4-30

TELCOM SEMICONDUCTOR, INC.

The relationship between output voltage and control

voltage for the TCM853 (Figure 5) is given by:

(

R2

R1

)

Figure 4. TCM850/851/852 Adjustable Application

NEGOUT

FB

OUT

All caps = 1.0

µF unless otherwise noted

TCM850

TCM851

TCM852

GND

+

–

SHDN

SHDN

OSC

R2 = 100k

R1 = 100k

10

µF

Figure 5. TCM853 Standard Application Circuit

DETAILED DEVICE DESCRIPTION

charge pump (using commutating capacitor C1 and reser-

voir capacitor C2). The negative voltage at NEGOUT is then

regulated by an internal linear regulator, the output of which

is connected to the OUT pin (Figure 1). The most negative

output voltage possible is the inverted input voltage (i.e.

regulator reduces combined output noise (charge pump

ripple plus incoming supply noise) to 2mVp-p for the TCM850/

1/2 and 1mVp-p for the TCM853.

APPLICATIONS INFORMATION

Setting the Output Voltage

The TCM850/1/2 operate in the fixed output voltage

mode (OUT = – 4.1V) when the FB input is grounded (Figure

3). The output voltage can be adjusted by connecting FB to

the midpoint of a resistive voltage divider from OUT to GND

(Figure 4). Care must be taken to allow a minimum of 1.0V

across the linear regulator for proper regulation. The output

voltage is calculated using the formula below (R2 should be

chosen to be between 100k

Ω and 400kΩ):

R2

R1

)

NEGOUT

FB

OUT

All caps = 1.0

µF unless otherwise noted

TCM850

TCM851

TCM852

10

µF

GND

+

–

SHDN

SHDN

OSC

Figure 3. TCM850/851/852 Standard Application

NEGOUT

CONT

OUT

VCTRL (0 to 10V)

All caps = 1.0

µF unless otherwise noted

R2 = 100k

R1 = 100k

TCM853

10

µF

GND

+

–

SHDN

TCM850

TCM851

TCM852

TCM853

REGULATED GaAs FET BIAS SUPPLY