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ISL58781 Scheda tecnica(PDF) 10 Page - Intersil Corporation |
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ISL58781 Scheda tecnica(HTML) 10 Page - Intersil Corporation |
10 / 15 page 10 FN6909.3 December 3, 2015 Submit Document Feedback Applications Information IOUT The data sheet values for oscillator current, and write current are based on an RSET of 620 when PMAX and WriteDAC are both set to full scale. The user may choose RSET to match the output current needs of the application. The PMAX DAC is biased by IRSET (= VRSET/RSET). See the “Typical Performance Curves” on page 10. The write channel output capability for a typical part is shown in Figure 1. The amount of IOUT will be limited by the available headroom voltage at the IOUTx pins. A four input DAC (Reg 1-0A bits 3, 2, 1, 0) can be used to control the amount of RC snubbing applied to the outputs. Read current may be controlled by either the Read DAC or the IAPC input. When set by PREAD, IREAD is limited to the data sheet value, whereas the IAPC input will allow a significantly higher value to be obtained. The ReadDAC and IAPC currents sum together. Glitches could occur if two or three WEN lines are changed simultaneously, and the propagation delay is different for the two lines between the inner circuits of the controller and the inner circuits of the LDD. Because the WEN lines are encoded, the selected write current will be correct before the change in code, and again after the code changes. But some other output could result momentarily if the propagation delays are not matched. The skew detector detects the first rising edge at the LVDS outputs. FOSC Both FOSC and RFREQ may be chosen to accommodate the desired range or operating point of the HFMFREQ DAC. Although FOSC is relatively linear with DAC code, monotonicity is not guaranteed (see Figure 5). Under extreme conditions, where VSO = 4.5V and temperature is +125°C, the HFM frequency is only capable of 900MHz (see Figure 6). The oscillator may be turned on either by the OSCEN lines or by the WEN code selected. The particular code that selects the oscillator is under program control. The Pcool function is only available through the program control and WEN selection. The WEB enables write current. Thus WEN code 000 may also select a write current. Power The main power consumption is caused by the headroom voltage across the output stage (VSO - VIOUT) xIOUT. For IOUT1 and IOUT2, the VSO can be reduced below 5.0V (but above VSO-GOOD), as long as sufficient headroom voltage is available to obtain the desired output current. For the blue outputs, the built in ADC can be used to obtain the output voltage, which is also the headroom voltage. The HFM oscillator power consumption will increase with increasing frequency and amplitude (see Figure 7). Note that in the QFN package, the die is mounted directly on the thermal pad. This provides a very low thermal resistance Junction to thermal pad of just a few °C/W. The problem is in moving the heat from the thermal pad to some other heat sink. Figure 10 shows that when mounted well on a 4-layer PCB with 3 ground plane layers, and an area of 10cmx10cm, the JA is +37°C/W. The typical application will not afford this good of a heat sink. Typical Performance Curves FIGURE 1. IOUT WRITE CURRENT vs VSD vs IPMAX_BIAS FIGURE 2. IBLUE vs PMAX vs VHEADROOM (VSO =5.0V) (RSET = 620) (1-21 = FFh), (RLOAD = 10) VHEADROOM (V) (REG 1-21 = 88h) (VSO = 4.5V) IOUT2 vs VHEAD vs PMAX CODE 3FFh 300h 200h 100h 80h 40h 0.00.5 1.01.5 2.02.5 3.0 3.5 4.04.5 0 200 400 600 800 1000 1200 (RSET = 620) 02 5 7 VHEADROOM (V) 13 46 8 3FFh 300h 200h 100h 40h 001h 0 100 200 300 400 500 ISL58781 |
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