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XRP6142 Datasheet(PDF) 10 Page - Exar Corporation |
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XRP6142 Datasheet(HTML) 10 Page - Exar Corporation |
10 / 17 page X XR RP P6 61 14 42 2 S Sy yn ncch hrro on no ou us s S Stte ep p--D Do ow wn n C Co on nttrro olllle err w wiitth h D DD DR R M Me em mo orry y T Te errm miin na attiio on n © 2010 Exar Corporation Page 10 of 17 Rev. 1.0.0 voltage ripple for regulating the output. To ensure stable operation two constraints must be met: First the COUT must have sufficient ESR in order to get enough voltage ripple at feedback pin. It is recommended that XRP6142 be operated with at least 25mV ripple at feedback pin. Assuming majority of output voltage ripple is from ESR, we get: IL mV ESR Δ ≥ 25 …………. (1) Where ∆IL is inductor current ripple nominally set at 30% of IOUT. Note that VOUT ripple, is attenuated by the resistor divider R1/R2, and a smaller ripple is seen at FB pin. For example if VOUT ripple is 25mV and R1=R2=10k, then the voltage ripple at FB is only 12.5mV. One solution to this problem is to increase the output ripple accordingly, such that ripple at FB is 25mV. A more desirable solution is to provide a high- frequency/low-impedance path for the output ripple to be transmitted to FB without attenuation. This can be done by placing a small feed-forward capacitor CFF in parallel with R1. As a starting point calculate CFF from: fs R CFF × × × = 1 2 10 π Where fs is the switching frequency In general, a CFF of 1nF should provide satisfactory feed-forward for most applications based on the XRP6142. The second constraint for stability establishes a relation between ESR and COUT. OUT C Ton ESR ≥ …………. (2) Once ESR is calculated from equation (1), equation (2) can be used to calculate COUT. The aforementioned are in addition to the usual requirements for COUT for a buck converter. The usual constraint in order to meet load step transient requirement is given by: 2 2 2 1 2 2 OUT OUT V Vos I I C − − ≥ Where: I2 is load step high-level current I1 is load step low-level current VOUT is output voltage including transient (nominally this is set 3% higher than VOUT) In general, the best capacitors are the ones with known and consistent ESR across operating temperature range. Examples include POSCAPs, Tantalums and certain Aluminum Electrolytics. OUTPUT INDUCTOR Select the output inductor for inductance and current rating. As a rule of thumb the DC current rating and saturation current should be at least 50% higher than maximum output current. Calculate the inductance from: () fs IL D V V L OUT IN × Δ × − = Where: D is duty cycle fs is switching frequency ∆IL is inductor current ripple nominally set at 30% of IOUT. INPUT CAPACITOR Select the input capacitor for capacitance, voltage rating and RMS current rating. As a rule of thumb, the voltage rating should be twice the maximum input voltage of the converter. RMS current rating can be approximated from: () D D I I OUT RMS − × = 1 Calculate CIN such that input voltage ripple does not exceed 2% of VIN. Ceramic input capacitors are recommended. This choice minimizes input voltage ripple due to ESL and ESR. Thus a simplified expression for CIN can be written: |
Similar Part No. - XRP6142 |
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Similar Description - XRP6142 |
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