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Motore di ricerca datesheet componenti elettronici |
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Motore di ricerca datesheet componenti elettronici |
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SP6136 Scheda tecnica(PDF) 8 Page - Exar Corporation |
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SP6136 Scheda tecnica(HTML) 8 Page - Exar Corporation |
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8 / 18 page  8 Oct 3-06 Rev L SP636 Synchronous Buck Controller © 2006 Sipex Corporation APPLICATION INFORMATION Increasing the Current Limit If it is desired to set Imax > {60mV / DCR} (in this case larger than 4.6A), then a resistor RS3 should be added as shown in figure 2. RS3 forms a resistor divider and reduces the voltage seen by the comparator. Since: 60mV (Imax • DCR) RS3 = {RS + RS2 + RS3} Solving for RS3 we get: RS3 = [60mV • (RS + RS2)] .......() [(Imax • DCR) – 60mV] As an example: if desired Imax is 7A, then RS3 = 63.4KW. Figure 2- Over-current detection circuit for Imax > 60mV / DCR Decreasing the Current Limit If it is required to set Imax < {60mV / DCR}, a resistor is added as shown in figure 3. RS3 increases the net voltage detected by the current-sense comparator. Voltage at the positive and negative terminal of compara- tor is given by: VSP = Vout + (Imax • DCR) VSN = Vout • {R S3 / (RS2 + RS3)} Since the comparator is triggered at 60mV: VSP-VSN = 60 mV Combining the above equations and solv- ing for RS3: RS3 = RS2 • [Vout - 60mV + (IMAX•DCR)].........(2) 60mV - (IMAX • DCR) As an example: for Imax of 2Aand Vout of 3.3V, calculated RS3 is .5MW (232KW standard). Figure 3- Over-current detection circuit for Imax < {60mV / DCR} Power MOSFET Selection There are four main criterion in selecting Power MOSFETs for buck conversion: ● Voltage rating BVdss ● On resistance Rds(on) ● Gate-to-drain charge Qgd ● Package type In order to better illustrate the MOSFET se- lection process, the following buck converter design example will be used: Vin = 2V, Vout = 3.3V, Iout = 10A, f = 2000KHz, DCR = 4.5m W (inductor DC resistance), efficiency = 94% and Ta = 40˚C. Select the voltage rating based on maximum input voltage of the converter. A commonly used practice is to specify BVdss at least twice the maximum converter input voltage. This is done to safeguard against switching transients that may break down the MOSFET. For converters with Vin of less than 0V, a RS3 63.4 K CSP 6.8nF SWN ISP ISN SP613X CS 0.uF RS2 5. K RS 5. K L = 2.7uH, DCR = 4.mOhm Vout CSP 6.8nF ISN SP613X SWN ISP CS 0.uF RS2 5. K RS 5. 11K Vout L = 2.7uH, DCR = 4.mOhm RS3 .5MOhm |
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