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SC1403ITSTR Datasheet(PDF) 12 Page - Semtech Corporation |
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SC1403ITSTR Datasheet(HTML) 12 Page - Semtech Corporation |
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12 / 31 page  12 2004 Semtech Corp. SC1403 PRELIMINARY POWER MANAGEMENT United States Patent No. 6,377,032 www.semtech.com Applications Information (Cont.) C R1 R2 L RL CSH CSL VO VLX (Phase node) (Output) The equation for the current sense signal, CSH - CSL, is given by: 1) (EQ R1 R2 sC R1 R2 R2 ) V (V V o LX CSL) (CSH ⋅ ⋅ + + ⋅ − = − where (V LX - Vo) is the voltage across the inductor terminals. The values for C, R2, and R1 can be found by comparing the above circuit with resistive sensing, which is shown below. Rsns sL VLX RL Vo (Output) (Phase node) CSH CSL With resistive sensing, the current sense signal CSH-CSL can be written in the complex s-domain as: 2) (EQ sL RL Rsns Rsns V V V o LX CSL CSH + + ⋅ − = − ) ( ) ( where (V LX - Vo) is the voltage across the inductor terminals. Note the similarity between EQ 1 and EQ 2. By choosing proper values for C, R1, and R2, the current-sense voltage (CSH-CSL) will track the inductor current. The following equations determine C, R1, and R2: 55mV C ILpk L R1 ⋅ ⋅ = () 55mV RL ILpk 55mV R1 R2 − ⋅ ⋅ = The recommended value for C is 1.0uF. RL inductor resistance is specified at 11.6 mohm typical. 55mV is the current sense thresh- old. For the reference design, the values are set to C = 1uF and R1 = R2 = 1.3K. This sets the current limit to approximately 10A. Two guidelines must be used when selecting C, R1, and R2: The values of R2 and R1 should not exceed approximately 1.5Kohm. The bias current from the CSH input flows through these resistors and creates an error term. The value for R1 should not be too small due to power consider- ations. During a switching cycle, the voltage across R1 is either (Vin - Vo) or (-Vo). This creates a power loss in R1: the power loss can be determined by: () () mW 45 k 1 R1 Vo - Vin Vo P 1 R = Ω − ⋅ = ⋅ = 3 . 3 . 3 21 3 . 3 Choosing the Main Switching mosfet The IRF7143 is used in the reference design. Before choosing the main (high-side) mosfet, we need to check three parameters: volt- age, power, and current rating. The maximum drain to source voltage of the mosfet is mainly determined by the switcher topology. Since this is a buck topology, V 21 V V MAX _ IN MAX _ DS = = The IRF7413 is a 30V device, which allows for 70% derating at 21V operation. The mosfet power dissipation has three components: conduction losses, switching losses, and gate drive losses. The conduction loss is determined using the RMS mosfet current; the equation is shown below. The mosfet current is a trapezoid waveform with values equal to: 2 L LOAD MIN I I I ∆ − = 2 L LOAD MAX I I I ∆ + = L fs D Vo IL ⋅ − ⋅ = ∆ ) 1 ( Vin Vo D = () 2 2 MAX MAX MIN MIN RMS I I I I D I + ⋅ + ⋅ = As input voltage decreases, the duty cycle increases and the ripple current decrease, and overall the RMS mosfet current will increase. The conduction losses are then given by the formula below, where Rds(on) is 18m-ohm for the IRF7413 at room temperature. Note that Rds(on) increases with temperature. 2 ) ( RMS on ds CONDUCTION I R P ⋅ = The mosfet switching loss is estimated according to: G OUT S IN RSS SWITCHING I I f V C P ⋅ ⋅ ⋅ = 2 Crss is the reverse transfer capacitance of the mosfet, which is 240pF for IRF7413. Ig is the gate driver current, which is 1A for SC1403. The mosfet gate drive loss is estimated from: |
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