Load Balance Controller, EZ1900 and Low Dropout Regulators March 11, 1998 AN96-4 TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com Using the Load Balance Controller, EZ1900, with Low Dropout Regulators for a Flexible Motherboard Design Introduction Principle of Operation The EZ1900 load balance controller is a flexible, low cost device, providing an automatic power supply upgrade from single to split-voltage plane processors, when used with two low dropout regulators. Single plane processors, such as the Intel Pentium® Processor P54C, Cyrix 6x86™, AMD AMD5K86™ and the PowerPC™ require a single supply voltage, normally 3.3 or 3.5V. In single plane operation, the EZ1900 controls the output of two regulators so that they provide the same output voltage, sharing current for processors whose I/O and core power planes are connected together. In this mode, the two regulators operate as master and slave. New split plane processors, such as the Pentium P55C and versions of the 6x86, AMD5K86 & PowerPC 603/604EV require two supply voltages: VI/O for the I/O circuitry, at 3.3 or 3.5V; and VCC2 for the CPU core, at between 2.5 and 2.8V. The EZ1900 can be used with almost any three- or five-terminal voltage regulators. This application note describes how to use the EZ1900 to provide an automatic upgrade path, avoiding costly production changes or jumpers for different processors. SLAVE VIN U1 R6 3 + INPUT See Table1 C1 470uF/16V 2 R3 ADJ/GND EZ1083 VIN OUTPUT Microprocessors, such as those listed above, have a pin, VCC2DET, or similar, which, indicates to the EZ1900 in which mode to operate. Pin 1 (SEL) of the EZ1900 detects this signal: floating (or open) indicates singleplane (current sharing) and low switches the outputs for split-plane operation. A typical application circuit is shown in figure 1. The slave regulator powers the CPU core and the master supplies the I/O circuitry. The EZ1900 can be used with almost any three- or five-terminal voltage regulator, even with VCC2 devices of different rated current. + C2 470uF/10V R4 +V 1 SEL -IN 2 3 +IN SOUT -V 6 4 R5 See Table1 + C3 470uF/16V MASTER U3 3 VIO INPUT OUTPUT ADJ/GND EZ1083 2 R1 + 1 C4 470uF/16V REF DND06066.DSN Current Sharing The EZ1900 controls current sharing by sensing the input current to the two regulators by means of the two sense resistors, R5 & R6. For balanced currents, the voltage drop across each resistor should be of the order of 50 - 100mV [around 10 - 20mΩ=for a 5A current]. 1 7 EZ1900 U2 MODE When operating in split-plane mode, the EZ1900 switches the output of the slave regulator to provide a lower voltage VCC2. The two regulators’ outputs are independent of each other. R2 Figure 1: Typical Application Circuit Pentium is a registered trademark of Intel Corporation; 6x86 is a trademark of Cyrix Corporation; AMD5K86 is a trademark of AMD; PowerPC is a trademark of IBM. For most split-plane regulators, the CPU core requires no more than 6A, with 3A or less for the I/O. The most economical solution is to use a 6A regulator, such as the EZ1585DCT for the core (slave), and a 3A regulator, such as the EZ1085 for the I/O (master). This would give a maximum total current in single-plane mode of 9A; enough to power all versions of the Cyrix 6x86. When two different regulators are installed, the currents must be shared proportionally, so that neither regulator enters current limit. This is done by ad- 1 © 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 Load Balance Controller, EZ1900 and Low Dropout Regulators AN96-4 March 11, 1998 justing the values of R5 & R6 so that the voltage drops across each at rated current are the same. In this case, R5 should be set to twice the value of R6 [3A x 2R = 6A x R]. 3. Determine the trace width required, based on the copper weight. 4. Determine the trace length required, based on the copper weight. Please see Semtech’s datasheet for the EZ1900 for application examples using the EZ1580 and EZ1087 five-terminal low dropout regulators. 5. Design a suitable resistor layout for the board. It is very likely that the resistor will have to be laid out in “serpentine” fashion, as shown in figure 2. Sense Resistor 6. Keep the two sense resistors as close to each other and the VIN as possible. The values of the sense resistors do not have to be controlled to any great degree of precision — it is the ratio of the two values which is important. As a result, the sense resistors can be constructed inexpensively using copper board traces. Any process related errors in setting the resistors will apply equally to both R5 & R6. Suitable resistor sizes are given in Table 1. Setting the Output Voltages The output of the two regulators can be set to accommodate different processor voltage requirements, by means of setting the values of resistors R1 through R4. The values are shown in table 2. How To Design a Circuit using the EZ1900 Conclusion 1. Select the maximum current required for each regulator.Suitable regulators are Semtech’s EZ1585DCT (6A) for CPU core and EZ1085CT (3A) for the I/O. 2. From table 1, determine the sense resistor value required. The EZ1900 can be used with any three or five terminal regulator to construct a flexible circuit which will automatically supply the correct voltages for powering single or split voltage plane processors. The circuit is low cost, eliminating costly production changes and jumpers to set different supply voltage requirements. Table 1: Copper Trace Sizes for EZ1900 Application Circuit Copper weight (oz) Current Resistance (Amps) (mΩ) Ω) Pd 0.5 (mW) 1 2 Copper weight (oz.) 3 0.5 Trace Width (in.) 1 2 3 Trace Length (in.) 1 80.0 80 0.010 0.010 0.010 0.010 0.815 1.629 3.259 4.888 2 40.0 160 0.010 0.010 0.010 0.010 0.407 0.815 1.629 2.444 3 26.7 240 0.015 0.010 0.010 0.010 0.407 0.543 1.086 1.629 4 20.0 320 0.027 0.013 0.010 0.010 0.543 0.543 0.815 1.222 5 16.0 400 0.042 0.021 0.010 0.010 0.679 0.679 0.679 0.679 6 13.3 480 0.060 0.030 0.015 0.010 0.815 0.815 0.815 0.815 7 11.4 560 0.082 0.041 0.020 0.014 0.950 0.950 0.950 0.950 8 10.0 640 0.107 0.053 0.027 0.018 1.086 1.086 1.086 1.086 9 8.9 720 0.135 0.068 0.034 0.023 1.222 1.222 1.222 1.222 8.0 800 0.167 0.083 0.042 0.028 1.358 1.358 1.358 1.358 10 2 2 Note: 0.5oz/ft copper is 18µm thick; Copper trace widths based on 1200A /oz.in, which is a conservative rating for a 40oC rise. R = (0.491 x L)/(B x W) where R=Trace Resistance (mΩ);L=Trace Length (in.);B=Copper weight (oz.);W=Trace Width (in.) 2 © 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 Load Balance Controller, EZ1900 and Low Dropout Regulators AN96-4 March 11, 1998 Fig 2: Resistor Layout Example SHOWN FULL SIZE Vin PLANE R5 R6 U3 Pin3 PLANE U1 Pin3 PLANE 1200A2/oz.in rating per Table 1: 8A per side 1oz copper 40oC rise in traces Pin3 Pin2 U2 Table 2: Resistor Values Mode Processor (SEL pin) VI/O VCC2 R1 R2 R3 R4 (Volts) (Volts) (Ω) (Ω) (Ω) (Ω) OPEN VRE 3.49 3.49(1) 133 237 133 165(2) OPEN STD/VR 3.384 3.384(1) 133 226 133 165(2) LOW P55C AMD5K86 3.3 2.8 133 215 133 165 LOW 6x86 AMD5K86 3.3 2.5 133 215 133 133 (1) Although the VCC2 setpoint is at 2.8V, the EZ1900 adjusts the slave output upwards to achieve load current balance at the VI/O setpoint. VCC2 and VI/O must be connected. (2) As required for split plane VCC2, e.g. 165Ω for 2.8V, 133Ω for 2.5V R1 to R4 are 1% tolerance resistors. 3 © 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320