DESIGN FEATURES VID Voltage Programmer for Intel Mobile Processors Microprocessor manufacturers’ relentless push for higher speed and lower power dissipation, especially in areas of mobile laptop computer processors, is forcing supply voltages to these processors to a level previously thought impossible or impractical. In fact, the supply voltage has become so critical that different microprocessors demand different yet precise supply voltage levels in order to function optimally. To accommodate this new generation of microprocessors, LTC introduces the LTC1706-19 VID (voltage identification) voltage programmer. This device is a precision, digitally programmable resistive divider designed for use with an entire family of LTC’s DC/DC converters with onboard 1.19V references. These converters include the LTC1433, LTC1434, LTC1435, LTC1435A, LTC1436, L TC1438, LTC1439, LTC1538-AUX, LTC1539 and LTC1624. (Consult the factory for future compatible DC/DC converter VIN 4.5V–22V 1 CSS 0.1µF 2 3 CC2 220pF 40k VID0 VCC VCC 40k VID1 SGND VCC 40k VID3 Figure 2. LTC1706-19 block diagram TG SW BOOST 6 SGND VOSENSE FB VID2 INTVCC 5 RFB2 SWITCH-CONTROL LOGIC RFB2 RUN/SS RC 10k RFB1 13k 40k VIN ITH SENSE VCC COSC CC 1000pF 51pF VCC LTC1435A COSC 43pF by Peter Guan BG PGND RF 4.7Ω 13 CF 0.1µF 16 + M1 Si4410DY CIN 10µF, 30V ×2 R SENSE VOUT 1.30V– 2.00V/7A 0.015Ω 14 L1 3.3µH DB* 12 0.22µF + + 11 10 SENSE– SENSE+ 7 8 1000pF SENSE VCC 15 4.7µF M2 Si4410DY D1 MBRS -140T3 FB LTC1706-19 VID VID VID VID 0 1 2 3 COUT 820µF 4V ×2 GND *DB = CMDSH-3 FROM µP Figure 1. Intel Mobile Pentium II processor VID power converter Linear Technology Magazine • August 1998 13 DESIGN FEATURES VIN 4.8V–20V VCC 2.7V–5.5V LTC1624 1000pF LTC1706-19 3 10µF VCC 7 6 VID0 SENSE 8 VID1 100pF 1 VID2 2 5 VID3 FB 470pF GND 4 6.8k 1 2 3 4 SENSE– VIN ITH/RUN BOOST VFB GND TG SW 8 7 0.1µF 6 RSENSE 0.05Ω + Si4412DY VOUT 1.3V–2.0V 5 10µH MBRS340T3 CIN 22µF 35V ×2 + COUT 100µF 10V ×2 Figure 3. High efficiency SO-8, N-channel switching regulator with programmable output products.) The LTC1706-19 is fully compliant with the Intel mobile VID specifications and comes in a tiny SO-8 package. Four digital pins are provided to program output voltages from 1.3V to 2.0V in 50mV steps with an accuracy of ±0.25%. Figure 1 shows a VID-programmed DC/DC converter for an Intel mobile processor that uses the LTC1435A and LTC1706-19 to deliver 7A of output current with a programmable VOUT of 1.3V to 2.0V from a VIN of 4.5V to 22V. Simply connecting the LTC170619’s FB and SENSE pins to the LTC1435A’s VOSENSE and SENSE– pins, respectively, closes the loop between the output voltage sense and the feedback inputs of the LTC1435A regulator with the appropriate resistive divider network, which is controlled by the LTC1706-19’s four VID input pins. Figure 2 shows a simplified block diagram of the LTC1706-19. A 40k resistor in series with a diode from VCC pulls up each VID input pin. Therefore, the VID pin must be grounded or driven low to produce a digital low input, whereas a digital high input can be generated by either floating the VID pin or connecting it to V CC. Series diodes from V CC are included to prevent the inputs from being damaged or clamped by a potential higher than VCC. This allows the LTC1706-19 to be fully TTL compatible and operational over a VID input voltage range that is much higher than VCC. When all the inputs are high, the LTC1706-19 has a typi- 14 cal quiescent current of 0.1µ A from VCC, because all active devices are turned off. However, due to the pullup resistors on each of the VID programming inputs, each grounded input contributes approximately 68µ A, (VCC – 0.6)/40k of supply current in a 3.3V system. The top feedback resistor in the block diagram, R FB1 , connected between SENSE and FB, is a 15k resistor whose value is not modified by the state of the VID program inputs. The bottom feedback resistor, RFB2, however, is modified by the four VID inputs. The precision of the ratio between RFB2 and RFB1 results in a ±0.25% output accuracy. Table 1 shows the VID inputs and their corresponding output voltages. VID3 is the most significant bit (MSB) and VID0 is the least significant bit (LSB). When all four inputs are low, the LTC1706-19 sets the regulator output voltage to 2.00V. Each increasing binary count is equivalent to decreasing the output voltage by 50mV. Therefore, to obtain a 1.30V output, the three MSBs are left floating while only VID0 is grounded. In cases where all four VID inputs are tied high or left floating, such as when no processor is present in the system, a regulated 1.25V output is generated at VSENSE. Figure 3 shows a combination of the LTC1624 and the LTC1706-19 configured as a high efficiency stepdown switching regulator with a programmable output of 1.3V to 2.0V from an input of 4.8V to 20V. Using only one N-channel power MOSFET, the two SO-8 packaged LTC parts offer an extremely versatile, efficient, compact regulated power supply. Table 1. VID inputs and corresponding output voltages Code VID3 VID2 VID1 VID0 Output 0000 GND GND GND GND 2.00V 0001 GND GND GND Float 1.95V 0010 GND GND Float GND 1.90V 0011 GND GND Float Float 1.85V 0100 GND Float GND GND 1.80V 0101 GND Float GND Float 1.75V 0110 GND Float Float GND 1.70V 0111 GND Float Float Float 1.65V 1000 Float GND GND GND 1.60V 1001 Float GND GND Float 1.55V 1010 Float GND Float GND 1.50V 1011 Float GND Float Float 1.45V 1100 Float Float GND GND 1.40V 1101 Float Float GND Float 1.35V 1110 Float Float Float GND 1.30V Linear Technology Magazine • August 1998 DESIGN FEATURES Figure 4 shows the LTC1436A-PLL and the LTC1706-19, a combination that yields a high efficiency low noise synchronous step-down switching regulator with programmable 1.3V to 2V outputs and external frequency synchronization capability. Besides the LTC family of 1.19Vreferenced DC/DC converters, the LTC1706-19 can also be used to pro- 10k VIN 4.5V–22V 1 24 PLL LPF PLLIN 2 CSS 0.1µF 3 4 COSC VIN RUN/SS TGL TGS LTC1436A-PLL ITH SW CC 510pF INTVCC RC 10k BOOST 6 8 put will range from 1.27V to 2.03V in steps of 50.8mV. The LTC1706-19 is the ideal companion chip to provide precise, programmable low-voltage outputs for an entire family of LTC DC/DC converters. Its compact size, compatibility and high accuracy are just the right features for today’s portable electronic equipment. EXTERNAL FREQUENCY SYNCHRONIZATION 0.1µF COSC 39pF 100pF gram the output voltages of regulators with different onboard references. Figure 5 shows the LTC1706-19 programming the output of the LT1575, an UltraFast™ transient response, low dropout regulator that is ideal for today’s power-hungry desktop microprocessors. However, since the LT1575 has a 1.21V reference instead of a 1.19V reference, the out- SGND BGL VOSENSE PGND 18 + CIN 22µF, 35V ×2 M1 Si4412DY 21 19 M3 IRLML2803 20 L1 3.3µH RSENSE 0.02Ω VOUT 1.30V– 2.00V/5A D B* 17 0.22µF + + 4.7µF 16 M2 Si4412DY 15 SENSE– SENSE+ 9 10 SENSE VCC 22 D1 MBRS -140T3 FB LTC1706-19 VID VID VID VID 0 1 2 3 COUT 100µF 10V ×2 GND *DB = CMDSH-3 FROM µP 1000pF Figure 4. High efficiency, low noise, synchronous step-down switching regulator with programmable output and external synchronization VIN 12V VCC 3.3V LT1575 LTC1706-19 3 7 8 1 2 VCC 6 SENSE VID0 1 2 VID1 1µF VID2 FBK VID3 5 3 4 SHDN IPOS VIN INEG GND GATE FB COMP 8 3.3V 7 6 5.1Ω IRFZ24 + 5 220µF GND 4 VOUT 1.27V–2.03 IN 50.8mV STEPS 7.5k 24µF 10pF 1000pF Figure 5. UltraFast transient response, low dropout regulator with adjustable output voltage Linear Technology Magazine • August 1998 15