10
LT1317/LT1317B
FREQUENCY COMPENSATION
The LT1317 has an external compensation pin (V
C
) which
allows the frequency response to be optimized for the
circuit configuration. In most cases, the values used in
Figure 1 will work. Some circuits may need additional
compensation and a simple trial and error method for
determining the necessary component values is given.
Figure 6 shows the test setup. A load step is applied and
the resulting output voltage waveform is observed. Fig-
ures 7 through 10 detail the response for various values of
R and C in the compensation network. The circuit of
Figure 7 starts with a large C and small R giving a highly
overdamped system. This system will always be stable but
the output voltage displays a long settling time of >5ms.
Figure 8’s circuit has reduced C giving a shorter settling
time but still overdamped. Figure 9 shows the results
when C is reduced to the point where the system becomes
underdamped. The output voltage responds quickly
(≈200µs to 300µs) but some ringing exists. Figure 10 has
APPLICATIONS INFORMATION
WUU
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optimum R and C values giving the best possible settling
time with adequate phase margin.
An additional 100pF capacitor (C
C2
) is connected to the V
C
pin and is necessary if the LT1317 is operated near current
limit. Also, C
C2
should be present when higher ESR output
capacitors are used.
Figure 6. Frequency Response Test Setup
V
IN
SW
FB
LT1317
10µH
MBR0520L
SHDN
R
604k
50Ω
V
OUT
3.3V
1M
15Ω
2W
C
C
C2
100pF
1317 F06
V
C
GND
V
IN
2V
47µF
47µF
++
1ms/DIV 1317 F09
I
LOAD
2mA TO
200mA
V
OUT
100mV/DIV
AC COUPLED
1ms/DIV 1317 F10
I
LOAD
2mA TO
200mA
V
OUT
100mV/DIV
AC COUPLED
5ms/DIV 1317 F07
I
LOAD
2mA TO
200mA
V
OUT
100mV/DIV
AC COUPLED
5ms/DIV
1317 F08
I
LOAD
2mA TO
200mA
V
OUT
100mV/DIV
AC COUPLED
Figure 8. Reducing C to 22nF
Speeds Up the Response. (R = 33k)
Figure 10. 3.3nF and 33k Gives the
Shortest Settling Time with No Ringing.
Figure 9. Using 680pF for C Results in an
Underdamped System with Ringing. (R = 33k)
Figure 7. With C = 56nF and R = 33k,
the System is Highly Overdamped.
