S in g le /Du a l/Qu a d , Mic ro p o w e r,
S in g le -S u p p ly Ra il-t o -Ra il Op Am p s
2/MAX495
The MAX492/MAX494/MAX495 can drive capacitive
To inc re a s e s ta b ility while d riving la rg e c a p a c itive
loa d s , c onne c t a p ull-up re s is tor a t the outp ut to
decrease the current that the amplifier must source. If
the amplifier is made to sink current rather than source,
stability is further increased.
loads in excess of 1000pF under certain conditions
(Figure 5). When driving capacitive loads, the greatest
p ote ntia l for ins ta b ility oc c urs whe n the op a mp is
sourcing approximately 100µA. Even in this case, sta-
bility is maintained with up to 400pF of output capaci-
tance. If the output sources either more or less current,
stability is increased. These devices perform well with a
1000pF pure capacitive load (Figure 6). Figure 7 shows
the performance with a 500pF load in parallel with vari-
ous load resistors.
Frequency stability can be improved by adding an out-
put isolation resistor (R ) to the voltage-follower circuit
S
(Figure 8). This resistor improves the phase margin of
the circuit by isolating the load capacitor from the op
amp’s output. Figure 9a shows the MAX492 driving
10,000pF (R ≥ 100kΩ), while Figure 9b adds a 47Ω
L
isolation resistor.
V
IN
50mV/div
R
S
V
OUT
MAX49_
C
L
V
IN
V
OUT
50mV/div
10µs/div
Figure 8. Capacitive-Load Driving Circuit
Figure 9b. Driving a 10,000pF Capacitive Load with a 47Ω
Isolation Resistor
+5V
V
CC
V
IN
50mV/div
2
3
7
1k
1k
6
V
OUT
MAX495
4
V
OUT
50mV/div
10µs/div
Figure 9a. Driving a 10,000pF Capacitive Load
Figure 10. Power-Up Test Configuration
______________________________________________________________________________________ 13
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
