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A8291SETTR-TB Datasheet(PDF) 8 Page - Allegro MicroSystems |
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A8291SETTR-TB Datasheet(HTML) 8 Page - Allegro MicroSystems |
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8 / 18 page  Single LNB Supply and Control Voltage Regulator A8291 8 Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com been removed. Consecutive startup attempts should allow at least 2 s of delay between restarts. In-Rush Current At start-up or during an LNB reconfiguration event, a tran- sient surge current above the normal DC operating level can be provided by the A8291. This current increase can be as high as 600 mA, typical, for as long as required, up to a maximum of 48 ms. Tone Detection A 22 kHz tone detector is provided in the A8291 solution. The detector extracts the tone signal and provides it as an open-drain signal on the TDO pin. The maximum tone out error is ±1 tone cycle, and the maximum tone out delay with respect to the input is 1 tone cycle. Detection thresholds are given in table 1. Tone Generation The A8291 solution offers four options for tone generation, providing maximum flexibility to cover every application. The EXTM pin (external modulation), in conjunction with the I2C™ control bits: TMODE (tone modulation) and TGATE (tone gate), provide the necessary control. The TMODE bit controls whether the tone source is either internal or external (via the EXTM pin). Both the EXTM pin and TGATE bit determine the 22 kHz con- trol, whether gated or clocked. Four options for tone generation are shown in figure 1. Note that when using option 4, when EXTM stops clocking, the LNB volts park at the LNB voltage, either plus or minus half the tone signal amplitude, depending on the state of EXTM. For example, if the EXTM is held low, the LNB dc voltage is the LNB pro- grammed voltage minus 325 mV (typical). With any of the four options, when a tone signal is generated, TDET is set in the status register. When the internal tone is used (options 1 or 2), the minimum tone detect amplitude is 400 mV, and when an external tone is used (options 3 or 4), the minimum tone detection amplitude is 300 mV. DiSEqC™ Bypass MOSFET A pair of N-channel MOSFETs are connected in parallel (source to drain and drain to source) to provide a low source output impedance during tone transmission. The MOSFETs are enabled either via the BFC input pin (ac- tive low) or by setting the BFC2 bit to 1 in the Control register. When the BFC pin is used instead of I2C™ control, it is not latched; a logic high or low turns the FET off or on. When the EXTM TMODE TGATE Tone (LNB Ref) LNB (V) EXTM TMODE TGATE Tone (LNB Ref) LNB (V) EXTM TMODE TGATE Tone (LNB Ref) LNB (V) EXTM TMODE TGATE Tone (LNB Ref) LNB (V) Option 1 – Use internal tone, gated by the TGATE bit. Option 2 – Use internal tone, gated by the EXTM pin. Option 3 – Use external tone, gated by the TGATE bit. Option 4 – Use external tone. Figure 1. Options for tone generation Table 1. Detection Thresholds for Tone Generation Options Transmit Receive Option (Fig. 1) 1 2 3 4 n.a. n.a. TMODE 1 1 0 0 0 1 TGATE Control 0/1 1 Control 0/1 1 At least one must be 0 to prevent tone transmission EXTM 1 Control 0/1 22 kHz logic signal, continuous Control gated 22 kHz logic signal Guaranteed Detection Threshold (mVPP) 400 400 300 300 300 400 Rejection Threshold (mVPP) 100 100 100 100 100 100 |
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