LINEAR TECHNOLOGY AUGUST 1998 IN THIS ISSUE… COVER ARTICLE The LTC®1625 Current Mode DC/DC Controller Eliminates the Sense Resistor ............................... 1 Christopher B. Umminger Issue Highlights ............................ 2 LTC in the News… ......................... 2 DESIGN FEATURES LTC1627 Monolithic Synchronous Step-Down Regulator Maximizes Single or Dual Li-Ion Battery Life ..................................................... 5 Jaime Tseng Clock-Tunable, High Accuracy, Quad 2nd Order, Analog Filter Building Blocks ............................. 8 Philip Karantzalis New DC/DC Controller Enables High Step-Down Ratios ........................ 11 Greg Dittmer VID Voltage Programmer for Intel Mobile Processors ................ 13 Peter Guan Fixed Frequency, 500kHz, 4.5A StepDown Converter in an SO-8 Operates from a 5V Input ............ 16 Karl Edwards 48 Volt Hot Swap™ Controller for Negative Voltages ................. 19 Henry Yun and Robert Reay Hot Swapping the PCI Bus Using the LTC1643 ............................... 22 Robert Reay Eliminate Pipeline Headaches with New 12-Bit 3Msps SAR ADC ......... 25 Dave Thomas and William C. Rempfer DESIGN INFORMATION Component and Measurement Advances Ensure 16-Bit DAC Settling Time (Part One) .............. 30 Jim Williams New 16-Bit Bipolar Output DAC in Narrow SO-16 Package ................ 33 Hassan Malik DESIGN IDEAS ....................... 34–37 (complete list on page 34) New Device Cameos ..................... 37 Design Tools ................................ 39 Sales Offices ............................... 40 VOLUME VIII NUMBER 3 The LTC1625 Current Mode DC/DC Controller Eliminates the by Christopher B. Umminger Sense Resistor Introduction Power supply designers have a new tool in their quest for ever higher efficiencies. In the past, when designing a step-down DC/DC converter, one had to choose between the high efficiency of voltage mode control and the many benefits of current mode control. Although voltage mode control offers high efficiency and a simple topology, it is difficult to compensate, has poor rejection of input-voltage transients and does not inherently limit output current under fault conditions, such as an output short circuit. Current mode control overcomes these problems by adding a control loop to regulate the inductor current in addition to the output voltage. Unfortunately, a sense resistor is required to measure this current, which adds cost and complexity while reducing converter efficiency. However, with the new LTC1625 No RSENSE™ controller, one can enjoy all of the benefits of current mode control without the penalties of using a sense resistor. The LTC1625 is a step-down DC/ DC switching regulator controller that incorporates popular features from the LTC1435 and related parts. It is capable of a wide range of operation with inputs from 3.7V to 36V. Fixed output voltages of 5V and 3.3V can be selected or an external resistive divider can be used to obtain output voltages from 1.19V up to nearly the full input voltage. The controller provides synchronous drive for N-channel power MOSFETs and retains the advantage of low dropout operation typically associated with P-channel MOSFETs. Burst Mode™ operation maintains efficiency at low load currents, but can be overridden to assist secondary-winding regulation by forcing continuous operation. In addition to eliminating the sense resistor, the LTC1625 further reduces the external parts count by incorporating the oscillator timing capacitor. The oscillator frequency can be set to 150kHz, 225kHz, or can be injection locked to any frequency between these points. Current Mode Control without a Sense Resistor How does one implement current mode control without a current sense resistor? The answer is to make the standard power MOSFET switches do double duty as current sense elements. By measuring the drain-tosource voltage of a MOSFET operating in the triode region, one can use the RDS(ON) of the device as a sense resistance. Although conceptually simple, this is tricky to implement in practice because inductor current information can only be obtained when a MOSFET is turned on. Figure 1 continued on page 3 , LTC and LT are registered trademarks of Linear Technology Corporation. Adaptive Power, Burst Mode, C-Load, FilterCAD, Hot Swap, Linear View, Micropower SwitcherCAD, No RSENSE, SwitcherCAD and UltraFast are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. t VIN Easing the Layout Challenge VS ∝ IL VT VSW DESIGN FEATURES t t + TA ×11 TK M1 – I Σ + SW IL – ITH M2 + – BA ×11 VB PGND t Figure 1. The LTC1625 senses VDS across the power MOSFETs to infer inductor current. LTC1625, continued from page 1 illustrates how this is accomplished in the LTC1625. The drain-to-source voltages of the power MOSFETs are sensed through the TK, SW and PGND pins. Sense amplifiers TA and BA measure and amplify these voltages only when the respective MOSFET is conducting current. The two resulting signals (VT and VB) are summed to create a sense voltage ramp (VS) that is proportional to the inductor current throughout the entire switching cycle. A current comparator (I) uses the sense voltage and a current threshold voltage (ITH) to control the power MOSFETs. Because the LTC1625 fully recovers the inductor current information, it behaves 10 IO(MAX) (A) Si4420DY 5 Si4410DY Si4412DY Si9936DY 0 0 50 RDS(ON) (mΩ) 100 DI_1068_02a. EPS Figure 2. Maximum output current is determined primarily by the on-resistance of the power MOSFETs. Linear Technology Magazine • August 1998 identically to traditional current mode controllers that use a sense resistor. An important feature of current mode control is its ability to inher ently limit load current simply by restricting the range of the currentthreshold voltage. The maximum allowed current is then inversely proportional to the sense resistance. With the LTC1625, the power MOSFET onresistance determines the maximum output current. An approximate graph of this relationship is shown in Figure 2, along with data points for some popular Siliconix MOSFETs. The onresistance is typically guaranteed to be below a maximum specified by the manufacturer, but it will vary substantially between lots and over temperature. Thus, a design accommodating the maximum possible on-resistance will have a significantly higher maximum output current when this resistance is lower than normal. To ensure reasonable current levels in case of a fault, such as an output short to ground, the LTC1625 will fold back the current if the output voltage falls significantly. Ensuring that switching transients do not interfere with the control loop is a common difficulty with high output-current power converters. A great deal of effort is often expended in board layout and component placement to subdue coupling effects, such as pulse pairing or grouping. Eliminating the sense resistor and its associated signal lines makes this job easier. In addition, the LTC1625 uses some internal blanking around the switching transitions to further reduce the possibility of jitter or pulse pairing in the control loop. The result is a controller that is relatively immune to these coupling effects and remarkably easy to lay out successfully. On the other hand, the addition of blanking increases the minimum time in which the controller can turn the top MOSFET on and off. This places a limit on the minimum duty cycle but does not unduly restrict the maximum input voltage for a given output voltage. Table 1 shows the many common input and output voltage combinations that the LTC1625 can accommodate. Additional Features The LTC1625 controller is designed for synchronous, step-down applications with two N-channel power MOSFETs. Using an N-channel MOSFET for the topside switch is more cost-effective than using a P-channel device, but it requires a floating topside driver. This driver is powered using an external bootstrap capacitor and diode. If the input voltage Table 1. Some common input and output voltage combinations available with fOSC = 150kHz VOUT (V) 1.5 1.8 2.5 3.3 5.0 12.0 VIN (V) 5 12 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ 18 24 30 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ 3 DESIGN FEATURES 4 age that limits the load current to Removing Output Voltage Constraints along with RSENSE approximately one third of its maxiThe LTC1625 can be configured for a wide variety of output voltages using the VPROG and VOSENSE pins. For basic 3.3V and 5V applications, one can use an internal resistive divider with the output connected directly to the VOSENSE pin. Connect the VPROG pin to INTVCC for a 5V output or to ground for a 3.3V output. Leaving the VPROG pin open disables the internal resistive divider and connects the error amplifier feedback node directly to the VOSENSE pin. With an external resistive divider, the LTC1625 is capable of regulating an output voltage anywhere between the input voltage and the 1.19V internal reference. This represents a significant improvement over previous current mode controllers, which were constrained by the common mode range of the current sense signal lines. Start-up and shutdown of the LTC1625 can be controlled via the RUN/SS pin. This pin is connected to an external capacitor that is charged from an internal 3µ A current source. The controller will be shut down if this pin is held below 1.4V. After the pin is released, the part stays shut down while the capacitor is charged, permitting a controlled delay for sequencing the power supply startup. When the voltage rises above 1.4V, the controller will start but with a clamp on the current threshold volt- mum value. Limiting the load current reduces the inrush current into the converter. As the voltage increases above 1.4V, the clamp is gradually raised to its normal value. Design Examples Figure 3 shows the LTC1625 in an application supplying a 2.5V output using an external feedback divider. Si4410DY MOSFETs from Siliconix allow this converter to deliver up to 5A of load current. Ripple current is 1.8A (36% of full load) and current limit occurs around 6A. Note also that the EXTVCC pin is connected to an external 5V supply. This increases efficiency by drawing the roughly 7mA gate charge current from a supply lower than VIN. An efficiency plot of this circuit is shown in Figure 4. An LTC1435 with identical components in the power path is also plotted for comparison. At lower output voltages such as this, the sense resistor is responsible for an increasing share of the total power loss. By eliminating this source of loss, the LTC1625 is easily able to deliver an efficiency greater than 90% at high load current. The benefit of reduced I2R loss is readily apparent at the highest loads. The controller makes a transition to Burst Mode operation below around 1.1 A which keeps the efficiency high at moderate loads. continued on page 18 RF 4.7Ω 5V 1 2 CSS 0.1µF 3 4 CC1 820pF RC1 5 10k 6 CC2 220pF 7 8 SYNC TK RUN/SS SW FCB TG ITH BOOST SGND INTVCC VOSENSE VPROG BG PGND 15 M1 Si4410DY 14 CB O.22µF 13 11 VIN 5V–28V CIN 10µF 30V ×2 L1 7µH D1 MBRS140T3 *DB 12 10 + CF 0.1µF LTC1625 16 EXTVCC VIN + drops close to the output voltage, the LTC1625 will begin skipping cycles, leaving the top MOSFET on longer in order to maintain low dropout operation. Once in dropout, the boost capacitor is recharged by turning off the top MOSFET and turning on the bottom MOSFET briefly every tenth cycle. This maintains a duty cycle greater than 99% in dropout. Another external part was eliminated by incorporating the oscillator timing capacitor into the LTC1625. The controller runs at a nominal 150kHz frequency that can be increased 50% by taking the SYNC pin above 1.2V. Frequency-sensitive applications can synchronize the oscillator by applying a clock signal between 165kHz and 200kHz to this pin. Providing gate charge to the power MOSFET switches is one of the main sources of efficiency loss in switching regulators. The LTC1625 includes two features to minimize this loss. Normally, both MOSFETs are turned on and off once per oscillator cycle. When the load current drops to near zero, the gate charge required for switching becomes a significant fraction of the total input current. The LTC1625 addresses this problem with Burst Mode operation. As the load current decreases below approximately one fifth of its maximum value, both MOSFETs are turned off for a few cycles while the output capacitors support the load. In this way, gate charge is saved at the expense of fixed frequency operation. For applications that require it, fixed frequency operation can be maintained by tying the FCB pin low to force continuous operation at low load currents. This pin can also be used to adaptively override Burst Mode operation when regulating the output of a secondary winding. Another problem with gate charge is that it is normally drawn from the high voltage input supply. However, power can be saved by obtaining the gate charge from a high efficiency external supply (between 5V and 7V) connected to the EXTVCC pin. This could be the output voltage of the regulator, a boosted version of the output or some other system supply, if available. CVCC 4.7µF R2 11k 1% + R1 10k 1% 9 VOUT 2.5V/5A COUT 100µF 10V 0.065Ω ESR ×3 M2 Si4410DY * DB = CMDSH-3 Figure 3. 2.5V/5A adjustable-output supply Linear Technology Magazine • August 1998 DESIGN FEATURES tantalum compensation value of 1.5nF is increased to 22nF (×3) for the ceramic output capacitor. If synchronization is not used and the internal oscillators free run, the circuit will operate correctly, but ripple cancellation will not occur. Input and output capacitors must be ripple rated for the individual output currents. Redundant Operation The circuit shown in Figure 3 is fault tolerant when operating at less than 8A of output current. If one power stage fails open circuit, the output will remain in regulation. The feedback loop will compensate by raising the voltage on the VC pin, increasing the switch current of the two remaining devices. 5V to 3.3V at 2.5A on 0.25in2 of board space, 0.125in High higher ripple current can be tolerated, allowing the use of small, low value, high current inductors. A ceramic output capacitor also reduces board area and improves voltage ripple. Using Figure 1’s circuit with the SO-8 LT1506 and the component changes in Table 1, a very small, low profile, step-down converter can be implemented. In many space-sensitive applications, the component that dominates both board area and overall height is the inductor. One of the factors affecting inductor value choice is maximum ripple current. Using the high current switch rating of the LT1506, Conclusion Table 1. Component changes for a low profile version of Figure 1’s circuit Part Value C1, C3 22µF, 10V CC 22nF L1 2.2µH The LT1506 is a compact, easy to use, monolithic switcher. The internal 4.5A switch covers a wide range of medium power applications. Its input operating range of 4V to 15V and availability in SO-8 or DD packages make it ideal for very space-efficient, local onboard DC/DC converters. Vendor/ Part# Tokin 1210ZG226Z Sumida CD43 2R2 Authors can be contacted at (408) 432-1900 RF 4.7Ω 1 VIN = 20V VOUT = 2.5V INTVCC EFFICIENCY (%) 95 CSS LTC1625 0.1µF 2 3 4 90 CC1 1nF LTC1435 RC1 5 10k 6 CC2 220pF 85 7 8 LTC1625 16 EXTVCC VIN SYNC TK RUN/SS SW FCB TG ITH BOOST SGND INTVCC VOSENSE VPROG BG PGND 15 13 12 1 2 3 LOAD CURRENT (A) 4 **DB 11 10 CB O.1µF VIN 12V–28V CIN 22µF 35V ×2 L1* 39µH D1 MBRS140T3 CVCC 4.7µF R2 35.7k 1% + R1 3.92k 1% 9 80 0 M1 Si4412DY 14 + 100 + CF 0.1µF VOUT 12V/2.2A COUT 100µF 16V 0.030Ω ESR M2 Si4412DY 5 DI_1068_02a. EPS * L1 = SUMIDA CDRH127-390MC ** DB = CMDSH-3 Figure 4. Efficiency vs load current Figure 5. 12V/2.2A adjustable-output supply LTC1625, continued from page 4 Conclusion A circuit demonstrating the wide output range of the LTC1625 is shown in Figure 5. This application uses Si4412DY MOSFETs to deliver a 12V output at up to 2.2A. Note that the SYNC pin is tied high for 225kHz operation in order to reduce the inductor size and ripple current. The LTC1625 step-down DC/DC controller offers true current mode control without the expense and difficulty of using a sense resistor. Popular features from Linear Technology’s other controllers, such as fixed frequency operation, N-channel MOSFET drive, Burst Mode operation, soft-start and output voltage programming make 18 this controller useful in a variety of applications. By eliminating the power loss in the sense resistor, even higher efficiencies can be achieved than were previously possible, making the LTC1625 an excellent choice for DC/DC converter designs requiring the highest performance. Linear Technology Magazine • August 1998