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LMR70503 Datasheet(PDF) 11 Page - Texas Instruments |
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LMR70503 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 24 page Switching frequency Peak Inductor Current IPEAK-MAX Dummy Load 0 Load current Const Frequency (2) Const Peak Current (3) Ton-min (1) FSW-MIN |VOUT| 0 0 0 Load current Load current Load current LMR70503 www.ti.com SNVS850A – JUNE 2012 – REVISED APRIL 2013 Figure 31. The LMR70503 Operation Modes vs. Load Current Minimum Switching Frequency Operation In a typical peak current mode controlled DC-DC converter, the peak current limit is constant and the switching frequency decreases when load current reduces. To maintain low noise operation and avoid audio frequency interference, the minimum switching frequency of the LMR70503 is limited at 500 kHz typically. At heavy load, the peak current limit remains constant and the switching frequency varies with the load to regulate the output voltage. With reduced loading, the absolute output voltage is going to be charged higher than regulation if the switching frequency cannot decrease accordingly. Therefore, to regulate the output voltage with minimum frequency at light load, the peak current limit is reduced, in proportional to the output voltage offset. In this mode, as shown in Figure 31, the switching frequency is fixed to the minimum switching frequency, the peak inductor current increases with load current, and the output voltage magnitude has a small offset above regulation. Minimum ON-Time and Dummy Load When load current is near zero, the peak inductor current can not reduce further due to TON-MIN of the high side switch. Under such conditions, an internal dummy load is turned on by sensing excessive output voltage offset, which removes the extra charge from the output capacitor(s). In this condition, the switching frequency is fixed to the minimum value. The peak inductor current value is at its minimum value, as shown in Figure 31. The dummy load current is zero when the LMR70503 operates with on time higher than TON-MIN. The minimum peak inductor current is determined by IPEAK-MIN = TON-MIN × VIN / L where • VIN is the supply voltage • L is the inductance value (1) The peak inductor current is higher with higher VIN. The inductor current falling slew rate is determined by SRFALLING = (|VOUT| + VF) / L where • |VOUT| is the absolute value of the output voltage • VF is the forward voltage drop of the power diode (2) At lower |VOUT|, it takes longer time to discharge the inductor current to zero. Therefore, there is more energy to charge the output capacitor(s). The output voltage will have more offset at higher VIN and lower VOUT. The dummy load current is a function of the FB voltage: the more the offset at the FB node, the higher the dummy load current, as shown in Figure 32. Copyright © 2012–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMR70503 |
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