A solution for a -2.7-V/3-mA GaAs bias supply is proposed in Figure 9-11. The input voltage of 3.3 V is first

inverted with a TPS60403 and stabilized using a TLV431 low-voltage shunt regulator. Resistor RP with capacitor

CP is used for filtering the output voltage.

TPS60400

OUT

C1+

IN

C1–

GND

1

2

3

5

4

C(fly)

0.1 µF

CO

1 µF

CI

0.1 µF

VO (–2.7 V/3 mA)

GND

VI (3.3 V)

GND

RP

TLV431

R2

R1

CP

Figure 9-11. GaAs Supply

A 0.1-µF capacitor was selected for C(fly). By this, the output resistance of the inverter is about 52 Ω.

RPMAX can be calculated using the following equation:

V

O + * 1 )

R1

R2

V

ref * R1

I

I(ref)

(14)

A 100-Ω resistor was selected for RP.

The reference voltage across R2 is 1.24 V typical. With 5-µA current for the voltage divider, R2 gets:

R

PMAX +

V

CO *

V

O

I

O

* RO

With: VCO = –3.3 V; VO = –2.7 V; IO = –3 mA

RPMAX = 200 Ω – 52 Ω = 148 Ω

(15)

With CP = 1 µF the ratio VO/VI of the RC post filter is:

R2 +

1.24 V

5 mA

[ 250 kW

R1 +

2.7 * 1.24 V

5 mA

[ 300 kW

(16)

V

O

V

I

+

1

1 ) (2p125000Hz

100W

1 mF)

2

[ 0.01

(17)

The application generates an output voltage of 1/2 of the input voltage.

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 CO in series are

charged. Assuming the same capacitance, the voltage across C(fly) and CO is split equally between the

capacitors. In the second cycle, S2 and S4 close and both capacitors with VI/2 across are connected in parallel.

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SLVS324C – JULY 2001 – REVISED OCTOBER 2020

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