V I S H A Y I N T E R T E C H N O L O G Y, I N C . LOW VALUE POWER METAL STRIP® RESISTORS DC/DC Converter DC/DC Converter Utilizes Current Sensing Resistors with Ultra Low Ω, Low TCR, Tight Tolerance, High Current Capability, and Low Thermal EMF Due to the increase in the sales of Notebook PC, the demand for the DC/DC Converter has been showing rapid growth because of its high energy conversion efficiency (95% vs. 50 to 70% for the Conventional IC Regulator), and its precise current-limiting capability. However, to assure the performance of the Multiple Output DC/DC Converter, the currentlimiting voltage must be detected precisely to protect the expensive Notebook PC from an overload, which in general, is due to shorts within the capacitors used in these devices. For high energy conversion efficiency, various control ICs have been developed that utilize resistive components. (See page 2 for a typical design.) To achieve the perfect cutoff mode of the DC Load Current at the programmed current-limiting voltage of the Control IC*, a very stable and accurate sense resistor with the following characteristics is required for precise voltage comparison: • Tight Tolerance For maximizing the current supply within the limit of acceptable current, the tolerance of the sense resistor must be ± 1% or tighter. • Low TCR Since the maximum normal DC load current is in excess of 3 amps, in general, the maximum ∆R due to self-heating must be as low as 1000ppm across the ambient temperature range of 0°C to + 60°C. Thus, a low TCR is required for current sensing. w w w . v i s h a y. c o m * Current level is programmed by the user by choosing ohmic value of the current sense resistor. • Resistance Value For maximizing energy conversion efficiency of the DC/DC Converter, the value of the current sense resistor is calculated according to the worst-case-low current-limiting threshold voltage of the Control IC and the peak inductor current by the following equation: VL Rs = I peak Where: Rs = Sense Resistor (Ω) I peak = Peak Inductor Current* = Maximum DC Load Current) x 1.15 VL = Worst-case-low current-limiting threshold voltage The key factor in determining the resolution (current-limiting threshold voltage) of the Control IC is its semiconductor noise. Typical resistance values utilized in various control ICs are 20 to 25 milli-ohms. The Vishay Dale® WSL-0805, WSL-1206, WSL2010, WSL-2512, WSL-0805-18, WSL-1206-18, WSL-2010-18, WSL-2512-18, WSR2 and WSR3 can provide extremely low values down to 1 milliohms with one resistor. This saves mounting costs and space, and improves the MTBF of the DC/DC Converter. In comparison, with conventional cermet chips, four to six chips are needed to achieve very low ohmic values; and with conventional thin film chips, at least two chips are needed due to their low power capability. VSD-AP0001-0301 APPLICATION NOTE • Very Low Ohmic Value For maximizing energy conversion efficiency of the DC/DC Converter the ohmic value, for minimizing power consumption at the current sense resistor, should be below 25 milli-ohms. • Low Thermal EMF For an accurate comparison between the programmed current-limiting voltage (typically, ≤ 100mV) of the Control IC and the detected voltage by the sense resistor, the error due to thermal EMF of the sense resistor must be minimized. Application Note WSL and WSR for DC/DC Converters Vishay Dale • Terminal Construction Down to 1 milli-ohms, two terminal construction is accepted for the DC/DC Converter. However, when better accuracy is required the four-terminal WSK-2512 is available. Fourterminal construction will reduce as mounted resistance tolerance variations: • reduce lead resistance • reduce TCR of copper terminal • reduce TCR of solder joint. • High Current Capability The maximum DC load current of an up-to-date Notebook PC with high-speed CPU, large memory (D-RAM) and the CD-ROM is in excess of 3 amps. In this case, the sense resistor must be able to withstand a high continuous current of at least 1.15 times the DC load current (peak inductor current) due to the effect of the inductor connected in series for smoothing the circuit. Otherwise, the DC/DC Converter will fail to function due to a failed sense resistor. • High Temperature Capability Utilization of DC/DC converters in industrial and automotive applications may expose components to higher temperature environments. The sense resistor must be capable of operating in high temperature conditions with minimal reduction (derating) of rated power. The Vishay Dale® WSL-0805, WSL-1206, WSL-2010, WSL2512, WSL-0805-18, WSL-1206-18, WSL-2010-18, WSL2512-18, WSR2 and WSR3 will withstand high temperatures. In comparison cermet chips have poor high temperature capability. See Chart 1 for high temperature comparison. • Vishay Dale® sense resistors provide characteristics required and demanded by DC/DC Converter operations that cannot be met with other conventional cermet and thin film resistive chip components. * For smoothing circuit. Typical Control IC for Dual Output Step-down DC/DC Converter The two key functions of a Control IC are to supply dual stepdown DC potential energy and to perform accurate current limiting which is programmed by the user. A. A voltage drop across a sense resistor connected in series between the N-channel Power MOSFET and the output terminal is monitored in order to compare the worst-caselow current-limiting threshold voltage of the Control IC with the current level programmed by the user via sense resistor. B. The sense resistor also functions as a part of the smoothing circuit to supply high linear DC current. The sequence of the Dual Output Step-down Converter's steady state operations is first the gate (Ga) turns on the Nchannel Power MOSFET (A) and sinks the DC current into the load until the terminal voltage reaches the programmed positive voltage of the Control IC, then, second, (Ga) cuts off (A), and (Gb) turns on (B), and leaks the DC current to the load until the terminal voltage reaches the programmed negative voltage, then, third (Gb) cuts off (B) • • •. INPUT 5.5V to 30V Gb Ga • TCR b OUTPUT #2 5.0V A R1 R2 CONTROL IC At steady state conditions, Notebook PCs generally require input currents in excess of 3 amps. For accurate currentlimiting control by the Control IC, the DR of the sense resistor, due to self-heating, must be low enough to ensure that the accuracy of comparison between the worst-case-low current-limiting threshold voltage of the Control IC and the sense resistor voltage is ± 5%. b B SOFT-START CAPACITORS To meet this standard current limit, the dynamic tolerance (no load, static resistance tolerance + % ∆R caused by self-heating + apparent % ∆R caused by thermal EMF) allocated to the sense resistor is 2%; and, to assure 2% tolerance of the sense resistor across the temperature range of 0°C to + 100°C (ambient temperature + self-heating), the resistor must have a low TCR. The Vishay Dale® WSL-0805, WSL-1206, WSL2010, WSL-2512, WSL-0805-18, WSL-1206-18, WSL-201018, WSL-2512-18, WSR2 and WSR3 have the lowest TCR in the industry. See Chart 2 for TCR comparison. for reducing surge current at start up R1 and R2: Current Surge Resistors A and B: N-Channel Power MOSFETs V Ga V Gb b I 0 b www.vishay.com 2 For technical questions, contact [email protected] OUTPUT#2 3.3V Application Note WSL and WSR for DC/DC Converters Vishay Dale CHART 1 % Power POWER METAL STRIP - HIGH TEMPERATURE CAPABILITY Temperature °C CHART 2 Resistance Temperature Coefficient (ppm/°C) POWER METAL STRIP - TCR PERFORMANCE Resistance Value (mΩ) For technical questions, contact [email protected] www.vishay.com 3 WORLDWIDE SALES CONTACTS DISCRETE SEMICONDUCTORS AND PASSIVE COMPONENTS THE AMERICAS VISHAY AMERICAS ONE GREENWICH PLACE SHELTON, CT 06484 UNITED STATES PH: +1-402-563-6866 FAX: +1-402-563-6296 ASIA VISHAY INTERTECHNOLOGY ASIA PTE LTD. 25 TAMPINES STREET 92 KEPPEL BUILDING #02-00 SINGAPORE 528877 PH: +65-6788-6668 FAX: +65-6788-0988 JAPAN VISHAY JAPAN CO., LTD. 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