Datasheet MIC5159 (Microchip) - 8

Micrel, Inc. MIC5159 θ 2.0 JA = 25.71°C/W Constant Current Limit 1.8 The heatsink and MOSFET must have a combined 1.6 thermal resistance to meet the above criteria. 1.4 The typical thermal resistance from the junction to the AGE (V) T 1.2 case (θJC) of a TO-263 (D2 pack) is 6°C/W. Adding 1.0 0.2°C/W for case to sink thermal resistance (θ 0.8 CS), the heatsink must have a sink to ambient thermal resistance 0.6 (θ 0.4 SA) of: OUTPUT VOL 0.2 Re-Entrant θ Current Limit SA = θJA– (θJC + θCS) 0 θSA = 25.71°C/W – (6°C/W + 0.2°C/W) 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 θ OUTPUT CURRENT (A) SA = 19.51°C/W According to the calculations, the heatsink must have a
Figure 2. Output Voltage Characteristics
θSA of 19.51°C/W or better.
Re-Entrant Current Limit
For a full discussion of heat sinking and thermal effects on voltage regulators, refer to the “ Regulator Thermals” section of Micrel’s 5 Designing with Low-Dropout Voltage 4.5 Constant Current Regulators handbook. Limiting 4 3.5
Short-Circuit Current Limit
3 The above thermal design calculations apply to normal 2.5 operation. In the case where the P-Channel MOSFET Re-Entrant 2 Current Limiting must survive extended periods of short-circuit current, 1.5 another approach for thermal design must be 1 considered. Due to the fact that the MIC5159 delivers POWER DISSIPATION (W) 0.5 constant current limiting, power dissipated by the 00 0.5 1 1.5 MOSFET is equal to the input voltage multiplied by the OUTPUT VOLTAGE (V) maximum output current. Figure 1 shows a simple, inexpensive circuit that allows
Figure 3. Power Dissipation vs. Output Voltage
the current limiting to be re-entrant. This reduces power dissipation in current limited conditions. As the output
Enable/Shutdown
voltage begins to drop, the differential voltage across the The MIC5159 comes with an active-high enable pin that input and output increases. This pulls the current sense allows the regulator to be disabled. Forcing the enable voltage lower, reducing the amount of output current to pin low disables the regulator and sends it into a low off- maintain 50mV across the sense resistor. This reduction mode-current state. Forcing the enable pin high enables in output current equates to a reduction in power the output voltage. This part is CMOS and the enable pin dissipation in the MOSFET. Figures 2 and 3 show a cannot be left floating; a floating enable pin may cause comparison of linear current limiting versus the re- an indeterminate state on the output. entrant current limiting scheme implemented in Figure 1. RVOUT
Output Capacitor
The MIC5159 requires an output capacitor to maintain 3.3 V RVIN Q1,2,3 stability and improve transient response. Proper IN Si4433DYx3 1.8 VOUT 1.5A selection is important to ensure proper operation. The RSENSE MIC5159 output capacitor selection is highly dependent
MIC5159-1.8BM6/YM6
upon the components and the application. With a very C1 C2 high gate charge (gate capacitance) MOSFET, the 10µF ISENSE GATE 47µF output requires a much larger valued ceramic capacitor VIN ADJ for stability. As an alternative to a large valued ceramic capacitor, a smaller-valued tantalum capacitor can be used to provide stability. At higher load currents, lower RDS(ON) MOSFETs are used; these MOSFETs typically having much larger gate charge. If the application does
Figure 1. Re-Entrant Current Limit
not require ultra-low-dropout voltage, smaller values of ceramic capacitance may be used. June 2006 8 M9999-062706