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CS5165A Datasheet(PDF) 15 Page - ON Semiconductor |
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CS5165A Datasheet(HTML) 15 Page - ON Semiconductor |
15 / 18 page CS5165A http://onsemi.com 15 Droop Resistor Tolerance Tolerance due to sheet resistivity variation 16% Tolerance due to L/W error 1.0% Tolerance due to temperature variation 12% Total tolerance for droop resistor 29% In order to determine the droop resistor value the nominal voltage drop across it at full load has to be calculated. This voltage drop has to be such that the output voltage full load is above the minimum DC tolerance spec. VDROOP(TYP) + [VDAC(MIN) * VDC(MIN)] 1 ) RDROOP(TOLERANCE) Example: for a 300 MHz PentiumII, the DC accuracy spec is 2.74 < VCC(CORE) < 2.9 V, and the AC accuracy spec is 2.67 V < VCC(CORE) < 2.9 3V. The CS5165A DAC output voltage is +2.812 V < VDAC < +2.868 V. In order not to exceed the DC accuracy spec, the voltage drop developed across the resistor must be calculated as follows: VDROOP(TYP) + [VDAC(MIN) * VDC PENTIUMII(MIN)] 1 ) RDROOP(TOLERANCE) + 2.812 V * 2.74 V 1.3 + 56 mV With the CS5165A DAC accuracy being 1.0%, the internal error amplifier’s reference voltage is trimmed so that the output voltage will be 40 mV high at no load. With no load, there is no DC drop across the resistor, producing an output voltage tracking the error amplifier output voltage, including the offset. When the full load current is delivered, a drop of −56 mV is developed across the resistor. Therefore, the regulator output is pre−positioned at 40 mV above the nominal output voltage before a load turn−on. The total voltage drop due to a load step is DV−40 mV and the deviation from the nominal output voltage is 40 mV smaller than it would be if there was no droop resistor. Similarly at full load the regulator output is pre−positioned at 16 mV below the nominal voltage before a load turn−off. The total voltage increase due to a load turn−off is DV−16 mV and the deviation from the nominal output voltage is 16 mV smaller than it would be if there was no droop resistor. This is because the output capacitors are pre−charged to value that is either 40 mV above the nominal output voltage before a load turn−on or, 16 mV below the nominal output voltage before a load turn−off (see Figure 15). Obviously, the larger the voltage drop across the droop resistor ( the larger the resistance), the worse the DC and load regulation, but the better the AC transient response. Design Rules for Using a Droop Resistor The basic equation for laying an embedded resistor is: RAR + ò L A or R + ò L (W t) where: A = W × t = cross−sectional area ρ = the copper resistivity (mW − mil) L = length (mils) W = width (mils) t = thickness (mils) For most PCBs the copper thickness, t, is 35 mm (1.37 mils) for one ounce copper. ρ = 717.86 mW−mil For a Pentium II load of 14.2 A the resistance needed to create a 56 mV drop at full load is: Response Droop + 56 mV IOUT + 56 mV 14.2 A + 3.9 mW The resistivity of the copper will drift with the temperature according to the following guidelines: DR + 12% @ TA +) 50°C DR + 34% @ TA +) 100°C Droop Resistor Width Calculations The droop resistor must have the ability to handle the load current and therefore requires a minimum width which is calculated as follows (assume one ounce copper thickness): W + ILOAD 0.05 where: W = minimum width (in mils) required for proper power dissipation, and ILOAD Load Current Amps. The Pentium®II maximum load current is 14.2 A. Therefore: W + 14.2 A 0.05 + 284 mils + 0.7213 cm Droop Resistor Length Calculation L + RDROOP W t ò + 0.0039 284 1.37 717.86 + 2113 mil + 5.36 cm Output Inductor The inductor should be selected based on its inductance, current capability, and DC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade transient response. Inductor Ripple Current Ripple Current + [(VIN * VOUT) VOUT] (Switching Frequency L VIN) Example: VIN = +5.0 V, VOUT = +2.8 V, ILOAD = 14.2 A, L = 1.2 mH, Freq = 200 kHz Ripple Current + [(5.0 V * 2.8 V) 2.8 V] [200 kHz 1.2 mH 5.0 V] + 5.1 A Output Ripple Voltage VRIPPLE + Inductor Ripple Current Output Capacitor ESR Example: VIN = +5.0 V, VOUT = +2.8 V, ILOAD = 14.2 A, L = 1.2 mH, Switching Frequency = 200 kHz Output Ripple Voltage = 5.1 A × Output Capacitor ESR (from manufacturer’s specs) ESR of Output Capacitors to limit Output Voltage Spikes ESR + D VOUT DIOUT |
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