Datasheet LTC1702A (Analog Devices) - 8

LTC1702A
U U W U APPLICATIONS INFORMATION 2-Step Conversion
regulation system happens in the 5V supply, which is “2-step” architectures use a primary regulator to convert usually located away from the CPU. The power lost to heat the input power source (batteries or AC line voltage) to an in the LTC1702A section of the system is relatively low, intermediate supply voltage, often 5V. This intermediate minimizing the added heat near the CPU. voltage is then converted to the low voltage, high current See the Optimizing Performance section for a detailed supplies required by the system using a secondary regu- explanation of how to calculate system efficiency. lator— the LTC1702A. 2-step conversion eliminates the need for a single converter that converts a high input
2-Phase Operation
voltage to a very low output voltage, often an awkward The LTC1702A dual switching regulator controller also design challenge. It also fits naturally into systems that features the considerable benefits of 2-phase operation. continue to use the 5V supply to power portions of their Notebook computers, hand-held terminals and automo- circuitry, or have excess 5V capacity available as newer tive electronics all benefit from the lower input filtering circuit designs shift the current load to lower voltage requirement, reduced electromagnetic interference (EMI) supplies. and increased efficiency associated with 2-phase Each regulator in a typical 2-step system maintains a operation. relatively low step-down ratio (5:1 or less), running at high Why the need for 2-phase operation? Up until the efficiency while maintaining a reasonable duty cycle. In LTC1702A, constant-frequency dual switching regulators contrast, a regulator taking a single step from a high input operated both channels in phase (i.e., single-phase opera- voltage to a 1.xV or 2.xV output must run at a very narrow tion). This means that both topside MOSFETs turned on at duty cycle, mandating trade-offs in external component the same time, causing current pulses of up to twice the values and compromising efficiency and transient amplitude of those for one regulator to be drawn from the response. The efficiency loss can exceed that of using a input capacitor. These large amplitude current pulses 2-step solution (see the 2-Step Efficiency Calculation increased the total RMS current flowing from the input section and Figure 14). Further complicating the calcula- capacitor, requiring the use of more expensive input tion is the fact that many systems draw a significant capacitors and increasing both EMI and losses in the input fraction of their total power off the intermediate 5V supply, capacitor and input power supply. bypassing the low voltage supply. 2-step solutions using the LTC1702A usually match or exceed the total system With 2-phase operation, the two channels of the LTC1702A efficiency of single-step solutions, and provide the addi- are operated 180 degrees out of phase. This effectively tional benefits of improved transient response, reduced interleaves the current pulses coming from the switches, PCB area and simplified power trace routing. greatly reducing the overlap time where they add together. The result is a significant reduction in total RMS input 2-step regulation can buy advantages in thermal manage- current, which in turn allows less expensive input capaci- ment as well. Power dissipation in the LTC1702A portion tors to be used, reduces shielding requirements for EMI of a 2-step circuit is lower than it would be in a typical 1- and improves real world operating efficiency. step converter, even in cases where the 1-step converter has higher total efficiency than the 2-step system. In a Figure 7 shows example waveforms for a single switching typical microprocessor core supply regulator, for ex- regulator channel versus a 2-phase LTC1702A system ample, the regulator is usually located right next to the with both sides switching. A single-phase dual regulator CPU. In a 1-step design, all of the power dissipated by the with both sides operating would exhibit double the single core regulator is right there next to the hot CPU, aggravat- side numbers. In this example, 2-phase operation reduced ing thermal management. In a 2-step LTC1702A design, the RMS input current from 9.3ARMS (2 × 4.66ARMS) to a significant percentage of the power lost in the core 4.8ARMS. While this is an impressive reduction in itself, 1702afa 8