ADP3605 Data Sheet
Rev. B | Page 10 of 12
PUMP CAPACITOR
The ADP3605 alternately charges C
P
to the input voltage when
C
P
is switched in parallel with the input supply, and then transfers
charge to C
O
when C
P
is switched in parallel with C
O
.
During the time C
P
is charging, the peak current is approximately
two times the output current. During the time C
P
is delivering
charge to C
O
, the supply current drops down to about 3 mA.
A low ESR capacitor has a much greater impact on performance
for C
P
than C
O
because current through C
P
is twice the C
O
current.
Therefore, the voltage drop due to C
P
is about four times the
ESR of C
P
times the load current. While the ESR of C
O
affects
the output ripple voltage, the voltage drop generated by the
ESR of C
P
, combined with the voltage drop due to the output
source resistance, determines the maximum available V
OUT
.
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 = (V
IN
− |V
OUT
|) I
OUT
+ (V
IN
) I
S
where:
I
OUT
and I
S
are output current and supply current, respectively.
V
IN
and V
OUT
are input and output voltages, respectively.
For example, assuming worst-case conditions, V
IN
= 6 V, V
OUT
=
−2.9 V, I
OUT
= 120 mA, and I
S
= 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 V
SENSE
and V
OUT
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 s
ingle 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.
Figure 16. Adjustable Regulated Output Voltage
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
where V
OUT
is in volts and R is in kΩ.
–5
–4
–3
0 20 6040 80 100
R = 24kΩ
R = 29kΩ
120
11135-016
V
OUT
(V)
LOAD CURRENT (mA)
1
3
4
2
5
8 7
ADP3605
V
IN
= 5.0V
R
V
OUT
8
1
7
5
3
4 2
V
IN
= 5V
C
IN
4.7µF
C
O
4.7µF
C1
4.7µF
10µF
D2
1N5817
D1
1N5817
C
P
4.7µF
SD GND
ADP3605
R1
44.2kΩ
C
P
+
V
IN
V
OUT
V
SENSE
C
P
–
+
+
+
+
+
11135-017
