ADP3605(2012) Ver la hoja de datos (PDF) - Analog Devices
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ADP3605 Datasheet PDF : 12 Pages
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ADP3605
Data Sheet
PUMP CAPACITOR
The ADP3605 alternately charges CP to the input voltage when
CP is switched in parallel with the input supply, and then transfers
charge to CO when CP is switched in parallel with CO.
During the time CP is charging, the peak current is approximately
two times the output current. During the time CP is delivering
charge to CO, the supply current drops down to about 3 mA.
A low ESR capacitor has a much greater impact on performance
for CP than CO because current through CP is twice the CO current.
Therefore, the voltage drop due to CP is about four times the
ESR of CP times the load current. While the ESR of CO affects
the output ripple voltage, the voltage drop generated by the
ESR of CP, combined with the voltage drop due to the output
source resistance, determines the maximum available VOUT.
SHUTDOWN MODE
The output of the ADP3605 can be disabled by pulling the SD
pin (Pin 4) high to a TTL/CMOS logic compatible level that
stops the internal oscillator. In shutdown mode, the quiescent
current is reduced to 2 µA (typical). Applying a digital low level or
tying the SD pin to ground turns on the output. If the shutdown
feature is not used, Pin 4 must be tied to the ground pin.
POWER DISSIPATION
The power dissipation of the ADP3605 circuit must be limited
such that the junction temperature of the device does not exceed
the maximum junction temperature rating. Total power dissipation
is calculated as
P = (VIN − |VOUT|) IOUT + (VIN) IS
where:
IOUT and IS are output current and supply current, respectively.
VIN and VOUT are input and output voltages, respectively.
For example, assuming worst-case conditions, VIN = 6 V, VOUT =
−2.9 V, IOUT = 120 mA, and IS = 5 mA. Calculated device power
dissipation is
P ≈ (6 V − |−2.9 V|) 0.12 + (6 V) 0.005 A = 402 mW
This is far below the 660 mW power dissipation capability of
the ADP3605.
GENERAL BOARD LAYOUT GUIDELINES
Because the internal switches of the ADP3605 turn on and off
very fast, good printed circuit board (PCB) layout practices are
critical to ensure optimal operation of the device. Improper layouts
results in poor load regulation, especially under heavy loads.
Output performance can be improved by following these
simple layout guidelines:
• Use adequate ground and power traces or planes
• Use single point ground for device ground and input and
output capacitor grounds
• Keep external components as close to the device as possible
• Use short traces from the input and output capacitors to
the input and output pins, respectively
ADP3605 REGULATED ADJUSTABLE OUTPUT
VOLTAGE
The regulated output voltage is programmed by a resistor that is
inserted between the VSENSE and VOUT pins, as illustrated in
Figure 16. The inherent limit of the output voltage of a single
inverting charge pump stage is −1 times the input voltage.
The inverse (that is, negative) scaling factor of 1.00 is reduced
somewhat due to losses that increase with output current. To
increase the scaling factor to attain a more negative output voltage,
an external pump stage can be added with passive components,
as is shown in Figure 17. This single stage increases the inverse
scaling factor to a limit of two, although the diode drops limits
the ability to attain that exact 2.00 scaling factor noticeably. Even
further increases can be achieved with additional external
pump stages.
–5
R = 29kΩ
–4
VIN = 5.0V
8
7
ADP3605
1
R
VOUT
R = 24kΩ
3
4
5
2
–3
0
20
40
60
80
100
120
LOAD CURRENT (mA)
Figure 16. Adjustable Regulated Output Voltage
VIN = 5V CIN +
4.7µF
CP +
4.7µF
ADP3605
8 VIN
VOUT 7
1 CP+
3 CP–
R1
44.2kΩ
VSENSE 5
SD GND
4
2
CO
+ 4.7µF
D1
1N5817
+
C1
4.7µF
D2
1N5817
10µF
+
Figure 17. Regulated −7 V from a 5 V Input
High accuracy on the adjustable output voltage is achieved with
the use of precision trimmed internal resistors, which eliminate the
need to trim the external resistor or add a second resistor to form a
divider. The adjustable output voltage is set by
VOUT
=
1.5
9.5 kΩ
R
where VOUT is in volts and R is in kΩ.
Rev. B | Page 10 of 12
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