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TPS54226 Datasheet(PDF) 8 Page - Texas Instruments |
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TPS54226 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 32 page TPS54226 SLVSA14E – OCTOBER 2009 – REVISED JULY 2011 www.ti.com Current Protection The output over-current protection (OCP) is implemented using a cycle-by-cycle valley detect control circuit. The switch current is monitored by measuring the low-side FET switch voltage between the SW pin and GND. This voltage is proportional to the switch current. To improve accuracy, the voltage sensing is temperature compensated. During the on time of the high-side FET switch, the switch current increases at a linear rate determined by Vin, Vout, the on-time and the output inductor value. During the on time of the low-side FET switch, this current decreases linearly. The average value of the switch current is the load current Iout. If the measured voltage is above the voltage proportional to the current limit, then the device constantly monitors the low-side FET switch voltage, which is proportional to the switch current, during the low-side on-time. The converter maintains the low-side switch on until the measured voltage is below the voltage corresponding to the current limit at which time the switching cycle is terminated and a new switching cycle begins. In subsequent switching cycles, the on-time is set to a fixed value and the current is monitored in the same manner. There are some important considerations for this type of over-current protection. The load current one half of the peak-to-peak inductor current higher than the over-current threshold. Also when the current is being limited, the output voltage tends to fall as the demanded load current may be higher than the current available from the converter. This may cause the output under-voltage protection circuit to be activated. When the over current condition is removed, the output voltage returns to the regulated value. This protection is non-latching. Over/Under Voltage Protection The TPS54226 detects over and undervoltage conditions by monitoring the feedback voltage (VFB). This function is enabled after approximately 1.7 times the soft-start time. When the feedback voltage becomes higher than 120% of the target voltage, the OVP comparator output goes high and the circuit latches the high-side MOSFET driver turns off and the low-side MOSFET turns on. When the feedback voltage becomes lower than 70% of the target voltage, the UVP comparator output goes high and an internal UVP delay counter begins. After 250 μs, the device latches off both internal top and bottom MOSFET. UVLO Protection Undervoltage lock out protection (UVLO) monitors the voltage of the VREG5 pin. When the VREG5 voltage is lower than UVLO threshold voltage, the TPS54226 is shut off. This is protection is non-latching. Thermal Shutdown Thermal protection is self-activating. If the junction temperature exceeds the threshold value (typically 150 °C), the TPS54226 shuts off. This protection is non-latching. 8 Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Link(s): TPS54226 |
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