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ISL62870 Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL62870 Datasheet(HTML) 7 Page - Intersil Corporation |
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7 / 16 page  7 FN6708.0 August 14, 2008 Soft-Start Delay Circuit Description When the voltage on the VCC pin has ramped above the rising power-on reset voltage VVCC_THR, and the voltage on the EN pin has increased above the rising enable threshold voltage VENTHR, the SREF pin releases its discharge clamp, and enables the reference amplifier VSET. The soft-start current ISS is limited to 20µA and is sourced out of the SREF pin and charges capacitor CSOFT until VSREF equals VREF. The regulator controls the PWM such that the voltage on the FB pin tracks the rising voltage on the SREF pin. The elapsed time from when the EN pin is asserted to when VSREF has charged CSOFT to VREF is called the soft-start delay tSS which is given by Equation 3: Where: -ISS is the soft-start current source at the 20µA limit -VSREF is the buffered VREF reference voltage The end of soft-start is detected by ISS tapering off when capacitor CSOFT charges to VREF. The internal SSOK flag is set, the PGOOD pin goes high, and diode emulation mode (DEM) is enabled. Component Selection For CSOFT Capacitor Choosing the CSOFT capacitor to meet the requirements of a particular soft-start delay tSS is calculated using Equation 4, which is written as follows: Where: -tSS is the soft-start delay -ISS is the 20µA soft-start current source at the 20µA limit -VSREF is the buffered VREF reference voltage Fault Protection Overcurrent The overcurrent protection (OCP) setpoint is programmed with resistor ROCSET, which is connected across the OCSET and PHASE pins. Resistor RO is connected between the VO pin and the actual output voltage of the converter. During normal operation, the VO pin is a high impedance path, therefore there is no voltage drop across RO. The value of resistor RO should always match the value of resistor ROCSET. Figure 5 shows the overcurrent set circuit. The inductor consists of inductance L and the DC resistance DCR. The inductor DC current IL creates a voltage drop across DCR, which is given by Equation 5: The IOCSET current source sinks 10µA into the OCSET pin, creating a DC voltage drop across the resistor ROCSET, which is given by Equation 6: The DC voltage difference between the OCSET pin and the VO pin, which is given by Equation 7: The IC monitors the voltage of the OCSET pin and the VO pin. When the voltage of the OCSET pin is higher than the voltage of the VO pin for more than 10µs, an OCP fault latches the converter off. Component Selection For ROCSET and CSEN The value of ROCSET is calculated with Equation 8 which is written as follows: Where: -ROCSET (Ω) is the resistor used to program the overcurrent setpoint -IOC is the output DC load current that will activate the OCP fault detection circuit - DCR is the inductor DC resistance FIGURE 4. ISL62870 VOLTAGE PROGRAMMING CIRCUIT SREF VSET + − VREF EA + − FB RFB VOUT VCOMP t SS V SREF CSOFT ⋅ I SS ------------------------------------------- = (EQ. 3) C SOFT t SS ISS ⋅ V SREF ----------------------- = (EQ. 4) FIGURE 5. OVERCURRENT PROGRAMMING CIRCUIT PHASE CO L VO ROCSET CSEN OCSET VO RO DCR IL 10¬µ + _ VDCR + _ VROCSET V DCR I L DC R ⋅ = (EQ. 5) V ROCSET 10 μAR OCSET ⋅ = (EQ. 6) V OCSET V – VO V DCR V – ROCSET I L DC R ⋅ I OCSET ROCSET ⋅ – == (EQ. 7) (EQ. 8) R OCSET I OC DCR ⋅ I OCSET ---------------------------- = ISL62870 |
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