STPM32 datasheet(35/121 Pages) STMICROELECTRONICS

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DS10272 Rev 8

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STPM32, STPM33, STPM34

Theory of operation

121

The sigma-delta modulators convert the input signals into a continuous serial stream of “1”

and “0” at a rate determined by the sampling clock. In the STPM3x, the oversampling clock

is equal to 4 MHz.

1-bit DAC in the feedback loop is driven by the serial data stream. DAC output is subtracted

from the input signal and from the integrated error. If the loop gain is high enough, the

average value of DAC output (and therefore the bitstream) can approach to the input signal

level. When a large number of samples are averaged, a very precise value of the analog

signal is obtained. This average is described in DSP section.

The converted sigma-delta bitstreams of voltage and current channels are fed to the internal

hardwired DSP unit, which decimates, filters and processes those signals in order to boost

the resolution and to yield all necessary signals for computations.

8.2.3

Clock generator

All the internal timing of the STPM3x is based on the input clock signal, namely 16 MHz.

This signal can be provided in two different ways:

External quartz: the oscillator works with an external crystal

External clock: the XTAL2 pin can be fed by an external 16 MHz clock signal

For specific features of crystal oscillator or CLKIN signal please refer to Table 5.

The clock generator is powered by the analog supply and is responsible for two tasks. The

former delays the turn-on of some function blocks after POR in order to help a smooth start

of external power supply circuitry by keeping off all major loads. The latter provides all

necessary clocks for analog and digital parts.

Figure 26. Different oscillator circuits (a): with quartz; (b): with external source

From the external 16 MHz clock, the entire clock tree is generated. All internal clocks have

50% duty cycle.

CLKIN/XTAL2

XTAL1

CLKOUT/ZCR

15 pF

1MΩ

CLKIN/XTAL2

XTAL1

CLKOUT/ZCR

16 MHz

GIPG1303141329LM

STPM32 Datasheet(PDF) 35 Page - STMicroelectronics