L9788 datasheet(81/264 Pages) STMICROELECTRONICS

81 / 264 page

DS12308 Rev 4

81/264

L9788

Multi-voltage regulator supplies

263

6.9

Main oscillator

The device uses an internal oscillator, called MAIN Oscillator, to synchronize internal logic

and internal charge pump. A second oscillator is used to check internally if the Main

oscillator is working correctly. The Main Oscillator output is continuously verified by the

Auxiliary Oscillator output. The two clocks frequency are measured and compared so that

out of range frequency values can be detected. In case of frequency error greater than 20%

a flag "freq_err" (Upstream Read11) is set and read and cleared by MSC. In case of

frequency error greater than 30% another flag "clk_mon_fault" (Upstream Read11) is set

and read and cleared by MSC, drivers are disabled when this error occurs. The monitor

circuit is able also to detect a stuck condition in the reference clock, moreover the check is

done on both the main and the auxiliary oscillator so that a failure in any of the two

oscillators is able to set the fault flag.

Figure 29. Clock monitor block diagram

The Main and Auxiliary oscillators are kept independent using redundant circuits, biased by

independent current sources, and layouted in different floor plan regions kept isolated

thanks to deep trench usage. The only common point between the oscillators is the digital

supply that is however monitored by the independent mechanisms.

Aux oscillator is used just for safety purpose and connected to a small digital portion (no

impact on EMC performance). The main oscillator has spread spectrum solution applied by

default to reduce emissions; it means that oscillator frequency has and average value equal

to MainOSCIL with a triangular modulation applied, so final frequency is MainOSCIL ± 3.5%

and a triangular variation of about 111 kHz frequency

Conditions:

4.8 ≤ VB_IN ≤ 18V, Tj -40 to 175 °C unless otherwise specified;

GADG0112161042PS

Clock divider

Window counter

OSC_fail_SPI_flag

Fault logic

management

oscillator

Auxiliary

oscillator

Main

Clock divider

To high

To low

Stuck at

Window counter

To high

To low

Stuck at

Table 40. Clock monitor characteristics

Symbol

Parameter

Test condition

Min

Typ

Max

Unit

MainOSCIL

Internal MAIN oscillator

frequency

VB_IN = 4.8 V-18 V

14.25

15.75

MHz

MainOSCIL

Internal redundant

oscillator frequency

VB_IN = 4.8 V-18 V

MHz

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