Datasheet MIC5159 (Microchip) - 7

Micrel, Inc. MIC5159
Application Information
⎛ (2.5V − 1 8 . V)⎞ R = ⎜ ⎟ − m 10 Ω DS(ON) The MIC5159 is a high performance voltage regulator ⎝ 5A ⎠ controller. When used with an external P-Channel RDS(ON) = 130mΩ MOSFET and a tiny ceramic output capacitor, it forms a According to the above calculation, the minimum R wide variety of simple, inexpensive ultra-low-dropout DS(ON) is 130mΩ for a 2.5V to 1.8V LDO with 5A of output voltage regulators. current. For this design, the R DS(ON) for the FETs should
Current Sense Resistor Selection
maintain better than 130mΩ over the required temperature, current, and voltage conditions. A current sense resistor placed between the input and the current sense pin (IS) allows for programmability of Placing two or more P-Channel FETs in parallel can the current limit. This resistor can simply be calculated reduce the total R DS(ON) of the regulator. This also aids by: thermal dissipation by sharing the current and heat between the multiple FETs. ⎛ 50mV ⎞ R = SENSE ⎜⎜ ⎟⎟ ⎝ I
Thermal Considerations
OUT ⎠ Linear regulators are simple to use. The most Where IOUT is the maximum output current. For example, complicated design parameters to consider are thermal the current sense resistor for a 2.5VIN to 1.8VOUT, 5A, characteristics. Since the MIC5159 offers no thermal linear regulator calculates as follows: protection, thermal design requires the following ⎛ 50mV ⎞ application-specific parameters: R = SENSE ⎜ ⎟ ⎝ 5A ⎠ • Maximum ambient temperature (TA) R • Output current (I SENSE = 10mΩ OUT) • Output voltage (VOUT)
P-Channel MOSFET Selection
• Input voltage (V The P-Channel MOSFET selected for use with the IN) MIC5159 must satisfy the following requirements: First, calculate the maximum power dissipation of the regulator: • Input voltage P • D = (VIN – VOUT) × IOUT Gate threshold Ground current can generally be ignored. The amount of • Load current power dissipated by ground current and input voltage is • Dropout voltage (input-to-output differential) minimal. Minimum θJA for the MOSFET can be • Thermal performance calculated using the following formula: ⎛ (T − J(MAX) TA ) To prevent damage to the P-Channel MOSFET, the ⎞ θ = ⎜ ⎟ maximum input voltage (V JA ⎜ ⎟ IN(max)) must be less than its ⎝ D P ⎠ drain-source breakdown voltage (BVDS). In addition, the minimum input voltage (V Where T IN(min)) must be greater than or J(max) is equal to the maximum die temperature equal to the gate threshold voltage (V of the P-Channel. GS) of the P- Channel MOSFET. For a given output current and θJA = θJC + θCS + θSA dropout requirement, the ON-resistance (RDS(ON)) of the
Example
P-Channel MOSFET must also be determined. The minimum R For the same regulator, 2.5VIN to 1.8VOUT at 5A with an DS(ON) of the P-Channel MOSFET is calculated as follows: ambient temperature of 60°C: ⎛ ( − ⎞ PD = (2.5V–1.8V) × 5A IN(MIN) V VOUT ) R = ⎜ ⎟ − P DS(ON) RSENSE ⎜ I ⎟ D = 3.5W ⎝ OUT(MAX) ⎠ Where VIN is the maximum VIN and IOUT is the maximum Where I I OUT(max) is the maximum output current and OUT. RSENSE is the current sense resistor. The P-Channel MOSFET must be able to dissipate For example, the MIC5159-1.8BM6 is used with an 3.5W. The minimum θJA to maintain a maximum TJ of external MOSFET to form a 5A LDO with an input of 150°C (max.) TJ according to a typical MOSFET data 2.5V. Either a 2.5V or 1.8V gate threshold MOSFET can sheet is as follows: be selected. The minimum RDS(ON) is calculated as: (150 C ° − 60 C ° ) θ = JA 3.5W June 2006 7 M9999-062706