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MIC5159BM6 Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC5159BM6 Datasheet(HTML) 8 Page - Micrel Semiconductor |
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8 / 23 page  MIC5159 Micrel M9999-041204 8 April 2004 The typical thermal resistance from the junction to the case ( θ JC) of a TO-263 (D 2 pack) is 6 °C/W. Adding 0.2°C/W for case to sink thermal resistance ( θ CS), the heatsink must have a sink to ambient thermal resistance ( θ SA) of: θ SA = θJA– (θJC + θCS) θ SA = 25.71°C/W – (6°C/W + 0.2°C/W) θ SA = 19.51°C/W According to the calculations, the heatsink must have a θ SA of 19.51 °C/W or better. For a full discussion of heat sinking and thermal effects on voltage regulators, refer to the “Regulator Thermals” section of Micrel’s Designing with Low-Dropout Voltage Regulators handbook. Short-Circuit Current Limit The above thermal design calculations apply to normal operation. In the case where the P-Channel MOSFET must survive extended periods of short-circuit current, another approach for thermal design must be considered. Due to the fact that the MIC5159 delivers constant current limiting, power dissipated by the MOSFET is equal to the input voltage multiplied by the maximum output current. Figure 1 shows a simple, inexpensive circuit that allows the current limiting to be re-entrant. This reduces power dissipa- tion in current limited conditions. As the output voltage begins to drop, the differential voltage across the input and output increases. This pulls the current sense voltage lower, reduc- ing the amount of output current to maintain 50mV across the sense resistor. This reduction in output current equates to a reduction in power dissipation in the MOSFET. Figures 2 and 3 show a comparison of linear current limiting versus the re- entrant current limiting scheme implemented in Figure 1. MIC5159-1.8BM6 RVOUT 1k9 RSENSE 25m9 C1 10µF 3.3 VIN RVIN 109 Q1,2,3 Si4433DYx3 ISENSE VIN GATE VOUT C2 47µF 1.8 VOUT 1.5A Figure 1. Re-Entrant Current Limit 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 OUTPUT CURRENT (A) Constant Current Limit Re-Entrant Current Limit Figure 2. Output Voltage Characteristics Re-Entrant Current Limit 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 OUTPUT VOLTAGE (V) Re-Entrant Current Limiting Constant Current Limiting Figure 3. Power Dissipation vs. Output Voltage Enable/Shutdown The MIC5159 comes with an active-high enable pin that allows the regulator to be disabled. Forcing the enable pin low disables the regulator and sends it into a low off-mode- current state. Forcing the enable pin high enables the output voltage. This part is CMOS and the enable pin cannot be left floating; a floating enable pin may cause an indeterminate state on the output. Output Capacitor The MIC5159 requires an output capacitor to maintain stabil- ity and improve transient response. Proper selection is impor- tant to ensure proper operation. The MIC5159 output capaci- tor selection is highly dependent upon the components and the application. With a very high gate charge (gate capaci- tance) MOSFET, the output requires a much larger valued ceramic capacitor for stability. As an alternative to a large valued ceramic capacitor, a smaller-valued tantalum capaci- tor can be used to provide stability. At higher load currents, lower R DS(ON) MOSFETs are used; these MOSFETs typi- cally having much larger gate charge. If the application does not require ultra-low-dropout voltage, smaller values of ce- ramic capacitance may be used. Input Capacitor An input capacitor of 1.0 µF or greater is recommended when the device is more than 4 inches away from the bulk AC supply capacitance or when the supply is a battery. Small, surface mount, ceramic capacitors can be used for bypassing the input to the regulator, further improving the integrity of the output voltage. Larger input capacitors may be required depending on the impedance of the source and the output load requirements. |
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