ADP3605(2012) Ver la hoja de datos (PDF) - Analog Devices
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ADP3605 Datasheet PDF : 12 Pages
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Data Sheet
ADP3605
APPLICATIONS INFORMATION
CAPACITOR SELECTION
The high internal oscillator frequency of the ADP3605 permits
the use of small capacitors for both the pump and the output
capacitors. For a given load current, factors affecting the output
voltage performance are the following:
• Pump (CP) and output (CO) capacitance
• ESR of the CP and CO
When selecting the capacitors, keep in mind that not all
manufacturers guarantee capacitor ESR in the range required
by the circuit. In general, the ESR of the capacitor is inversely
proportional to its physical size; therefore, larger capacitance
values and higher voltage ratings tend to reduce ESR. Because
the ESR is also a function of the operating frequency, when
selecting a capacitor, ensure that its value is rated at the
operating frequency of the circuit.
Temperature is another factor affecting capacitor performance.
Figure 14 illustrates the temperature effect on various capacitors. If
the circuit has to operate at temperatures significantly different
from 25°C, the capacitance and the ESR values must be carefully
selected to adequately compensate for the change. Various
capacitor technologies offer improved performance over
temperature; for example, certain tantalum capacitors provide
good low temperature ESR; however, at a higher cost. Table 3
provides the ratings for different types of capacitor technologies
to help the designer select the right capacitors for the application.
The exact values of CIN and CO are not critical. However, low
ESR capacitors, such as solid tantalum and multilayer ceramic
capacitors, are recommended to minimize voltage loss at high
currents. Table 4 shows a partial list of the recommended low
ESR capacitor manufacturers.
10
ALUMINUM
1
CERAMIC
TANTALUM
0.1
ORGANIC SEMIC
INPUT CAPACITOR
A small 1 µF input bypass capacitor, preferably with low ESR,
such as tantalum or multilayer ceramic, is recommended to
reduce noise and supply transients and to supply part of the
peak input current drawn by the ADP3605. A large capacitor is
recommended if the input supply is connected to the ADP3605
through long leads, or if the pulse current drawn by the device
may affect other circuitry through supply coupling.
OUTPUT CAPACITOR
The output capacitor (CO) is alternately charged to the CP voltage
when CP is switched in parallel with CO. The ESR of CO introduces
steps in the VOUT waveform whenever the charge pump charges
CO, which contributes to VOUT ripple. Thus, ceramic or tantalum
capacitors are recommended for CO to minimize ripple on the
output. Figure 15 illustrates the output ripple voltage effect for
various capacitance and ESR values. 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. Because output
current is supplied solely by the output capacitor, CO, during
one-half of the charge pump cycle, peak-to-peak output ripple
voltage is calculated by
VRIPPLE
=
2×
IL
fS × CO
+2× IL
× ESRCO
where:
IL = load current
fS = 250 kHz nominal switching frequency
CO = 10 µF with an ESR of 0.15 Ω
120 mA
VRIPPLE = 2 × 250 kHz ×10 μ F + 2 ×120 mA × 0.15 = 60 mV
Multiple smaller capacitors can be connected in parallel to yield
lower ESR and lower cost. For lighter loads, proportionally smaller
capacitors are required. To reduce high frequency noise, bypass
the output with a 0.1 µF ceramic capacitor in parallel with the
output capacitor.
100
ADP3605
–3.0V OUTPUT
80
0.01
–50
0
50
TEMPERATURE (°C)
Figure 14. ESR vs. Temperature
60
100
40
20
150mΩ
100mΩ
50mΩ
Rev. B | Page 9 of 12
0
0
20
40
60
80 100 120 140 160
CAPACITANCE (µF)
Figure 15. Output Ripple Voltage vs. Capacitance and ESR
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