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TPS5405 Datasheet(PDF) 9 Page - Texas Instruments |
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TPS5405 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 19 page ss ref ss ss C (nF) V (V) t (ms) I ( A) ´ = m TPS5405 www.ti.com SLVSBF7B – MAY 2012 – REVISED APRIL 2013 Pulse Skipping Mode The TPS5405 is designed to operate in pulse skipping mode at light load currents to boost light load efficiency. When the peak inductor current is lower than 300 mA typically, the COMP pin voltage falls to 0.5 V typically and the device enters pulse skipping mode. When the device is in pulse skipping mode, the COMP pin voltage is clamped at 0.5 V internally which prevents the high side integrated MOSFET from switching. The peak inductor current must rise above 300 mA for the COMP pin voltage to rise above 0.5 V and exit pulse skipping mode. Since the integrated current comparator catches the peak inductor current only, the average load current entering pulse skipping mode varies with the applications and external output filters. Voltage Reference (VSENSE) The voltage reference system produces a ±2% initial accuracy voltage reference (±4% over temperature) by scaling the output of a temperature stable bandgap circuit. The typical voltage reference is designed at 0.8 V. Bootstrap Voltage (BOOT) The TPS5405 has an integrated boot regulator and requires a 0.1-µF ceramic capacitor between the BOOT and LX pins to provide the gate drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R or X5R grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve drop out, the TPS5405 is designed to operate at 100% duty cycle as long as the BOOT to LX pin voltage is greater than 2.1 V typically. Programmable Slow Start Using SS Pin It is recommended to program the slow start time externally because no slow start time is implemented internally. The TPS5405 effectively uses the lower voltage of the internal voltage reference or the SS pin voltage as the power supply’s reference voltage fed into the error amplifier and will regulate the output accordingly. A capacitor (CSS) on the SS pin to ground implements a slow start time. The TPS5405 has an internal pull-up current source of 2 μA that charges the external slow start capacitor. The equation for the slow start time (10% to 90%) is shown in Equation 2. The internal Vref is 0.8 V and the ISS current is 2 μA. (2) The slow start time should be set between 1 ms to 10 ms to ensure good start-up behavior. The slow start capacitor should be no more than 27 nF. If during normal operation, the input voltage drops below the VIN UVLO threshold, or a thermal shutdown event occurs, the TPS5405 stops switching. Error Amplifier The TPS5405 has a transconductance amplifier for the error amplifier. The error amplifier compares the VSENSE voltage to the internal effective voltage reference presented at the input of the error amplifier. The transconductance of the error amplifier is 92 μA/V during normal operation. Frequency compensation components are connected between the COMP pin and ground. Slope Compensation To prevent the sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the TPS5405 adds a built-in slope compensation which is a compensating ramp to the switch current signal. Overcurrent Protection and Frequency Shift The TPS5405 implements current mode control that uses the COMP pin voltage to turn off the high-side MOSFET on a cycle by cycle basis. Every cycle the switch current and the COMP pin voltage are compared; when the peak inductor current intersects the COMP pin voltage, the high-side switch is turned off. During overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high, causing the switch current to increase. The COMP pin has a maximum clamp internally, which limits the output current. Copyright © 2012–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: TPS5405 |
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