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TPS54540-Q1 Datasheet(PDF) 11 Page - Texas Instruments |
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TPS54540-Q1 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 44 page  OUT(max) (max) IN(min) OUT(max) DS(on) OUT(max) L V D x (V - I x R VF) - VF I x R = + + TPS54540-Q1 www.ti.com SLVSC56A – SEPTEMBER 2013 – REVISED OCTOBER 2013 When in Eco-mode, the COMP pin voltage is clamped at 600 mV and the high side MOSFET is inhibited. Since the device is not switching, the output voltage begins to decay. The voltage control loop responds to the falling output voltage by increasing the COMP pin voltage. The high side MOSFET is enabled and switching resumes when the error amplifier lifts COMP above the pulse skipping threshold. The output voltage recovers to the regulated value, and COMP eventually falls below the Eco-mode pulse skipping threshold at which time the device again enters Eco-mode. The internal PLL remains operational when in Eco-mode. When operating at light load currents in Eco-mode, the switching transitions occur synchronously with the external clock signal. During Eco-mode operation, the TPS54540-Q1 senses and controls peak switch current, not the average load current. Therefore the load current at which the device enters Eco-mode is dependent on the output inductor value. The circuit in enters Eco-mode at about 25.3 mA output current. As the load current approaches zero, the device enters a pulse skip mode during which it draws only 146 μA input quiescent current. Low Dropout Operation and Bootstrap Voltage (BOOT) The TPS54540-Q1 provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and SW pins provides the gate drive voltage for the high side MOSFET. The BOOT capacitor is refreshed when the high side MOSFET is off and the external low side diode conducts. The recommended value of the BOOT capacitor is 0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is recommended for stable performance over temperature and voltage. When operating with a low voltage difference from input to output, the high side MOSFET of the TPS54540-Q1 will operate at 100% duty cycle as long as the BOOT to SW pin voltage is greater than 2.1V. When the voltage from BOOT to SW drops below 2.1V, the high side MOSFET is turned off and an integrated low side MOSFET pulls SW low to recharge the BOOT capacitor. To reduce the losses of the small low side MOSFET at high output voltages, it is disabled at 24 V output and re-enabled when the output reaches 21.5 V. Since the gate drive current sourced from the BOOT capacitor is small, the high side MOSFET can remain on for many switching cycles before the MOSFET is turned off to refresh the capacitor. Thus the effective duty cycle of the switching regulator can be high, approaching 100%. The effective duty cycle of the converter during dropout is mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, the low side diode voltage and the printed circuit board resistance. The start and stop voltage for a typical 5 V output application is shown in Figure 22 where the Vin voltage is plotted versus load current. The start voltage is defined as the input voltage needed to regulate the output within 1% of nominal. The stop voltage is defined as the input voltage at which the output drops by 5% or where switching stops. During high duty cycle (low dropout) conditions, inductor current ripple increases when the BOOT capacitor is being recharged resulting in an increase in output voltage ripple. Increased ripple occurs when the off time required to recharge the BOOT capacitor is longer than the high side off time associated with cycle by cycle PWM control. At heavy loads, the minimum input voltage must be increased to insure a monotonic startup. Equation 1 can be used to calculate the minimum input voltage for this condition. R (1) Where: Dmax ≥ 0.9 IB2SW = 100 µA VF = Forward Drop of the Catch Diode TSW = 1 / Fsw VB2SW = VBOOT + VF VBOOT = (1.41 x VIN - 0.554 - VF / TSW - 1.847 x 10 3 x IB2SW) / (1.41 + 1 / Tsw) RDS(on) = 1 / (-0.3 x VB2SW 2 + 3.577 x VB2SW - 4.246) spacer Copyright © 2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: TPS54540-Q1 |
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