LTC3619EMSE#TRPBF

LTC3619 400mA/800mA Synchronous Step-Down DC/DC with Average Input Current Limit Description Features Programmable Average Input Current Limit: ±5% Accuracy n Dual Step-Down Outputs: Up to 96% Efficiency ® n Low Ripple (> 100/clock frequency (2.25MHz) as a starting point, R being RLIM, C being CLIM. VOUT 2V/DIV VOUT 200mV/DIV IVIN 500mA/DIV VIN 1V/DIV AC-COUPLED VRLIM 1V/DIV IOUT 500mA/DIV IL 1A/DIV IIN 500mA/DIV 50ms/DIV 3619 F01a 1ms/DIV 3619 F01b VIN = 5V, 500mA COMPLIANT, RLIM = 116k, CLIM = 0.1µF ILOAD = 0A TO 1.1A, COUT = 2.2mF, VOUT = 3.3V ILIM = 475mA, CHANNEL 1 UNLOADED VIN = 5V, 500mA COMPLIANT, RLIM = 116k, CLIM = 2200pF ILOAD = 0A TO 2A, COUT = 2.2mF, VOUT = 3.3V ILIM = 475mA, CHANNEL 1 NOT LOADED Figure 1a. Input Current Limit Within 100ms Load Pulses Figure 1b. Input Current Limit Within 577µs, 2A Repeating Load Pulses 3619fa 11 LTC3619 Applications Information A general LTC3619 application circuit is shown in Figure 2. External component selection is driven by the load requirement, and begins with the selection of the inductor L. Once the inductor is chosen, CIN and COUT can be selected. Inductor Selection Although the inductor does not influence the operating frequency, the inductor value has a direct effect on ripple current. The inductor ripple current DIL decreases with higher inductance and increases with higher VIN or VOUT :  V  V (1) ∆IL = OUT •  1− OUT  fO • L  VIN  Accepting larger values of DIL allows the use of low inductances, but results in higher output voltage ripple, greater core losses, and lower output current capability. A reasonable starting point for setting ripple current is 40% of the maximum output load current. So, for a 800mA regulator, DIL = 320mA (40% of 800mA). The inductor value will also have an effect on Burst Mode operation. The transition to low current operation begins when the peak inductor current falls below a level set by the internal burst clamp. Lower inductor values result in higher ripple current which causes the transition to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. Furthermore, lower inductance values will cause the bursts to occur with increased frequency. Inductor Core Selection Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price versus size requirements, and any radiated field/EMI requirements, than on what the LTC3619 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3619 applications. Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT / VIN . To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: IRMS ≈IMAX VOUT (VIN − VOUT ) VIN Where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current, IMAX = ILIM – DIL /2. This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case is commonly used to design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours lifetime. This makes it advisable to further VIN 2.5V TO 5.5V CIN RUN2 VIN RUN1 PGOOD2 PGOOD1 LTC3619 L2 VOUT2 CF2 COUT2 R4 R3 SW2 SW1 VFB2 RLIM VFB1 GND CLIM L1 CF1 R1 R2 RLIM VOUT1 COUT1 3619 F02 Figure 2. LTC3619 General Schematic 3619fa 12 LTC3619 Applications Information Table 1. Representative Surface Mount Inductors MANUFACTURER PART NUMBER VALUE MAX DC CURRENT DCR HEIGHT Coilcraft LPS4012-152ML LPS4012-222ML LPS4012-332ML LPS4012-472ML LPS4018-222ML LPS4018-332ML LPS4018-472ML 1.5µH 2.2µH 3.3µH 4.7µH 2.2µH 3.3µH 4.7µH 2200mA 1750mA 1450mA 1450mA 2300mA 2000mA 1800mA 0.070Ω 0.100Ω 0.100Ω 0.170Ω 0.070Ω 0.080Ω 0.125Ω 1.2mm 1.2mm 1.2mm 1.2mm 1.8mm 1.8mm 1.8mm FDK FDKMIPF2520D FDKMIPF2520D FDKMIPF2520D 4.7µH 3.3µH 2.2µH 1100mA 1200mA 1300mA 0.11Ω 0.1Ω 0.08Ω 1mm 1mm 1mm Murata LQH32CN4R7M23 4.7µH 450mA 0.2Ω 2mm Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm Sumida CDRH2D18/LD CDH38D11SNP-3R3M CDH38D11SNP-2R2M 4.7µH 3.3µH 2.2µH 630mA 1560mA 1900mA 0.086Ω 0.115Ω 0.082Ω 2mm 1.2mm 1.2mm Taiyo Yuden CB2016T2R2M CB2012T2R2M CB2016T3R3M NR30102R2M NR30104R7M 2.2µH 2.2µH 3.3µH 2.2µH 4.7µH 510mA 530mA 410mA 1100mA 750mA 0.13Ω 0.33Ω 0.27Ω 0.1Ω 0.19Ω 1.6mm 1.25mm 1.6mm 1mm 1mm TDK VLF3010AT4R7-MR70 VLF3010AT3R3-MR87 VLF3010AT2R2-M1R0 VLF4012AT-2R2M1R5 VLF5012ST-3R3M1R7 VLF5014ST-2R2M2R3 4.7µH 3.3µH 2.2µH 2.2µH 3.3µH 2.2µH 700mA 870mA 1000mA 1500mA 1700mA 2300mA 0.28Ω 0.17Ω 0.12Ω 0.076Ω 0.095Ω 0.059Ω 1mm 1mm 1mm 1.2mm 1.2mm 1.4mm derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet the size or height requirements of the design. An additional 0.1µF to 1µF ceramic capacitor is also recommended on VIN for high frequency decoupling when not using an all-ceramic capacitor solution. Output Capacitor (COUT) Selection The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple DVOUT is determined by:  1  ∆VOUT ≈ ∆IL  ESR + 8fOCOUT   where fO = operating frequency, COUT = output capacitance and DIL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since DIL increases with input voltage. If tantalum capacitors are used, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet KO-CAP, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3619 control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. However, care must be taken when ceramic capacitors are used at the input. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through 3619fa 13 LTC3619 Applications Information long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. For more information, see Application Note 88. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. Setting the Output Voltage The LTC3619 regulates the VFB1 and VFB2 pins to 0.6V during regulation. Thus, the output voltage is set by a resistive divider, Figure 2, according to the following formula:  R2  VOUT = 0.6V  1+   R1 (2) Keeping the current small (< 10µA) in these resistors maximizes efficiency, but making it too small may allow stray capacitance to cause noise problems or reduce the phase margin of the error amp loop. CF2) can be added to improve the high frequency response, as shown in Figure 2. Capacitor CF provides phase lead by creating a high frequency zero with R2 which improves the phase margin. The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual overall supply performance. For a detailed explanation of optimizing the compensation components, including a review of control loop theory, refer to Application Note 76. In some applications, a more severe transient can be caused by switching in loads with large (>1µF) input capacitors. The discharged input capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the switch connecting the load has low resistance and is driven quickly. The solution is to limit the turn-on speed of the load switch driver. A Hot Swap™ controller is designed specifically for this purpose and usually incorporates current limiting, short-circuit protection, and soft-starting. Efficiency Considerations To improve the frequency response of the main control loop, a feedback capacitor (CF) may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line. The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: Checking Transient Response % Efficiency = 100% – (L1 + L2 + L3 + ...) The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to DILOAD • ESR, where ESR is the effective series resistance of COUT. DILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. where L1, L2, etc., are the individual losses as a percentage of input power. The initial output voltage step may not be within the bandwidth of the feedback loop, so the standard second order overshoot/DC ratio cannot be used to determine the phase margin. In addition, feedback capacitors (CF1 and Although all dissipative elements in the circuit produce losses, four sources usually account for the losses in LTC3619 circuits: 1) VIN quiescent current, 2) switching losses, 3) I2R losses, 4) other system losses. 1. The VIN current is the DC supply current given in the Electrical Characteristics which excludes MOSFET driver and control currents. VIN current results in a small (