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MIC3775-1.65BMM Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC3775-1.65BMM Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 13 page  Micrel, Inc. MIC3775 December 2006 11 M9999-121906 with an input as low as 3.0V or 2.5V respectively. This gives the PNP-based regulators a distinct advantage over older, NPN-based linear regulators. Minimum Load Current The MIC3775 regulator is specified between finite loads. If the output current is too small, leakage currents dominate and the output voltage rises. A 10mA minimum load current is necessary for proper regulation. Adjustable Regulator Design ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = R2 R1 1 1.240V VOUT Figure 2. Adjustable Regulator with Resistors The MIC3775 allows programming the output voltage anywhere between 1.24V and the 6V maximum operating rating of the family. Two resistors are used. Resistors can be quite large, up to 1MΩ, because of the very high input impedance and low bias current of the sense comparator. The resistor values are calculated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = 1 1.240 V R2 R1 OUT Where VO is the desired output voltage. Figure 2 shows component definition. Applications with widely varying load currents may scale the resistors to draw the minimum load current required for proper operation (see above). Power MSOP-8 Thermal Characteristics One of the secrets of the MIC3775’s performance is its power MSOP-8 package featuring half the thermal resistance of a standard MSOP-8 package. Lower thermal resistance means more output current or higher input voltage for a given package size. Lower thermal resistance is achieved by joining the four ground leads with the die attach paddle to create a single-piece electrical and thermal conductor. This concept has been used by MOSFET manufacturers for years, proving very reliable and cost effective for the user. Thermal resistance consists of two main elements, θJC (junction-to-case thermal resistance) and θCA (case-to- ambient thermal resistance). See Figure3. θJC is the resistance from the die to the leads of the package. θCA is the resistance from the leads to the ambient air and it includes θCS (case-to- sink thermal resistance) and θSA (sink-to-ambient thermal resistance). Using the power MSOP-8 reduces the θJC dramatically and allows the user to reduce θCA. The total thermal resistance, θJA (junction-to-ambient thermal resistance) is the limiting factor in calculating the maximum power dissipation capability of the device. Typically, the power MSOP-8 has a θJA of 80°C/W, this is significantly lower than the standard MSOP-8 which is typically 160°C/W. θCA is reduced because pins 5 through 8 can now be soldered directly to a ground plane which significantly reduces the case-to-sink thermal resistance and sink-to- ambient thermal resistance. Low-dropout linear regulators from Micrel are rated to a maximum junction temperature of 125°C. It is important not to exceed this maximum junction temperature during operation of the device. To prevent this maximum junction temperature from being exceeded, the appropriate ground plane heatsink must be used. Figure 3. Thermal Resistance Figure 4 shows copper area versus power dissipation with each trace corresponding to a different temperature rise above ambient. From these curves, the minimum area of copper necessary for the part to operate safely can be deter- mined. The maximum allowable temperature rise must be calculated to determine operation along which curve. Figure 4. Copper Area vs. Power-MSOP Power Dissipation (∆TJA) |
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