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MCP16251 Datasheet(PDF) 16 Page - Microchip Technology |
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MCP16251 Datasheet(HTML) 16 Page - Microchip Technology |
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16 / 32 page  MCP16251/2 DS20005173B-page 16 2013 - 2016 Microchip Technology Inc. Several parameters are used to select the inductor correctly: maximum rated current, saturation current and copper resistance (ESR). For boost converters, the inductor current can be much higher than the output current. The lower the inductor ESR, the higher the efficiency of the converter, a common trade-off in size versus efficiency. The saturation current typically specifies a point at which the inductance has rolled off a percentage of the rated value. This can range from a 20-to-40% reduction in inductance. As the inductance rolls off, the inductor ripple current increases, as does the peak switch current. It is important to keep the inductance from rolling off too much, causing switch current to reach the peak limit. 5.6 One-Cell Application Considerations The MCP16251/2 was designed to operate with a wide input voltage range after start-up, down to 0.35V, to accommodate a large variety of input sources. When considering a primary power solution for a design, the battery type and load current needs must be carefully selected. The MCP16251/2 start-up voltage is typical 0.82V at 1 mA load but this does not act as an UVLO start-up threshold. The start-up sequence is detailed in Section 4.2.1 “Low-Voltage Start-up” and begins with the charging of the output capacitor at limited, constant current until the output voltage equals the input voltage. The device starts draining current to bias its internal circuitry before the 0.82V input and cannot start-up or operate well with high-impedance sources because their voltage varies in time, from zero to over 0.82V (i.e., energy harvesting). Start-up voltage is the point where the device starts switching in closed loop and the output is regulated and depends on load and temperature as shown in Figures 2-12, 2-13 and 2-14. There are a few aspects to deal with when designing a step-up converter supplied from one alkaline or rechargeable cell. Batteries are available in a variety of sizes and chemistries and can support a variety of drain rates. No matter the chemistry, most batteries have several things in common. They should not be drained below their specified FEP (Functional End Point or Cut-Off Voltage). Below this point, if the battery has a load applied to it, there will not be enough energy to deliver power because all usable capacity is used. For an alkaline cell, FEP is 0.9V or 0.8V. Using the alkaline cell below the FEP will increase the risk of leakage. There is an exception for alkaline batteries: if the battery voltage is strictly monitored, it can be drained down to 0.5V in one-cell applications only. For a rechargeable NiMH cell, the FEP value is usually 1.0V – 1.1V. As the battery discharges, its deliverable energy or capacity decreases and the internal resistance increases. For example, the internal resistance of an alkaline cell goes up to 1 when discharged causing a voltage drop of up to hundreds of mV on the battery terminals under load conditions. This aspect will result in the converter’s inability to start-up properly in applications which require short periods of ON time and long periods of Sleep. When the load is removed, the battery voltage slowly recovers. These long cycles may bring the battery voltage close to its nominal value. However, a nearly depleted battery will not be capable of maintaining its voltage once the heavier load is applied during the next cycle. At each attempt, the converter drains a large amount of current to restart (see Figure 2-21), weakening the battery even further. As the battery voltage recovers in time, the converter will try to start-up as soon as its minimum input voltage-vs.-load condition is reached. In conclusion, with a battery discharged down to its FEP, a boost converter may start-up and work well under light load (in PFM mode), but will stop or lose regulation when a high load current is required. FIGURE 5-1: Example of a Typical Constant-Current Load Discharge Profile for an Alkaline and NiMH Cell. FIGURE 5-2: MCP16251 3.3VOUT/100 mA Boost Converter Waveforms Powered from One Alkaline Cell Discharged to 1.25V Open Load Voltage (Cell Internal Resistance Is Approximately 0.7 Ohms). 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 0 5 10 15 20 25 Run Time (h) I BATTERY = 100 mA 2000 mAh NiMH ----- AA Alkaline I BATTERY = 200 mA VBATTERY = 1.25V VBATTERY = 1.09V VOUT EN IL = Boost Inductor Current IL IBATTERY = Average of IL Boost Converter Load = 100 mA 1V/div 1V/div 200 mV/div 200 mV/div EN Turn ON/OFF Boost Converter Load = 100 mA |
Similar Part No. - MCP16251_16 |
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Similar Description - MCP16251_16 |
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