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ISL6532A Datasheet(PDF) 14 Page - Intersil Corporation |
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ISL6532A Datasheet(HTML) 14 Page - Intersil Corporation |
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14 / 17 page  14 FN9099.5 May 5, 2008 To properly compensate the LDO system, a 100k Ω 1% resistor and a 680pF X5R ceramic capacitor, represented as R10 and C25 in Figure 7, are used. This compensation will insure a stable system with any MOSFET given the following conditions: Maximum bandwidth will be realized at full load while minimum bandwidth will be realized at no load. Bandwidth at no load will be maximized as τ becomes closer to 10μs. Output Voltage Selection The output voltage of the VDDQ PWM converter can be programmed to any level between VIN and the internal reference, 0.8V. An external resistor divider is used to scale the output voltage relative to the reference voltage and feed it back to the inverting input of the error amplifier, see Figure 5. However, since the value of R1 affects the values of the rest of the compensation components, it is advisable to keep its value less than 5k Ω. Depending on the value chosen for R1, R4 can be calculated based on the Equation 7: If the output voltage desired is 0.8V, simply route VDDQ back to the FB pin through R1, but do not populate R4. The output voltage for the internal VTT linear regulator is set internal to the ISL6532A to track the VDDQ voltage by 50%. There is no need for external programming resistors. As with the VDDQ PWM regulator, the AGP linear regulator output voltage is set by means of an external resistor divider as shown in Figure 7. For stability concerns described earlier, the recommended value of the feedback resistor, R8, is 249 Ω. The voltage programming resistor, R9 can be calculated based on the Equation 8: Component Selection Guidelines Output Capacitor Selection - PWM Buck Converter An output capacitor is required to filter the inductor current and supply the load transient current. The filtering requirements are a function of the switching frequency and the ripple current. The load transient requirements are a function of the slew rate (di/dt) and the magnitude of the transient load current. These requirements are generally met with a mix of capacitors and careful layout. DDR memory systems are capable of producing transient load rates above 1A/ns. High frequency capacitors initially supply the transient and slow the current load rate seen by the bulk capacitors. The bulk filter capacitor values are generally determined by the ESR (Effective Series Resistance) and voltage rating requirements rather than actual capacitance requirements. High frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance components. Consult with the manufacturer of the load on specific decoupling requirements. Use only specialized low-ESR capacitors intended for switching-regulator applications for the bulk capacitors. The bulk capacitor’s ESR will determine the output ripple voltage and the initial voltage drop after a high slew-rate transient. An aluminum electrolytic capacitor’s ESR value is related to the case size with lower ESR available in larger case sizes. However, the Equivalent Series Inductance (ESL) of these capacitors increases with case size and can reduce the usefulness of the capacitor to high slew-rate transient loading. Unfortunately, ESL is not a specified parameter. Work with your capacitor supplier and measure the capacitor’s impedance with frequency to select a suitable component. In most cases, multiple electrolytic capacitors of small case size perform better than a single large case capacitor. Output Capacitor Selection - LDO Regulators The output capacitors used in LDO regulators are used to provide dynamic load current. The amount of capacitance and type of capacitor should be chosen with this criteria in mind. Output Inductor Selection The output inductor is selected to meet the output voltage ripple requirements and minimize the converter’s response time to the load transient. The inductor value determines the converter’s ripple current and the ripple voltage is a function of the ripple current. The ripple voltage and current are approximated by the following equations: DRIVE2 FB2 ISL6532A VAGP VDDQ FIGURE 7. COMPENSATION AND OUTPUT VOLTAGE SELECTION OF THE LINEAR 650 Ω R9 R10 C25 + - 0.8V + OUTPUT IMPEDANCE R8 REFERENCE ESR COUT RLOAD V AGP 0.8 1 R 8 R 9 ------- + ⎝⎠ ⎜⎟ ⎛⎞ × = τ C OUT ESR ⋅ 10 μs > = R FB R 8 249 Ω == (EQ. 6) R 4 R1 0.8V × V DDQ - 0.8V ------------------------------------ = (EQ. 7) R 9 R 8 0.8V × V AGP - 0.8 V ----------------------------------- = (EQ. 8) ΔI= VIN - VOUT Fs x L VOUT VIN ΔV OUT = ΔI x ESR x (EQ. 9) ISL6532A |
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