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ISL9104 Datasheet(PDF) 11 Page  Intersil Corporation 

ISL9104 Datasheet(HTML) 11 Page  Intersil Corporation 
11 / 13 page 11 FN6829.3 July 9, 2009 Overcurrent Protection The overcurrent protection is provided on ISL9104, ISL9104A when overload condition happens. It is realized by monitoring the CSA output with the OCP comparator, as shown in Figure 22. When the current at PChannel MOSFET is sensed to reach the current limit, the OCP comparator is trigged to turn off the PChannel MOSFET immediately. ShortCircuit Protection ISL9104, ISL9104A has a ShortCircuit Protection (SCP) comparator, which monitors the FB pin voltage for output shortcircuit protection. When the output voltage is sensed to be lower than a certain threshold, the SCP comparator reduces the PWM oscillator frequency to a much lower frequency to protect the IC from being damaged. Undervoltage Lockout (UVLO) When the input voltage is below the Undervoltage Lock Out (UVLO) threshold, ISL9104, ISL9104A is disabled. SoftStart The softstart feature eliminates the inrush current during the circuit startup. The softstart block outputs a ramp reference to both the voltage loop and the current loop. The two ramps limit the inductor current rising speed as well as the output voltage speed so that the output voltage rises in a controlled fashion. Low Dropout Operation The ISL9104, ISL9104A features low dropout operation to maximize the battery life. When the input voltage drops to a level that ISL9104, ISL9104A can no longer operate under switching regulation to maintain the output voltage, the PChannel MOSFET is completely turned on (100% duty cycle). The dropout voltage under such condition is the product of the load current and the ONresistance of the PChannel MOSFET. Minimum required input voltage VIN under this condition is the sum of output voltage plus the voltage drop cross the inductor and the PChannel MOSFET switch. Thermal Shut Down The ISL9104, ISL9104A provides builtin thermal protection function. The thermal shutdown threshold temperature is +130°C (typ) with a 30°C (typ) hysteresis. When the internal temperature is sensed to reach +130°C, the regulator is completely shut down and as the temperature drops to +100°C (typ), the ISL9104, ISL9104A resumes operation starting from the softstart. Applications Information Inductor and Output Capacitor Selection To achieve better steady state and transient response, ISL9104, ISL9104A typically uses a 1.0µH inductor. The peak topeak inductor current ripple can be expressed in Equation 1: In Equation 1, usually the typical values can be used but to have a more conservative estimation, the inductance should consider the value with worst case tolerance; and for switching frequency fS, the minimum fS from the “Electrical Specifications” table on page 3 can be used. To select the inductor, its saturation current rating should be at least higher than the sum of the maximum output current and half of the delta calculated from Equation 1. Another more conservative approach is to select the inductor with the current rating higher than the PChannel MOSFET peak current limit. Another consideration is the inductor DC resistance since it directly affects the efficiency of the converter. Ideally, the inductor with the lower DC resistance should be considered to achieve higher efficiency. Inductor specifications could be different from different manufacturers so please check with each manufacturer if additional information is needed. For the output capacitor, a ceramic capacitor can be used because of the low ESR values, which helps to minimize the output voltage ripple. A typical value of 4.7µF/6.3V ceramic capacitor should be enough for most of the applications and the capacitor should be X5R or X7R. Input Capacitor Selection The main function for the input capacitor is to provide decoupling of the parasitic inductance and to provide filtering function to prevent the switching current from flowing back to the battery rail. A 4.7µF/6.3V ceramic capacitor (X5R or X7R) is a good starting point for the input capacitor selection. Output Voltage Setting Resistor Selection For ISL9104, ISL9104A adjustable output option, the voltage resistors, R1 and R2, as shown in Figure 21, set the desired output voltage values. The output voltage can be calculated using Equation 2: where VFB is the feedback voltage (typically it is 0.8V). The current flowing through the voltage divider resistors can be calculated as VO/(R1 + R2), so larger resistance is desirable to minimize this current. On the other hand, the FB pin has leakage current that will cause error in the output voltage setting. The leakage current has a typical value of 0.1µA. To minimize the accuracy impact on the output voltage, select the R2 no larger than 200kΩ. For adjustable output versions, C3 (shown in Figure 21) is highly recommended for improving stability and achieving better transient response. Table 1 provides the recommended component values for some output voltage options. ΔI V O 1 V O V IN  – ⎝⎠ ⎜⎟ ⎛⎞ • Lf S •  = (EQ. 1) V O V FB 1 R 1 R 2  + ⎝⎠ ⎜⎟ ⎛⎞ • = (EQ. 2) ISL9104, ISL9104A 
