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MP155 Datasheet(PDF) 10 Page - Monolithic Power Systems |
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MP155 Datasheet(HTML) 10 Page - Monolithic Power Systems |
10 / 20 page MP155 – ENERGY EFFICIENT OFF-LINE REGULATOR MP155 Rev. 1.12 www.MonolithicPower.com 10 10/28/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2013 MPS. All Rights Reserved. MOS Diode IL Ipeak Io Vo VFB 2.5V Figure 3: VFB vs. Vout Monitoring the sampling capacitor regulates the output voltage can be regulated, as per the following equation: + =× R1 R2 Vo 2.5V R2 Frequency Foldback Under light load or no load conditions, the output drops very slowly, which increases the time for the MOSFET to turn ON again; i.e., frequency decreases as the load decreases. So the MP155 can maintain a high efficiency under light load condition by reducing the switching frequency automatically. The switching frequency can be obtained as: in o o s peak o in (V V ) V f 2L(I I ) V − =⋅ − , for CCM − =⋅ in O o o s 2 peak in 2(V V ) I V f LI V , for DCM At the same time, the peak current limit decreases from 290mA as the OFF time increases. In standby mode, the frequency and the peak current are both minimized, allowing for a small dummy load. As a result, the peak- current-compression function helps to reduce no- load consumption. Determine the peak current limitation from the following equation where τoff is the power module OFF time: =− μ × τ − μ Peak off I 290mA (1mA / S) ( 18 S) Minimum off time limitation The MP155 implements a minimum OFF time limit. During the normal operation, the minimum OFF time limit is 18µs; during start up, the minimum OFF time limit gradually drops from 72µs, to 36µs, then to 18 µs (see Figure 4). Each minimum OFF time has 128 switching cycles. This soft-start function allows for safe start-up. 128 Switching cycle 72us ≥ 128 Switching cycle 36us ≥ 18us ≥ Driver Figure 4: τminoff at Start-Up EA Compensation - + VFB - + Vref 2.5V EA + + - + FB Comparator Vramp Vramp Ipeak Figure 5: EA and Ramp Compensation To improve load regulation, the MP150 implements an error amplifier (EA) compensation function ( Figure 5 ). The MP155 samples the feedback voltage 6µs after the MOSFET turns off. EA compensation regulates the 2.5V reference voltage with the load, thus improving the power module regulation. RAMP Compensation An internal ramp compensation circuit precisely maintains the output voltage. An additional exponential voltage sinking source pulls down the feedback comparator’s reference voltage as shown in Figure 5. The ramp compensation is relative to the load conditions: Under full-load conditions, the compensation is ~1mV/µs; with a decreasing load, the compensation increases exponentially. |
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