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TPS65235-1RUKR Datasheet(PDF) 10 Page - Texas Instruments |
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TPS65235-1RUKR Datasheet(HTML) 10 Page - Texas Instruments |
10 / 39 page . SS TCAP SS C (nF) t (ms) 0 8 x I ( A) P SW (OCP) I 5 x I 0.8A SW (OCP) I 3 x I 0.8A 10 TPS65235-1 SLVSDP1 – JANUARY 2017 www.ti.com Product Folder Links: TPS65235-1 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Feature Description (continued) 7.3.3 Boost Converter Current Limit The boost converter has the cycle-by-cycle peak current limit on the internal Power MOSFET switch to serve as the secondary protection when LNB output is hard short. With ISW bit default setting “0” on I2C control register 0x01, the switch current limit ISW is proportional as LDO current limit I(OCP) set by ISET pin resistor, and the relationship can be expressed as: (2) For the 5 V VIN, if LNB current load is up to 1 A, the ISW bit should be written as “1”, the switch current limit ISW for the internal Power MOSFET is: (3) While due to the high power loss at 5 V, VIN, it has a chance to trigger the thermal shutdown before the loading is up to 1 A, especially the VLNB output is high. 7.3.4 Charge Pump The charge pump circuitry generates a voltage to drive the NMOS of the linear regulator. The voltage across the charge pump capacitor between VLNB and VCP is about 5.4 V, so the absolute value of the VCP voltage will be VLNB + 5.4 V. 7.3.5 Slew Rate Control When LNB output voltage transits from 13.4 V to 18.2 V or 18.2 V to 13.4 V, the cap at pin TCAP controls the transition time. This transition time makes sure the boost converter output to follow LNB output change. Usually boost converter has low bandwidth and can’t response fast. The voltage at TCAP acts as the reference voltage of the linear regulator. The boost converter’s reference is also based on TCAP with additional fixed voltage to generate a 0.8 V above the LNB output. The charging and discharging current is 10 µA, thus the transition time can be estimated as: (4) A 22-nF capacitor generates about 2 ms transition time. In light load conditions, when LNB output voltage is set from 18.2 V to 13.4 V, the voltage drops very slow, which causes wrong VOUT_GOOD (Bit 0 at status register 0x02) logic for LNB output voltage detection. TPS65235-1 has integrated a pull down circuit to pull down the output during the transition. This ensures the voltage change can follow the voltage at TCAP. When the 22-kHz tone signal is superimposing on the LNB output voltage, the pull down current can also provide square wave instead of a distorted waveforms. 7.3.6 Short Circuit Protection, Hiccup and Overtemperature Protection The LNB output current limit can be set by an external resistor. When short circuit conditions occur or current limit is triggered, the output current is clamped at the current limit for 4 ms with LDO on. If the condition retains, the converter will shut down for 128 ms and then restart. This hiccup behavior prevents IC from being overheat. The hiccup ON/OFF time can be set by I2C register. Refer to Control Register 1 for detail. The low side MOSFET of the boost converter has a peak current limit threshold which serves as the secondary protection. If boost converter’s peak current limit is triggered, the peak current will be clamped as high as 3.8 A when setting ISW default and LNB current limit up to 1 A. If loading current continues to increase, output voltage starts to drop and output power drops. Thermal shutdown prevents the chip from operating at exceedingly high temperatures. When the junction temperature exceeds 160°C, the output shuts down. When the die temperature drops below its lower threshold typically 140°C, the output is enabled. |
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