5.

Voltage Regulator

FS501 Regulator

309K

800K

GNDR

VDD

S81250

OUT

IN

20 uF

20 uF

VDD

VSS

9V Battery

VBAT

AGND

20 uF

VCC

VDD

ENGNDR_

The block diagram of FS501 voltage regulator is shown in Fig. 3. In the circuit, we need a low cost voltage

regulator, S81250, to regulate the battery voltage (more than 6.8V) to 6.4V VDD. The VDD power supply is

used for: (1) analog circuit power supply, (2) as the reference voltage of the voltage regulator.

In Fig. 3, voltages VDD and REFH are used as reference voltages to generate VCCR, GNDR, and VSS for

FS501. If VSS is set to 0V, then VCCR and GNDR will be regulated to 5V and 3.2V respectively.

Users can select the power supply source for FS501 by themselves, either generated internally or supplied

externally. The power sources of analog circuits of FS501 are supported through terminals of VDDA, AGND,

and VSSA. Therefore, we can directly connect the voltage regulator outputs of VCCR, GNDR, and VSS to

terminals of VCC, AGND, and VSSA respectively. If the user can find stable power source externally, the power

sources can be applied to terminals of VCC, AGND, and VSSA directly instead of internal power sources of

VCCR and GNDR.

Most of the power consumption of the analog circuit is due to the static current. In FS501 the current of analog

part is designed to be less than 1mA, and the current of digital part is less than 0.5mA .

Bias Current Source Generator

FS501 Current Bias

AGND

ENGNDR_

CSFB

VSS

Current

Bias

3uA

412K

VDD

LM385/1.23V

82K

AGND

The bias current source generator as shown in Fig. 4 generates the bias current for all the analog circuits of

FS501. If the embedded op amp works, CSFB will be pulled to AGND by the feedback; there are 1.2V in

resistor 412K, and 3uA bias current can be obtained. If the value of the 412K-resistor is reduced, the working

current of the chip will be increased, and some of the specs can be improved.