOUT
SGM8541
VOUT
Vn
Vp
SGM8541
VOUT
VIN
CL
SGM8541
10µF
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.
0.1µF
VSS(GND)
VSS
Figure 3. Amplifier with Bypass Capacitors
CF
RF
RISO
SGM8541
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.
７
SGM8541/8542/8544
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
