STPMC1 datasheet(27/77 Pages) STMICROELECTRONICS

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STPMC1

Theory of operation

Doc ID 15728 Rev 6

27/77

information about this signal is also available in the status bit

BIL, one per each phase (see

Table 32).

The three phase status bit

BIL is the AND of each phase status bit BIL.

The no-load condition occurs when the product between UX and IX register values is below

a given value. This value can be set by the LTCH configuration bits. Four different no-load

threshold values can be chosen according to the two LTCH bits as reported in

Table 14.

When a no-load condition occurs (

BIL = 1) the integration of power is suspended. The no-

load condition flag

BIL in standalone mode blocks generation of pulses for stepper and is

brought out to the output selector forcing SCLNLC pin low. In peripheral mode, the

BIL

signal can be accessed through the SPI interface.

The minimum output frequency (at no-load threshold) is given as % of the full-scale (FS)

output frequency, where FS internal AW frequency is 1370 Hz per phase.

Example 3: No-load condition threshold calculation

An energy meter has a power constant of C = 64000 pulses/kWh on LED pin.

It is valid the following relation:

C = 3600000 * f / P

where 3600000 is the factor between kWh and Ws and f is the output frequency on the

LED pin if P power is applied to the meter.

The minimum output frequency if LTCH [0] = LTCH [1] = 1 is:

f = 0,010 * 1370 Hz = 0,137 Hz

which gives a no-load condition power threshold equal to:

P = 3600000 * 0,137 Hz / 64000imp/kWh = 7,7 W

In this example, the no-load threshold is equivalent to 7,7 W of load or to a start-up

current of 32 mA at 240 V.

In NLC function is also implemented an hysteresis. When the current is falling the threshold

is half lower than that described above.

9.9

Error detection (status bits:

BCF, PIN)

The STPMC1 has two error detection circuits that checks:

●

the

ΔΣ signals

●

the state of output pins

The first error detection circuit checks if any of the

ΔΣ signals from the analog part is stuck at

1 or 0 within the period of observation (250 µs). In case of detected error the corresponding

ΔΣ signal is replaced with an idle ΔΣ signal, which represents a constant value 0. When this

Table 14.

No-load detection thresholds

LTCH (2 bits)

NLC threshold

0,00125*FS

0,0025*FS

0,005*FS

0,010*FS

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