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ISL6559CR-T Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6559CR-T Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 21 page 11 FN9084.8 December 29, 2004 Operation Initialization Before converter operation is initialized, proper conditions must exist on the enable and disable inputs. Once these conditions are met, the controller begins a soft-start interval. Once the output voltage is within the proper window of operation, the PGOOD output changes state to update an external system monitor. Enable and Disable The PWM outputs are held in a high-impedance state to assure the drivers remain off while in shutdown mode. Four separate input conditions must be met before the ISL6559 is released from shutdown mode. First, the bias voltage applied at VCC must reach the internal power-on reset (POR) circuit rising threshold. Once this threshold is met, the EN input signal becomes the gate for soft-start initialization. Hysteresis between the rising and falling thresholds insures that once enabled, the ISL6559 will not inadvertently turn off unless the bias voltage drops substantially. See Electrical Specifications for specifics on POR rising and falling thresholds. Second, the ISL6559 features an enable input (EN) for power sequencing between the controller bias voltage and another voltage rail. The enable comparator holds the ISL6559 in shutdown until the voltage at EN rises above 1.23V. The enable comparator has about 90mV of hysteresis to prevent bounce. It is important that the driver ICs reach their POR level before the ISL6559 becomes enabled. The schematic in Figure 7 demonstrates sequencing the ISL6559 with the HIP660X family of Intersil MOSFET drivers which require 12V bias. Third, the frequency select\disable input (FS/DIS) will shutdown the converter when pulled to ground. Under this condition, the internal oscillator is disabled. The oscillator resumes operation upon release of FS/DIS and a soft-start sequence is initiated. The 11111 VID code is reserved as a signal to the controller that no load is present. The controller will enter shutdown mode after receiving this code and will start up upon receiving any other code. This code is not intended as a means of enabling the controller when a load is present. To enable the controller, VCC must be greater than the POR threshold; the voltage on EN must be greater than 1.23V; FS/DIS must not be grounded; and VID cannot be equal to 11111. Once these conditions are true, the controller immediately initiates a soft-start sequence. Soft-Start The soft-start time, tSS, is determined by an 11-bit counter that increments with every pulse of the phase clock. For example, a converter switching at 250kHz per phase has a soft-start time of During the soft-start interval, the soft-start voltage, VRAMP, increases linearly from zero to 140% of the programmed DAC voltage. At the same time a current source, IRAMP, is decreasing from 160 µA down to zero. These signals are connected as shown in Figure 8 (IOUT may or may not be connected to FB depending on the particular application). The ideal diodes in Figure 8 assure that the controller tries to regulate its output to the lower of either the reference voltage or VRAMP. Since IRAMP creates an initial offset across RFB of (RFB x 160µA), the first PWM pulse will not be seen until VRAMP is greater than the RFB IRAMP offset. This produces a delay after the ISL6559 enables before the output voltage starts moving. For example, if VID = 1.5V, RFB = 1kΩ and TSS = 8.3ms, the delay time can be expressed using Equation 10. FIGURE 7. POWER SEQUENCING USING THRESHOLD- SENSITIVE ENABLE (EN) FUNCTION - + 1.23V (± 2%) EXTERNAL CIRCUIT ISL6559 INTERNAL CIRCUIT EN +12V VCC +5V POR CIRCUIT OV LATCH SIGNAL 10.7k Ω 1.40k Ω ENABLE COMPARATOR T SS 2048 f SW -------------8.3ms == (EQ. 9) FIGURE 8. RAMP CURRENT AND VOLTAGE FOR REGULATING SOFT-START SLOPE AND DURATION - + IAVG REFERENCE EXTERNAL CIRCUIT ISL6559 INTERNAL CIRCUIT COMP CC RC RFB FB IOUT VDIFF ERROR AMPLIFIER VOLTAGE VCOMP VRAMP IRAMP IDEAL DIODES ISL6559 |
Similar Part No. - ISL6559CR-T |
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Similar Description - ISL6559CR-T |
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