ADP3604 Просмотр технического описания (PDF) - Analog Devices
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ADP3604 Datasheet PDF : 8 Pages
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ADP3604
Since output current is supplied solely by the output capacitor
C3 during one-half of the charge-pump cycle, peak-to-peak
output ripple voltage is calculated by using the following
formula:
VRIPPLE
=
2
(
F
IOUT
PUMP )(C2)
+
IOUT
(ESRC2 )
In Figure 15, output ripple voltage vs. capacitance and various
ESR are shown.
120
OUT
100
ADP3604
ESR
140mA V
C
80
60
40
150mΩ
100mΩ
50mΩ
20
0
0 20 40 60 80 100 120 140 160 180
CAPACITANCE – µF
Figure 15. Output Ripple Voltage (mV) vs. Capacitance
and ESR
Note that as the capacitor value increases beyond the point
where the dominant contribution to the output ripple is due to
the ESR, no significant reduction in VOUT ripple is achieved by
added capacitance.
A low ESR capacitor has much greater impact on performance
for C2 than C3 since current through C2 is twice the C3 cur-
rent. There is a voltage drop across CP’s ESR during the charge
as well as during discharges. Therefore, the voltage drop due to
C2 is about 4 times C2’s ESR times the load current. The volt-
age drop generated by C2’s ESR combined with the voltage
drop due to the output source resistance, determines the maxi-
mum available VOUT, while C3’s ESR affects the output voltage
ripple.
When selecting the capacitors, keep in mind that not all manu-
facturers guarantee capacitor ESR in the range required by the
circuit. In general, the capacitor’s ESR is inversely proportional
to its physical size, so larger capacitance values and higher volt-
age ratings tend to reduce ESR.
ESR is also a function of the operating frequency. When select-
ing a capacitor, make sure its value is rated at the circuit’s oper-
ating frequency. The other factor affecting the capacitor’s
performance is temperature. If the circuit has to operate at tem-
peratures significantly different than 25°C, the capacitance and
ESR values must be carefully selected to adequately compen-
sate for the change. Various capacitor technologies offer im-
proved performance over temperature, for example, certain
tantalum capacitors provide good low-temperature ESR but at
a higher cost.
Figure 16 demonstrates the effect temperature has on various
capacitors. ADP3604’s high internal oscillator frequency per-
mits the usage of smaller capacitance for both the pump and
the output capacitors.
10
ALUMINUM
1.0
CERAMIC
TANTALUM
0.1
ORGANIC SEMIC
TANTALUM
ORGANIC SEMIC
CERAMIC
ALUMINUM
0.01
–50
0
50
100
TEMPERATURE – °C
Figure 16. ESR vs. Temperature
Table I. Alternative Capacitor Technologies
Type
High
Life Freq Temp Size Cost
Aluminum
Electrolytic
Capacitor
Multilayer
Ceramic
Capacitor
Solid
Tantalum
Capacitor
OS-CON
Capacitor
Fair Fair
Long Good
Above Avg
Avg
Above Good
Avg
Fair
Poor
Avg
Good
Small Low
Fair High
Avg Avg
Good Avg
The following is a partial list of manufacturers providing low
ESR capacitors.
Table II. Recommended Capacitor Manufacturers
Manufacturer Capacitor
Capacitor Type
Sprague
Sprague
Nichicon
Mallory
TOKIN
muRata
672D, 673D,
674D, 678D
675D, 173D,
199D
PF & PL
TDC & TDL
MLCC
GRM
Aluminum Electrolytic
Tantalum
Aluminum Electrolytic
Tantalum
Multilayer Ceramic
Multilayer Ceramic
EXTERNAL OUTPUT FILTERING
In applications requiring very low power supply ripple and
noise, the circuit in Figure 18 provides low noise and ripple of
less than 2% of the output voltage over the full load current and
temperature.
REV. 0
–5–
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