L9780 datasheet(18/56 Pages) STMICROELECTRONICS

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Main functionalities

L9780

18/56

DocID026356 Rev 3

On all VCCS pins a short to battery diagnostic is always active: a short to battery condition

is detected if OUT or TG pins exceed VCCA voltage + 30 mV or if SNS pin exceed STBS3

threshold voltage (2.7 V typical value, possible range defined in ). The consequences of a

short to battery fault detection are specified in Section 5.1.

On SNS pin is also present a short to ground diagnostic circuitry, that is able to detect if SNS

pin is into the range SNS_NTH < SNS < SNS_PTH (SNS_NTH and SNS_PTH are voltage

thresholds specified in Table 13, their typical values are -200 mV and 200 mV). The

consequences of a sensor short to ground detection are specified in Section 5.3.

VCCS is designed to respect specification of minimum bandwidth (100 kHz) for different Rtot

values and is able to guarantee in all conditions a minimum phase margin of 60°. This is

possible thanks to the programmability of the VCCS internal compensation capacitances

(Cint). In SPI input frame, 6bit for capacitance configuration are present (VCCSCAP[5..0]);

these bits allow to drive the connection of internal compensation capacitances, that in this

way are selectable by the user. The higher is the value of capacitance selected and the

higher is the phase margin of the structure, but the lower is the consequent bandwidth; with

some simplifications the bandwidth of VCCS can be described with the following formula:

Ft = (Gm_in * Gm_outDC * Rtot)/ (2π * Cint)

where Gm_in * Gm_outDC is a constant parameter depending by on the structure used in

the design; Rtot is the external total resistance and Cint the internal selectable capacitance.

In the following table it is shown that a wide range of possible Rtot can be driven obtaining

the correct compromise between bandwidth and phase margin.

Table 6. Cint vs. possible Rtot to guarantee phase margin and bandwidth

Cint (pF)

Maximum Rtot [Ω] value to

guarantee a phase margin > 60°

Minimum Rtot [Ω] value to

guarantee a bandwidth > 100 kHz

104

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