Datasheet MIC5159 (Microchip) - 9

Micrel, Inc. MIC5159
Input Capacitor Adjustable Regulator Design
An input capacitor of 1.0µF or greater is recommended when the device is more than 4 inches away from the The MIC5159 allows programming the output voltage bulk AC supply capacitance or when the supply is a anywhere between 1.235V to VIN. Two resistors are battery. Small, surface mount, ceramic capacitors can be used. See Figure 4. The resistor values are calculated used for bypassing the input to the regulator, further by: improving the integrity of the output voltage. Larger input ⎛⎛ VOUT ⎞ ⎞ capacitors may be required depending on the R1 = R2 × ⎜⎜ − ⎜⎜ ⎟⎟ ⎟⎟ 1 ⎝1.235 ⎠ impedance of the source and the output load ⎝ ⎠ requirements. Where VOUT is the desired output voltage.
Layout Considerations
V V Input and output capacitor placement should be as close IN RS OUT Si3445 as possible to the input and output, respectively. Trace resistance between the current sense and the MOSFET
MIC5159-x.x
C source should be minimized. Trace resistance will OUT = 10µF R1 ceramic increase dropout voltage. This is more of a factor at IS GATE higher output currents. IN ADJ Also, a minimum amount of distance between the gate EN R2 pin, on the MIC5159, and the P-Channel MOSFET gate GND is recommended. A long trace can create a small parasitic inductor. This, coupled to the gate capacitance of the MOSFET, can create a high frequency tank circuit.
Figure 4. Adjustable Regulator Design
A small 50Ω resistor in series with the gate may be required to eliminate high-frequency noise. June 2006 9 M9999-062706