AMS682EP Просмотр технического описания (PDF) - Advanced Monolithic Systems Inc
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AMS682EP Datasheet PDF : 7 Pages
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APPLICATIONS (Continued)
Capacitor Selection
The output resistance of the AMS682 is determined in part by
the ESR of the capacitors used. An expression for ROUT is
derived as shown below:
ROUT = 2( RSW1+RSW2+ESRC1+ RSW3+RSW4+ESRC2)
+2(RSW1+RSW2+ESRC1+ RSW3+RSW4+ESRC2)
+1/ (fPUMP X C1) +1/ (fPUMP X C2) + ESRC3
Assuming all switch resistances are approximately equal:
ROUT = 16 RSW+ 4ESRC1+ 4ESRC2+ ESRC3
+1/ (fPUMP X C1) +1/ (fPUMP X C2)
ROUT is typically 140Ω at +25°C with VIN =+5V and 3.3µF
low ESR capacitors. The fixed term (16RSW) is about 80-
90Ω. Increasing or decreasing values of C1 and C2 will affect
efficiency by changing ROUT.
Table 1 shows ROUT for various values of C1 and C2 (assume
0.5Ω ESR). C1 must be rated at 6VDC or greater while C2 and
C3 must be rated at 12VDC or greater.
Output voltage ripple is affected by C3. Typically the larger
the value of C3 the less the ripple for a given load current. The
formula for p-p VRIPPLE is :
VRIPPLE = [1/[2(fPUMP X C3)]+2(ESRC3)] (IOUT)
For a 10µF (0.5Ω ESR), fPUMP = 10kHz and IOUT=10mA the
peak -to-peak ripple voltage at the output will be less than
60mV. In most applications (IOUT ≤ 10mA) a 10-20µF
capacitor and 1-5µF pump capacitors will be sufficient. Table
2 shows VRIPPLE for different values of C3 (assume 1Ω ESR).
Table 1. ROUT vs. C1, C2
C1, C2 (µF)
0.05
0.10
0.47
1.00
3.30
5.00
10.00
22.00
100.00
ROUT (Ω)
4085
2084
510
285
145
125
105
94
87
AMS682
Paralleling devices
Paralleling multiple AMS682 reduces the output resistance of
the converter. The effective output resistance is the output
resistance of one device divided by the number of devices.
Figure 5 illustrates how each device requires separate pump
capacitors C1 and C2, but all can share a single reservoir
capacitor.
-5V Regulated Supply From A Single 3V Battery
Figure 6 shows a -5V power supply using one 3V battery.
The AMS682 provides -6V at V-OUT , which is regulated to -5V
by the negative LDO. The AMS682 input can vary from 3V to
5.5V without affecting regulation significantly. A voltage
detector is connected to the battery to detect undervoltage. This
unit is set to detect at 2.7V. With higher input voltage, more
current can be drawn from the outputs of the AMS682.
With 5V at VIN , 10mA can be drawn from the regulated output.
Assuming 150Ω source resistance for the converter, with
IL=10mA, the charge pump will drop 1.5V.
Table 2. VRIPPLE Peak-to-Peak vs. C3 (IOUT =10mA)
C3(µF)
VRIPPLE (mV)
0.50
1020
1.00
520
3.30
172
5.00
120
10.00
70
22.00
43
100.00
25
Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723
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