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RT9202B Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT9202B Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 14 page  RT9202B 10 DS9202B-03 March 2007 www.richtek.com Both VSET and VDS are referenced to VIN and a small capacitor across ROCSET helps VOCSET tracking the variations of VIN due to MOSFET switching. The over- current function will be tripped at a peak inductor current (IPEAK) determined by : The OC trip point v aries with MOSFET's RDS(ON) temperature variations. The temperature coefficient of IOCSET is 2500ppm that is used to compensate RDS(ON) temperature variations. To avoid over-current tripping in the normal operating load range, determine the ROCSET resistor value from the equation above with: 1.The maxim um RSD(ON) at the highest junction temperature 2.The minimum IOCSET from the characteristics 3.Determine IPEAK for IPEAK > IOUT(MAX) + ( ∆I)/2 where ∆I is the output inductor ripple current. Under Voltage and Over Voltage Protection The voltage at FB pin is monitored and protected against OC (over current), UV (under voltage), and OV (over voltage). The UV threshold is 0.5V and OV-threshold is 1.0V. Both UV/OV detection have 30 µs triggered delay. When OC or UV trigged, a hiccup re-start sequence will be initialized, as shown in Figure 4. Only 3 times of trigger are allowed to latch off. Hiccup is disabled during soft- start interval. Shutdown Pulling low the OCSET pin can shutdown the RT9202B PWM controller as shown in typical application circuit. Inductor Selection The RT9202B was designed for VIN = 5V, step-down application mainly. Figure 5 shows the typical topology and waveforms of step-down converter. The ripple current of inductor can be calculated as follows: ILRIPPLE = (5V - VOUT)/L × TON Because operation frequency is fixed at 300kHz, TON = 3.33 × VOUT/5V The VOUT ripple is VOUT RIPPLE = ILRIPPLE × ESR ESR is output capacitor equivalent series resistor Table 1 shows the ripple voltage of VOUT : VIN = 5V GATE CONTROL + - PWM OC DRIVE VCC OCSET R OCSET V SET+ V IN = +5V V DS+ i D UGATE PHASE I OCSET 20uF OVER-CURRENT TRIP: V DS > VSET ×× D DS(ON) OCSET OCSET i R > IR V OCSET = V IN - V SET V PHASE = V IN - V DS 0A 0V 2V 4V T1 T2 T3 TIME COUNT = 1 COUNT = 2 COUNT = 3 OVERLOAD APPLIED T0 Figure 3 Figure 4 DS(ON) OCSET OCSET PEAK R R I I × = |
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