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9 / 16 page MC33470 9 MOTOROLA ANALOG IC DEVICE DATA the compensation pin to ground. The error amplifier input is tied to the sense pin which also has an internal 20 µA current source to ground. The current source is intended to provide a 24 mV offset when an external 1.2 k resistor is placed between the output voltage and the sense pin. The 24 mV offset voltage is intended to allow a greater dynamic load regulation range within a given specified tolerance for the output voltage. The offset may be increased by increasing the resistor value. The offset can be eliminated by connecting the sense pin directly to the regulated output voltage. The voltage reference consists of an internal, low temperature coefficient, reference circuit with an added offset voltage. The offset voltage level is the output of the digital–to–analog converter. Control bits VID0 through VID4 control the amount of offset voltage which sets the value of the voltage reference, as shown in Table 1. The VID0–4 input bits each have internal 10 k pullup resistances. Therefore, the reference voltage, and the output voltage, may be programmed by connecting the VID pins to ground for logic “0” or by an open for a logic “1”. Typically, a logic “1” will be recognized by a voltage > 0.67 x VCC. A logic “0” is a voltage < VCC/3. MOSFET Switch Outputs The output MOSFETs are designed to switch a maximum of 18 V, with a peak drain current of 2.0 A. Both G1 and G2 output drives are designed to switch N–channel MOSFETs. Output drive controls to G1 and G2 are phased to prevent cross conduction of the internal IC output stages. Output dead time is typically 100 nanoseconds between G1 and G2 in order to minimize cross conduction of the external switching MOSFETs. Current Limit and Soft–Start Controls The soft–start circuit is used both for initial power application and during current limit operation. A single external capacitor and an internal 10 µA current source control the rate of voltage increase at the error amplifier output, establishing the circuit turn on time. The G1 output will increase from zero duty cycle as the voltage across the soft–start capacitor increases beyond about 0.5 V. When the soft–start capacitor voltage has reached about 1.5 V, normal duty cycle operation of G1 will be allowed. An overcurrent condition is detected by the current limit amplifier. The current limit amplifier is activated whenever the G1 output is high. The current limit amplifier compares the voltage drop across the external MOSFET driven by G1, as measured at the IFB pin, with the voltage at the Imax pin. Because the Imax pin draws 190 µA of input current, the overcurrent threshold is programmed by an external resistor. Referring to Figure 13, the current limit resistor value can be determined from the following equation: R1 + [( I L(max) )( R DS(on) )] (Imax) where: I L(max) + I O ) I ripple 2 = Maximum load current = Inductor peak to peak ripple current IO Iripple OUTEN Input and OT Output Pins On and off control of the MC33470 may be implemented with the OUTEN pin. A logic “1” applied the OUTEN pin, where a logic “1” is above 2.0 V, will allow normal operation of the MC33470. The OUTEN pin also has multiple thresholds to provide over temperature protection. An negative temperature coefficient thermistor can be connected to the OUTEN pin, as shown in Figure 15. Together with RS, a voltage divider is formed. The divider voltage will decrease as the thermistor temperature increases. Therefore, the thermistor should be mounted to the hottest part on the circuit board. When the OUTEN voltage drops below 2.0 V typically, the MC33470 OT pin open collector output will switch from a logic “1” to a logic “0”, providing a warning to the system. If the OUTEN voltage drops below 1.7 V, both G1 and G2 output driver pins are latched to a logic “0” state. Figure 15. OUTEN/OT Overtemperature Function VCC RS NTC Thermistor MC33470 OUTEN OT 10 k VCC APPLICATIONS INFORMATION Design Example Given the following requirements, design a switching dc–to–dc converter: = 5.0 V = 12 V = 10111 – Output Voltage = 2.8 V = 0.3 A to 14 A VCC VCCP VID4–0 bits Output current Efficiency > 80% at full load Output ripple voltage ≈ 1% of output voltage 1. Choose power MOSFETs. In order to meet the efficiency requirement, MOSFETs should be chosen which have a low value of RDS(on). However, the threshold voltage rating of the MOSFET must also be greater than 1.5 V, to prevent turn on of the synchronous rectifier MOSFETs due to dv/dt coupling through the Miller capacitance of the MOSFET drain–to–source junction. Figure 16 shows the gate voltage transient due to this effect. |
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