L9780 datasheet(16/56 Pages) STMICROELECTRONICS

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Main functionalities

L9780

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DocID026356 Rev 3

VCCS is a voltage controlled current source and is able to sink or source current from the

pump cell, depending on its control voltage, that is the voltage present on C3 pin. Through

TG and OUT pins there is the VCCS output current flow, while SNS is a sense pin. VCCS

present in L9780 has 2 output channels (1 and 2), selectable via SPI (using bit VCCSOUT);

TG and OUT pins of the non selected channel are in high impedance condition. In VCCS

architecture there are 2 internal switches (TG switches) able to short OUT and TG pins of

the selected channel (see Figure 6); the actuation of these switches can be selected via SPI

(using bit TGEN).

Figure 6. VCCS external resistance configuration

If the switch is in off condition the TG pin of the selected channel is in high impedance state.

If the TG switch is in on state the output current of VCCS structure can flow both through TG

and OUT pins and can be described with the following expression:

Iout = - (C3 – REF2)/Rtot

where Rtot is the external total resistance, that can be obtained as a combination of all the

external resistances connected to the VCCS pins. Referring to Figure 6, Rtot can be

calculated as follows:

Rtot = Rout // (Rtg + TAG resistor)

where Rtg and Rout are the series resistances of the VCCS pins related to the selected

channel.

Otherwise, if TG switch is in off state, Rtot = Rout and consequently the VCCS output current

can be described with the following expression:

Iout = - (C3 – REF2)/Rout

The VCCS characteristic is shown in Figure 7. In this figure the relationship between SNS

voltage and the voltage on Rtot resistance (that is equal to the difference between OUT

voltage and SNS voltage) is defined; these two quantities are in agreement in sign as both

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