IR22771STRPBF【PDF 11/16 页】IC CURRENT SENSE 16SOIC

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Sheet目录40

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IR22771S/IR21771S(PbF)

www.irf.com

1.4 DC transfer functions

The working principle of the device can be easily

explained by Figure 10, in which the main signals

are represented.

Triangular

reference

SYNC

Vin

Figure 10: Main current sensor signals and

outputs

PWM out (PO pin) gives a duty cycle which is

inversely proportional to the input signal.

Eq. 2 gives the resulting D

of the PWM output (PO

pin):

Eq. 2

?/DIV>

where V

= V

inp

-V

inm

30%

25%

20%

15%

10%

PO duty cycle

Figure 11: PO Duty Cycle (D

)

1.5 Filter AC characteristic

IR21771/22771 signal path can be considered as

composed by three stages in series (see Figure 13).

The first two stages perform the filtering action.

Stage 1 (input filter) implements the filtering action

originating the transfer function shown in Figure 14.

The input filter is a self-adaptive reset integrator

which performs an accurate ripple cancellation. This

stage extracts automatically the PWM frequency

from Sync signal and puts transmission zeros at

even harmonics, rejecting the unwanted PWM

noise.

The following timing diagram shows the principle by

which even harmonics are rejected (Figure 12).

Figure 12: Even harmonic cancellation principle

As can be seen from Figure 14, the odd harmonics

are rejected as a first order low pass filter with a

single pole placed in f

PWM

The input filter group delay in the pass-band is very

low (see GD on AC electrical characteristics) due to

the beneficial action of the zeroes.

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