Datasheet
SLVS324A − JULY 2001 REVISED NOVEMBER 2004
19
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APPLICATION INFORMATION
GaAs supply (continued)
R
PMAX
can be calculated using the following equation:
R
PMAX
+
ǒ
V
CO
* V
O
I
O
* R
O
Ǔ
With: V
CO
= −3.3 V; V
O
= −2.7 V; I
O
= −3 mA
R
PMAX
= 200 Ω − 52 Ω = 148 Ω
A 100-Ω resistor was selected for R
P
.
The reference voltage across R2 is 1.24 V typical. With 5-µA current for the voltage divider, R2 gets:
R2 +
1.24 V
5 mA
[ 250 kW
R1 +
2.7 * 1.24 V
5 mA
[ 300 kW
With C
P
= 1 µF the ratio V
O
/V
I
of the RC post filter is:
Ť
V
O
V
I
Ť
+
1
1 )
(
2p125000Hz 100W 1 mF
)
2
Ǹ
[ 0.01
step-down charge pump
By exchanging GND with OUT (connecting the GND pin with OUT and the OUT pin with GND), a step-down
charge pump can easily be formed. In the first cycle S1 and S3 are closed, and C
(fly)
with C
O
in series are
charged. Assuming the same capacitance, the voltage across C
(fly)
and C
O
is split equally between the
capacitors. In the second cycle, S2 and S4 close and both capacitors with V
I
/2 across are connected in parallel.
C
(fly)
1 µF
S2
S1
S3
S4
C
O
1 µF
V
O
(V
I/2
)
GND
V
I
V
O
(V
I/2
)
+
Figure 34. Step-Down Principle
TPS60400
OUT C1+
IN
C1− GND
1
2
3
5
4
C
(fly)
1
µ
F
C
O
1 µF
C
I
1 µF
V
O
(V
I/2
)
GND
V
I
GND
Figure 35. Step-Down Charge Pump Connection
The maximum input voltage between V
I
and GND in the schematic (or between IN and OUT at the device itself)
must not exceed 6.5 V. For input voltages in the range of 6.5 V to 11 V, an additional Zener-diode is
recommended (see Figure 36).