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When Sinclair Radionics Ltd launched in August 1974 with the Sinclair Scientific a highly unusual 12-function calculator, it relied on three custom-specific Integrated Circuits (ICs) to optimize for both its size and manufacturing costs:

• TMC0805 - Custom-specific single-chip calculator circuit • IFC1 - Full-custom 7-channel digit driver • IFC2 - Full-custom DC/DC converter and 2-channel digit driver

QUICK-LINK to Display Drivers.

Item	Min	Typ	Max	Unit	Comments
VCC		6.0		V
fOSC		750		kHz
VDD	-10.5	-9.5		V
VGG	-16.3	-15.0		V
IL		35	50	mA	VI=6.0 V, VO=3.5 V

GENERAL INFORMATION

The applications information is based on the reverse-engineering of a Sinclair Scientific calculator utilizing a TMC0805 single-chip calculator circuit in concert with an IFC1 7-digit display driver, an IFC2 DC/DC Converter with 2-digit display driver and a 9-Digit 7-Segment LED display module manufactured by Bowmar Ali. We started off with the Assembly Instructions of the calculator, but ended up analyzing every detail of the circuitry with an oscilloscope to fully understand the brainchild of Sir Clive Sinclair († September 16, 2021)

DC/DC CONVERTER CHARACTERISTICS

The Sinclair Scientific is using three different voltage levels for its electronics and with the TMC0805 manufactured in a PMOS process, it needs to supply negative voltages V_DD (-9.5 V) and V_GG (-15.0 V) with respect to the most positive V_SS (0 V) rail. The IFC1 and IFC2 chips on the other hand are manufactured in a bipolar process and their supply voltage V_CC (6 V) needs to be the most positive voltage with respect to its GND pin. The engineers at Sinclair simply connected the positive battery terminal with V_SS of the PMOS chip and V_CC of the bipolar chips. The negative battery terminal is connected to the GND pin of the bipolar chips, with the IFC2 generating the negative V_DD and V_GG voltages of the PMOS chip.

The DC/DC Converter of the IFC2 chip integrates the switching transistor and drives directly the coil of the transformer used to generate the two negative voltages V_DD and V_GG. The V_DD voltage is fed back to the IFC2 and used to regulate it to its nominal value of -9.5 V. The V_GG voltage is not individually regulated but directly proportional to the V_DD voltage due to the transformer design. The design is using pulse-width modulation (PWM) in its control loop with a nominal frequency of about 750 kHz to allow for a small transformer.

DRIVER CHARACTERISTICS

To optimize operating time of battery-powered calculators using LED displays, it is common practice to drive the LED chips from the battery voltage. The 9-digit LED display of the Sinclair Scientific is using common cathode LED chips with the TMC0605 driving the segments directly and the IFC1 and IFC2 switching the corresponding digits to the negative battery terminal. To limit the operating current of the LED chips, one can either use resistors in series with the segments or the digits. Sinclair is making use of the sequential scanning of the digit drivers and decided to use only one resistor with 100 Ohm between the GND pin of the IFC1 chip and the negative battery terminal, effectively limiting the digit current to about 35 mA peak current. This approach was not realized with the IFC2 chip, its GND pin needs to be connected directly to the negative battery terminal for the operation of its DC/DC converter. Instead the Sinclair Scientific calculator uses two discrete resistors connected between the driver outputs and the LED display. One output of the IFC2 chip, connected to the minus sign of the exponent, is using a 470 Ohm resistor, while the other output driving the 10s digit of the exponent is using a 100 Ohm resistor. The IFC2 integrates for its digit drivers two NPN bipolar junction transistors (BJTs) in common-emitter topology with the base of the transistors directly connected to input pins and each complemented with a pull-down resistor to V_DD to allow the scanning of the keyboard matrix of the calculator.

• Technology

The IFC2 was manufactured in a 10 um metal gate Bipolar process (metal width = 0.4 mil / 10 um, metal spacing = 0.4 mil / 10 um).

The die size of the IFC1 is approximately 75 mils * 45 mils / 1.9 mm * 1.2 mm.

• Packaging

The IFC2 uses a standard 0.3” wide 16-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch).

• Pin Configuration

Pin	IO	Function	Pin	IO	Function
1	V	VCC	8	O	FB/VDD
2	V	GND	7	O	Switch output
3	I	Digit input 1	6	O	Digit output 1
4	I	Digit input 2	5	O	Digit output 2

If you have additions to the above datasheet please email: joerg@datamath.org.

© Joerg Woerner, September 02, 2024. No reprints without written permission.