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Design Solutions 7 April 1999 A Low Cost Dynamic VID Power Supply for Pentium® III Processors INTRODUCTION The latest Pentium® III mobile processors announced by Intel require a 2-level power supply voltage that must be dynamically switched. A power supply that satisfies these requirements can be constructed using a DC/DC controller containing an internal 5-bit DAC. However, substantial external circuitry is still required to switch between the two operating voltages. This paper presents a new solution that satisfies Intel’s dynamic VID requirements, but with lower cost, smaller PCB area and better performance than competing solutions. Many notebook computer manufacturers also prefer the VID feature because it provides flexibility to change output voltages to support future Intel processors, as well as processors from AMD and Cyrix. When using a DC/DC converter with a built-in 5-bit DAC, the output voltages must be changed by modifying jumpers or zero-ohm resistors on the motherboard. The alternative solution presented in this paper also provides complete voltage flexibility by simply changing the value of two resistors. In fact, even greater flexibility is available, since voltages are not limited to those specified in the 5-bit VID table. signal must stay low until the new output voltage has settled within specifications, but must not remain low for more than 100µs. The MAX1711 data sheet states: “The internal PGOOD detector circuit monitors only output undervoltage; PGOOD will probably go low during upward transitions, but not downward.” For this reason, a transition detector and timer circuit must be added. 2. Rapid Output Voltage Change The CPU core power supply output voltage must adjust to the new programmed value in less than 100µs. In order to meet this requirement, the MAX1711 must be forced into PWM mode for a high-to-low voltage transition. The MAX1711 data sheet states: “If the minimum load is very light, it may be necessary to assert forced PWM mode (via SKIP) during the transition period to guarantee some output sink current capability. Otherwise, the output voltage won’t ramp downwards until pulled down by external load current.” External circuitry must be added to drive the SKIP pin high during voltage transitions. 3. Fast Transient Response DYNAMIC VID USING THE MAX1711 The MAX1711 data sheet (Rev. 0; 11/98) shows an example of a mobile VID power supply (see Figure 1). Most of the Figure has been taken directly from Maxim’s data sheet (Figure 10, page 24); a 5-bit multiplexer and jumpers have been added since they are required for dynamic control of the output voltage. This circuit is complex and expensive for the following reasons (quotations are taken directly from the MAX1711 data sheet): 1. POWER_GOOD Generation The POWER_GOOD signal must go low during each change in output voltage. This is true whether changing from low-to-high or high-to-low. The POWER_GOOD Powering a Pentium III processor demands extremely good transient response from the DC/DC converter. Linear Technology’s OPTI-LOOPTM compensation technology allows use of fewer output capacitors, and without ESR limitations. Maxim explains the difficulty an engineer faces when trying to meet these requirements without OPTI-LOOP: “Selecting the output capacitors in dynamically adjusted VCORE applications can be tricky due to trade-offs between capacitor capacity and ESR…It may be necessary to mix capacitor types or use specialized capacitors such as those shown in Figure 7 in order to achieve the required ESR while staying within the min/max capacitance value window.” , LTC and LT are registered trademarks of Linear Technology Corporation. OPTI-LOOP is a trademark of Linear Technology Corporation. Pentium is a registered trademark of Intel Corporation. 1 Design Solutions 7 5V VBATT 10V TO 22V 0.1µF 7 ON/OFF 0.22µF 9 VSEL 470pF 5 5V 15 1 REF BST DH CC LX 0.1µF 3.3V 10 13 * * * * BX * 3 7 11 17 21 * * * * 1A1 24 VCC 1B1 2A1 2B1 3A1 3B1 4A1 4B1 5A1 5B1 2 2 20 6 19 10 18 16 17 20 16 8 * CMPSH3 V+ VDD VCC 10µF 25V ×6 CERAMIC 1µF 20Ω MAX1711 DL GND 22 2Ω 24 IRF7805 0.1µF 23 1µH 20A IRF7805 ×2 PGND D1 FB D2 FBS D3 GNDS D4 PGOOD TON ILIM SKIP 14 * * 1k 11 12 21 POWERGOOD 40k 1% 2N7002 * * * 4 14 18 22 * 20µF CERAMIC 4 200k 1% 8 * 220µF 4V ×10 OS-CON 3 6 SN74CBT 3383 3.3V + SHDN D0 VOUT 1.5V 15A 13 * * * 1A2 1B2 2A2 2B2 3A2 3B2 4A2 4B2 5B2 5A2 BE 1 GND 5 9 3.3V 0.1µF 15 TRANSITION DETECTOR 19 23 12 1k 12 13 3M A4 VCC B4 Y4 14 1k 10 4 5 A3 Y3 1k 2 – 8 2N7002 1N4148 74HC86 3.3V A2 B2 Y2 30k 1N4148 1000pF 30k 100k A1 B1 820pF 5% 6 1000pF 1 100k 1% MAX986 B3 GND 7 Y1 49.9k 1% + 1N4148 1000pF 1k 100k 1% 11 1000pF 9 *INSTALL ZERO-OHM RESISTORS AS NEEDED TO PROGRAM DESIRED VOUT(L) AND VOUT(H) 2N7002 100k 2N7002 3 1N4148 2N7002 VID F01 Figure 1. MAX1711 Dynamic VID Implementation Requires Additional Circuitry for Operation Figure 7 in the MAX1711 data sheet shows 6 × 47µF ceramic output capacitors in parallel. However, because the ESR of these capacitors is too low for feedback loop stability, the user must also insert at least 5mΩ of PCB trace resistance in series with the high current path. This PCB trace resistor consumes PCB area and also reduces the DC/DC converter efficiency by 3.5% with a 10A load! The dynamic VID circuit shown in the MAX1711 data sheet (Figure 10, page 24) uses 10 × 220 µF OS-CON capacitors plus a 20µF ceramic capacitor on 2 the output. This is expensive and consumes considerable PCB area. 4. Inrush Current Limiting Using large amounts of output capacitance has another side-effect: very large current pulses are required to change the output voltage. The MAX1711 data sheet states: “attempting to slew the output upward quickly causes large current surges at the battery as the IC goes into output current limiting during the transition.” Design Solutions 7 This is particularly troublesome in the case of the MAX1711 for two reasons: 1) large output capacitance means more charge must be transferred to change the output voltage, and 2) output current limiting is very inaccurate. This second problem is a result of using the lower MOSFET RDS(ON) for current limiting. The resultant wide current limit tolerance means that current drawn from the battery during a low-to-high transition may be much more than the maximum load current. In order to prevent very large surge currents, Maxim’s dynamic VID implementation must include extra circuitry to reduce the current limit threshold during voltage changes. below ground. The MAX1711 data sheet states: “If the load can’t tolerate being forced to a negative voltage, it may be desirable to place a power Schottky diode across the output to act as a reverse-polarity clamp.” DYNAMIC VID USING THE LTC1735-1 A simple dynamic VID power supply may also be constructed using the LTC1735-1 (see Figure 2). This solution has several important advantages: • Lower DC/DC converter cost • Significantly less PCB area is required • OPTI-LOOP compensation provides better transient response 5. Multiplexing VID Values The MAX1711 accepts a 5-bit VID digital programming word to set the output voltage. However, because this 5-bit input must be switched between two different values, a multiplexer and two sets of jumpers must be connected to the MAX1711’s VID input (see Figure 1). Two sets of resistors/jumpers and the 24-pin multiplexer require considerable additional PCB area. • Complete output voltage programming flexibility down to VOUT = 0.8V • “Soft-Latch” output overvoltage protection VID Resistor Selection Instead of a 5-bit VID DAC, the LTC1735-1 relies on an external 2-range voltage feedback divider. A voltage select signal changes the feedback divider ratio, and, therefore, the programmed output voltage. Any two output voltages may be selected, with VOUT as low as 0.8V. Selecting different output voltages simply requires changing the values of two external resistors. Programming resistors can be modified according to the following simple formulas: 6. Output Overvoltage Protection The MAX1711 uses a fixed output overvoltage threshold of 2.25V nominal (2.29V maximum). This may be high enough to cause damage to low voltage processors. In addition, if the overvoltage protection ever does activate, a negative voltage can be delivered to the processor as the output capacitor discharges and “rings” 4.7Ω PGOOD 0.1µF ON/OFF 47pF 330pF LTC1735CGN-1 1 0.1µF 2 33k 3 47pF 4 5 47pF VIN 4.5V TO 24V 1000pF 6 7 8 COSC TG BOOST RUN/SS ITH SW PGOOD VIN SENSE – INTVCC SENSE + BG VOSENSE PGND EXTVCC SGND 10Ω 10Ω 16 FDS6680A 10µF 50V CERAMIC ×2 0.22µF 15 14 1.2µH 13 12 11 + 4.7µF 10 9 VOUT 1.3V TO 1.5V 12A 0.004Ω CMDSH-3 5V INPUT 1µF MBRS340T3 FDS6680A ×2 47pF R1 10k 0.5% 47pF R3 31.6k 1% + 180µF 4V SP ×4 2N7002 VSEL = 1: VOUT = 1.55V VSEL = 0: VOUT = 1.3V R2 15.8k 0.5% GND VID F02 Figure 2. LTC1735-1 Dynamic VID Implementation is Very Simple Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 3 Design Solutions 7 R2 = R3 = 8k VOUT (L) – 0.8V 8k VOUT (H) – VOUT (L) For a typical case of VOUT(L) = 1.3V and VOUT(H) = 1.55V, R2 = 15.8k and R3 = 31.6k as shown in Figure 2. Resistors R1 and R2 should be 0.5% tolerance, but resistor R3 may be 1% tolerance. VOUT PGOOD VSEL VID F03 Figure 3. LTC1735-1 Voltage Programming Response at IOUT = 100mA POWER_GOOD Operation Transient Response Linear Technology’s OPTI-LOOP compensation allows optimization of transient response, without limitations on output capacitor ESR. Figure 5 compares the transient response of the LTC1735-1 with OPTI-LOOP versus the MAX1711. The MAX1711 transient response scope photograph was taken from the MAX1711 data sheet, and uses 3 x 470µF Kemet T510 tantalum output capacitors. VOUT PGOOD VSEL VID F04 Figure 4. LTC1735-1 Voltage Programming Response at IOUT = 9A LTC1735-1 VOUT (50mV/DIV) The PGOOD signal generated by the LTC1735-1 automatically goes low every time VOUT is changed by the voltage select signal. Because the LTC1735-1’s internal power good comparator operates on both low-to-high and highto-low transitions, no additional circuitry is required. Figures 3 and 4 show the LTC1735-1 making both low-tohigh and high-to-low transitions. Figure 3 shows the response with a 100mA load current, while Figure 4 uses a 9A load current. Regardless of the load current, the output voltage completes the voltage transition in less than 30µs, easily meeting the 100µs specification requirement. MAX1711 VIN = 15V VOUT = 1.6V IOUT = 30mA to 7A COUT = 1500µF Output Overvoltage Protection The LTC1735-1 contains internal overvoltage protection (OVP) circuitry that activates when the output rises to 7.5% above the programmed voltage. The OVP threshold is not a fixed value, but moves as the output voltage is changed. For example, if the output voltage is programmed to 1.55V, the OVP will limit the output to 1.67V. Demo Board Linear Technology’s exclusive “soft-latch” circuitry provides robust OVP protection, but does not result in negative output voltages as with the MAX1711. Consequently, no power Schottky diode is needed across the output. The LTC1735-1 circuit shown in Figure 2 is available as a demo board (part number DC267). Contact your LTC representative to obtain a sample demo board for evaluation in your own laboratory. 4 Linear Technology Corporation Time (10µs/DIV) VID F05 Figure 5. LTC1735-1 vs MAX1711 Transient Response dsol7 LT/TP 0499 2K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 1999