16XSD200 datasheet(39/60 Pages) FREESCALE

39 / 60 page

Analog Integrated Circuit Device Data



Freescale Semiconductor

16XSD200

FUNCTIONAL DEVICE OPERATION

OPERATION AND OPERATING MODES

synchronization of the device. An external sample and hold

(S/H) capacitor is not required. After turn on, the CSNS

output current reflects the channel’s load current with the

specified accuracy after occurrence of the negative edge on

the SYNC pin, as in synchronous mode (see Current Sense

Synchronization (SYNC)). However, at the switch-off instant,

the last observed CSNS current is sampled and its value

saved, thanks to an internal S/H capacitor. The SYNC pin

goes high (SYNC = 1). If the channel on which Track & Hold

current sensing is performed is changed to another, the

internal S/H hold capacitor is first emptied and then charged

again to allow current monitoring of the other channel.

Consequently, T&H current monitoring of a channel is lost

when this channel is in the Off state at the moment the

current is monitored on the other channel. Track & Hold mode

should not be used for frequencies below 60 Hz.

Figure 17. Current Sensing Ratio Versus Output Current

Current Sense Errors

Current sense accuracy is adversely affected by errors of

the internal circuitry’s current sense ratio and offset. The

value of the current sensing output current can be expressed

with sufficient accuracy by the following equation:

ICSNS = (I(HS[x])+ I_LOAD_ERR_SYS + I_LOAD_Err_RAND)*CSRx

(1)

with CSR0 = (1/1500+GAIN0) and CSR1 = (1/500+GAIN1).

The device’s offset error has a “systematic” and a

“random” component (I_LOAD_ERR_SYS, I_LOAD_ERR_RAND).

At low current levels, the random offset error may become

dominant. The systematic offset error is caused by

predictable variations with supply voltage and temperature,

and has a small but positive value with small spread. The

random offset error is a randomly distributed parameter with

an average value of zero, but with high spread. The random

offset error is subject to part-to-part variations and also

depends on the values of supply voltage and device

temperature. The device has a special feature called offset

compensation, allowing an almost complete compensation of

the random offset error (see ESR0_ERR). This offset

compensation technique greatly minimizes this error.

Computing the compensated current sensing value is

illustrated in the next sections.

Activation and Use of Offset Compensation

According to the settings of the OFP_s bit (in the

RETRYR_s register), opposite values of the random offset

error are generated. To compensate the random offset error,

two separate measurements with opposite values of the

random offset error are required. The measured values must

be saved by an external µ-processor. Compensation of the

random offset error is achieved by computing the average of

both. When a dedicated bit called Offset Positive (OFP = bit

D8 of the RETRYR_s register) is set to 1, the current sunk

through the CSNS pin (ICSNS) can be described by:

ICSNS1=CSRx *(ILOAD+ I_LOAD_ERR_SYS+ I_LOAD_ERR_RAND)(2)

When bit OFP is set to 0, ICSNS can be described by:

ICSNS2 = CSRx *(ILOAD+ I_LOAD_ERR_SYS - I_LOAD_ERR_RAND) (3)

The random offset term I_LOAD_ERR_RAND can be

computed from equations (2) and (3) as follows:

I_LOAD_ERR_RAND = (ICSNS1 - ICSNS2) / (2*CSRx)(4)

The compensated current sense value ICSNS,COMP can be

obtained by computing the average value of measurements

ICSNS1 and ICSNS2 as follows:

ICSNS,COMP = (ICSNS1 + ICSNS2) / 2

(5)

When equations 2 and 3 are substituted in equation 5, the

random offset error cancels out, as shows eq. 6:

ICSNS,COMP = (I_LOAD_ERR_SYS + ILOAD) * CSRx

(6)

The systematic offset error I_LOAD_ERR_SYS is referenced

at the operating point 28 V and 25 °C. It can eventually be

fine tuned by performing a calibration. Gain errors at 25 °C

(=current sense ratio errors, represented by

GAIN0 and

Gain1) can also be reduced by performing a calibration at a

point in the range of interest. If calibration can not be done, it

is recommended to use the typical value of I_LOAD_ERR_SYS

(see ESR0_ERR).

Current Sense Error Model

The figures of uncompensated and compensated current

sense accuracy mentioned in Table 3 have been obtained

applying the error model of eq. 7 to the data:

ICSNS_MODEL = (I(HS[x])+ I_LOAD_ERR_SYS) * CSRx

(7)

ESRx_ERR = (ICSNS1 - ICSNS_MODEL)/ICSNS_MODEL

(8)

ESRx_ERR(COMP)= (ICSNS,COMP - ICSNS_MODEL)/ICSNS_MODEL (9)

The computation has been applied to each of the specified

measurement points. Model parameters I_LOAD_ERR_SYS and

CSRx have the nominal values, specified in ESR0_ERR.

The load current can be computed from this model as:

I(HS[x]) = ICSNS / CSRx - I_LOAD_ERR_SYS

(10)

I(HS[x]) = ICSNS,Comp / CSRx - I_LOAD_ERR_SYS

(11)

16XSD200 Datasheet(PDF) 39 Page - Freescale Semiconductor, Inc

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