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參數(shù)資料
| 型號： | ADP3605AR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Secondary Over-Voltage Protection for 2-4 cell in series Li-Ion/Poly (4.45V) 8-SM8 -40 to 110 |
| 中文描述： | SWITCHED CAPACITOR REGULATOR, 288 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁數(shù)： | 5/8頁 |
| 文件大?。?/td> | 197K |
| 代理商： | ADP3605AR |
ADP3605
–5–
REV. A
THEORY OF OPERATION
The ADP3605 uses a switched capacitor principle to generate a
negative voltage from a positive input voltage. An onboard
oscillator generates a two phase clock to control a switching
network that transfers charge between the storage capacitors.
The switches turn on and off at a 250 kHz rate, which is gener-
ated from an internal 500 kHz oscillator. The basic principle
behind the voltage inversion scheme is illustrated in Figures 11
and 12.
S2
S1
V
IN
V
OUT
S3
S4
C
O
+
–
C
P
Figure 11. ADP3605 Switch Configuration Charging the
Pump Capacitor
During phase one, S1 and S2 are ON, charging the pump ca-
pacitor to the input voltage. Before the next phase begins, S1
and S2
are turned OFF as well as S3 and S4 to prevent any
overlap. S3 and S4
are turned ON during the second phase (see
Figure 12) and charge stored in the pump capacitor is trans-
ferred to the output capacitor.
+
–
S2
S1
S3
S4
C
O
C
P
V
IN
V
OUT
Figure 12. ADP3605 Switch Configuration Charging the
Output Capacitor
During the second phase, the positive terminal of the pump
capacitor is connected to ground through variable resistance
switch, S3, and the negative terminal is connected to the out-
put, resulting in a voltage inversion at the output terminal.
The ADP3605 block diagram is shown on the front page.
APPLICATION INFORMATION
Capacitor Selection
The ADP3605’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 (C
P
) and output (C
O
) capacitance.
ESR of the C
P
and C
O
.
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.
Temperature is another factor affecting capacitor performance.
Figure 13 illustrates the temperature effect on various capaci-
tors. If the circuit has to operate at temperatures significantly
different from 25
°
C, the capacitance and 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 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 applica-
tion. The exact values of C
IN
and C
O
are not critical. How-
ever, 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
μ
F 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 ADP3605. A large capacitor is rec-
ommended if the input supply is connected to the ADP3605
through long leads, or if the pulse current drawn by the device
might affect other circuitry through supply coupling.
Output Capacitor
The output capacitor (C
O
) is alternately charged to the C
P
volt-
age when C
P
is switched in parallel with C
O
. The ESR of C
O
introduces steps in the V
OUT
waveform whenever the charge
pump charges C
O
, which contributes to V
OUT
ripple. Thus,
ceramic or tantalum capacitors are recommended for C
O
to
minimize ripple on the output. Figure 14 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 V
OUT
ripple is achieved by
added capacitance. Since output current is supplied solely by
the output capacitor, C
O
,
during one-half of the charge-pump
cycle, peak-to-peak output ripple voltage is calculated by using
the following formula.
V
I
F
C
I
ESR
RIPPLE
L
S
O
L
CO
=
×
×
+
×
×
2
2
where:
I
L
= Load Current
F
S
= 250 kHz nominal switching frequency
C
O
= 10
μ
F with an ESR of 0.15
V
mA
kHz
F
mA
mV
RIPPLE
=
×
×
+
×
×
=
120
2
250
10
2
120
0 15
.
60
μ
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
μ
F ceramic capacitor in
parallel with the output capacitor.
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