ADP3610 Просмотр технического описания (PDF) - Analog Devices
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ADP3610 Datasheet PDF : 10 Pages
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ADP3610
APPLICATION INFORMATION
Capacitor Selection
The ADP3610’s high internal oscillator frequency permits the
use of small capacitors for both the pump and the output ca-
pacitors. For a given load current, factors affecting the output
voltage performance are:
• Pump (CP) and output (CO) capacitance
• ESR of the CP and CO
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. Since the ESR is also a function
of the operating frequency, when selecting a capacitor, make
sure its value is rated at the circuit’s operating frequency. An-
other factor affecting capacitor performance is temperature. Fig-
ure 19 illustrates the temperature effect on various capacitors.
Aluminium electrolytic capacitors lose their capacitance at
low temperatures and their ESR increases considerably. Some
capacitor technologies do offer improved performance over
temperature; for example, certain tantalum capacitors provide
good low temperature ESR but at a higher cost. Table II pro-
vides 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 III shows a partial list of the recommended low
ESR capacitor manufacturers.
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 supply part of the peak
input current drawn by the ADP3610. A large capacitor is rec-
ommended if the input supply is connected to the ADP3610
through long leads, or if the pulse current drawn by the device
might affect other circuitry through supply coupling.
Output Capacitor
The output capacitor (CO) is alternately charged to the sum of
input voltage and pump capacitor voltage when CP is switched
in series with CO. The ESR of CO introduces steps in the VOUT
waveform whenever the charge pump charges CO, which tends
to increase VOUT ripple. Thus, ceramic or tantalum capacitors
are recommended for CO to minimize ripple on the output.
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.
Multiple smaller capacitors can be connected in parallel to yield
lower ESR and potential cost savings. For lighter loads, propor-
tionally smaller capacitors are required. To reduce high fre-
quency noise, bypass the output with a 0.1 µF ceramic capacitor.
Pump Capacitor
The ADP3610 alternately charges CP to the input voltage when
it is switched in parallel with the input supply, and then trans-
fers charge to CO when it is switched in series with the input and
connected to the output.
10
ALUMINUM
CERAMIC
1.0
TANTALUM
ORGANIC SEMIC
TANTALUM
0.1
ORGANIC SEMIC
CERAMIC
ALUMINUM
0.01
–50
0
50
100
TEMPERATURE – ؇C
Figure 19. ESR vs. Temperature
Power Dissipation
The power dissipation of the ADP3610 circuit must be limited
so the junction temperature of the device does not exceed the
maximum junction temperature rating. Total power dissipation
is calculated as follows:
PD = (2 VIN – VOUT) IOUT + VIN (IS)
Where IOUT and IS are output current and supply current, VIN
and VOUT are input and output voltages respectively.
For example: assuming worst case conditions, VIN = 3 V,
VOUT = 5.62 V, IOUT = 320 mA and IS = 14 mA. Calculated
device power dissipation is:
PD ≈ (6 V – 5.62 V) × 0.32 + 3 × (0.014) = 163.6 mW
The proprietary thermal coastline package used in the ADP3610
has a thermal resistance of 102°C/W. Therefore, the rise in
junction temperature for this application would be:
TRISE = 0.164 W × 102°C/W = 16.7°C
General Board Layout Guidelines
Since the ADP3610’s internal switches turn on and off very fast,
good PC board layout practices are critical to ensure optimal
operation of the device. Improper layouts will result in poor load
regulation, especially under heavy loads. Following these simple
layout guidelines will improve output performance.
1. Use adequate ground and power traces or planes.
2. Use single point ground for device ground and input and
output capacitor grounds.
3. Keep external components as close to the device as possible.
4. Use short traces from the input and output capacitors to the
input and output pins respectively.
5. All multiple GND, VIN and VOUT pins must be connected
together for proper operation.
REV. A
–7–
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