317B 查看數據表（PDF） - Linear Technology
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317B Datasheet PDF : 16 Pages
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LT1317/LT1317B
APPLICATIONS INFORMATION
FREQUENCY COMPENSATION
The LT1317 has an external compensation pin (VC) 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
VIN
2V
+
47µF
10µH MBR0520L
VIN
SW
LT1317
SHDN
FB
VC
GND
1M
+
604k
VOUT
3.3V
15Ω
2W
47µF
CC2
R
100pF
50Ω
C
1317 F06
Figure 6. Frequency Response Test Setup
optimum R and C values giving the best possible settling
time with adequate phase margin.
An additional 100pF capacitor (CC2) is connected to the VC
pin and is necessary if the LT1317 is operated near current
limit. Also, CC2 should be present when higher ESR output
capacitors are used.
VOUT
100mV/DIV
AC COUPLED
VOUT
100mV/DIV
AC COUPLED
ILOAD
2mA TO
200mA
5ms/DIV
1317 F07
Figure 7. With C = 56nF and R = 33k,
the System is Highly Overdamped.
ILOAD
2mA TO
200mA
5ms/DIV
1317 F08
Figure 8. Reducing C to 22nF
Speeds Up the Response. (R = 33k)
VOUT
100mV/DIV
AC COUPLED
ILOAD
2mA TO
200mA
1ms/DIV
1317 F09
Figure 9. Using 680pF for C Results in an
Underdamped System with Ringing. (R = 33k)
10
VOUT
100mV/DIV
AC COUPLED
ILOAD
2mA TO
200mA
1ms/DIV
1317 F10
Figure 10. 3.3nF and 33k Gives the
Shortest Settling Time with No Ringing.
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