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XRP6124 Datasheet(PDF) 7 Page - Exar Corporation |
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XRP6124 Datasheet(HTML) 7 Page - Exar Corporation |
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7 / 12 page  X XR RP P6 61 12 24 4 N No on n--S Sy yn nc ch hrro on no ou us s P PF FE ET T S Stte ep p--D Do ow wn n C Co on nttrro olllle err © 2011 Exar Corporation 7/12 Rev. 1.1.0 THEORY OF OPERATION THEORY OF OPERATION The XRP6124 utilizes a proprietary Constant On-Time (COT) control with emulated ESR. The on-time is internally set and automatically adjusts during operation, inversely with the voltage VIN, in order to maintain a constant frequency. Therefore the switching frequency is independent of the inductor and capacitor size, unlike hysteretic controllers. The emulated ESR ramp allows the use of ceramic capacitors for output filtering. At the beginning of each cycle, the XRP6124 turns on the P-Channel FET for a fixed duration. The on-time is internally set and adjusted by VIN. At the end of the on-time the FET is turned off, for a predetermined minimum off time TOFF-MIN (nominally 250ns). After the TOFF-MIN has expired the voltage at feedback pin FB is compared to a voltage ramp at the feedback comparators positive input. Once VFB drops below the ramp voltage, the FET is turned on and a new cycle starts. This voltage ramp constitutes an emulated ESR and makes possible the use of ceramic capacitors, in addition to other capacitors, as output filter for the buck converter. VOLTAGE OPTIONS The XRP6124 is available in two voltage options as shown in table 1. The low-voltage and high-voltage options have TON of 0.5µs at 12VIN and 24VIN respectively. Note that TON is inversely proportional to VIN. The constant of proportionality K, for each voltage option is shown in table 1. Variation of TON versus VIN is shown graphically in figures 6 and 7. Voltage rating (V) Part Number TON (µs) K=TONxVIN (μs.V) 3-18 XRP6124ES0.5-F 0.5 @ 12VIN 6 4.5-30 XRP6124HVES0.5-F 0.5 @ 24VIN 12 Table 1 : XRP6124 voltage options For a buck converter the switching frequency fs can be expressed in terms of VIN, VOUT and TON as follows: ON IN OUT T V V fs × = Since for each voltage option, the product of VIN and TON is the constant K shown in table 1, then switching frequency is determined by VOUT as shown in table 2. VOUT Switching frequency fs(kHz) XRP6124ES0.5-F XRP6124HVES0.5-F 1.2 200 100 1.5 250 125 1.8 300 150 2.5 417 208 3.3 550 275 5.0 833 417 12 --- 1000 Table 2: Switching frequency fs for the XRP6124 voltage options Where it is advantageous, the high-voltage option may be used for low-voltage applications. For example a 12VIN to 5VOUT conversion using a low-voltage option will result in switching frequency of 833kHz as shown in table 2. If it is desired to increase the converter efficiency, then switching losses can be reduced in half by using a high-voltage option operating at a switching frequency of 417kHz. VOUT Maximum Output Current IOUT(A) XRP6124ES0.5-F XRP6124HVES0.5-F 3.3VIN 5.0VIN 12VIN 18VIN 24VIN 1.2 5 5 4 --- --- 1.5 5 5 4 4 --- 1.8 5 5 4 4 4 2.5 4 4 4 4 4 3.3 --- 4 3 4 4 5.0 --- --- 3 3 3 12 --- --- --- 2 2 Table 3: Maximum recommended IOUT SHORT -CIRCUIT PROTECTION The purpose of this feature is to prevent an accidental short-circuit at the output from damaging the converter. The XRP6124 has a short-circuit comparator that constantly monitors the feedback node, after soft-start is |
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