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MCP1640T-ICHY Datasheet(PDF) 15 Page - Microchip Technology |
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MCP1640T-ICHY Datasheet(HTML) 15 Page - Microchip Technology |
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15 / 32 page  2010-2015 Microchip Technology Inc. DS20002234D-page 15 MCP1640/B/C/D 5.0 APPLICATION INFORMATION 5.1 Typical Applications The MCP1640/B/C/D synchronous boost regulator operates over a wide input and output voltage range. The power efficiency is high for several decades of load range. Output current capability increases with input voltage and decreases with increasing output voltage. The maximum output current is based on the N-Channel peak current limit. Typical characterization curves in this data sheet are presented to display the typical output current capability. 5.2 Adjustable Output Voltage Calculations and Maximum Output Current To calculate the resistor divider values for the MCP1640/B/C/D, the following equation can be used, where RTOP is connected to VOUT, RBOT is connected to GND and both are connected to the FB input pin. EQUATION 5-1: EXAMPLE 1: EXAMPLE 2: The internal error amplifier is of transconductance type; its gain is not related to the resistors' value. There are some potential issues with higher-value resistors. For small surface-mount resistors, environment contamination can create leakage paths that significantly change the resistor divider ratio and modify the output voltage tolerance. Smaller feedback resistor values will increase the current drained from the battery by a few µA, but will result in good regulation over the entire temperature range and environment conditions. The feedback input leakage current can also impact the divider and change the output voltage tolerance. For boost converters, the removal of the feedback resistors during operation must be avoided. In this case, the output voltage will increase above the absolute maximum output limits of the MCP1640/B/C/D and damage the device. The maximum device output current is dependent upon the input and output voltage. For example, to ensure a 100 mA load current for VOUT = 3.3V, a minimum of 1.0-1.1V input voltage is necessary. If an application is powered by one Li-Ion battery (VIN from 3.0V to 4.2V), the minimum load current the MCP1640/B/C/D can deliver is close to 300 mA at 5.0V output and a maximum of 500 mA (Figure 2-3). 5.2.1 VIN >VOUT SITUATION For VIN >VOUT, the output voltage will not remain in regulation. VIN >VOUT is an unusual situation for a boost converter, and there is a common issue when two Alkaline cells (2 x 1.6V typical) are used to boost to 3.0V output. The Input-to-Output Bypass option is recommended to be used in this situation until the batteries’ voltages go down to a safe headroom. A minimum headroom of approximately 150 to 200 mV between VOUT and VIN must be ensured, unless a low- frequency, high-amplitude output ripple on VOUT is expected. The ripple and its frequency is VIN and load dependent. The higher the VIN, the higher the ripple and the lower its frequency. 5.3 Input Capacitor Selection The boost input current is smoothed by the boost induc- tor reducing the amount of filtering necessary at the input. Some capacitance is recommended to provide decoupling from the source. Low ESR X5R or X7R are well suited since they have a low temperature coefficient and small size. For most applications, 4.7 µF of capaci- tance is sufficient at the input. For high-power applica- tions that have high source impedance or long leads, connecting the battery to the input 10 µF of capacitance is recommended. Additional input capacitance can be added to provide a stable input voltage. Table 5-1 contains the recommended range for the input capacitor value. 5.4 Output Capacitor Selection The output capacitor helps provide a stable output voltage during sudden load transients and reduces the output voltage ripple. As with the input capacitor, X5R and X7R ceramic capacitors are well suited for this appli- cation. Using other capacitor types (aluminum or tanta- lum) with large ESR has an impact on the converter's efficiency and maximum output power (see AN1337). VOUT =3.3V VFB =1.21V RBOT = 309 k RTOP = 533.7 k (Standard Value = 536 k) VOUT =5.0V VFB = 1.21V RBOT =309 k RTOP =967.9 k (Standard Value = 976 k) R TOP R BOT V OUT V FB ---------------- 1 – = |
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