LTC3406 LTC3406-1.5/LTC3406-1.8 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC ®3406 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. The device is available in an adjustable version and fixed output voltages of 1.5V and 1.8V. Supply current during operation is only 20µA and drops to ≤1µA in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3406 ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. Automatic Burst Mode® operation increases efficiency at light loads, further extending battery life. High Efficiency: Up to 96% Very Low Quiescent Current: Only 20µA During Operation 600mA Output Current 2.5V to 5.5V Input Voltage Range 1.5MHz Constant Frequency Operation No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle 0.6V Reference Allows Low Output Voltages Shutdown Mode Draws ≤ 1µA Supply Current Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protected Low Profile (1mm) ThinSOTTM Package Switching frequency is internally set at 1.5MHz, allowing the use of small surface mount inductors and capacitors. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. Low output voltages are easily supported with the 0.6V feedback reference voltage. The LTC3406 is available in a low profile (1mm) ThinSOT package. U APPLICATIO S ■ ■ ■ ■ ■ Cellular Telephones Personal Information Appliances Wireless and DSL Modems Digital Still Cameras MP3 Players Portable Instruments , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. Protected by U.S. Patents, including 6580258, 5481178. U ■ TYPICAL APPLICATIO 95 90 4 CIN** 4.7µF CER VIN SW 3 2.2µH* COUT† 10µF CER LTC3406-1.8 1 RUN VOUT 5 3406 F01a GND 2 *MURATA LQH32CN2R2M33 **TAIYO YUDEN JMK212BJ475MG † TAIYO YUDEN JMK316BJ106ML VOUT 1.8V 600mA 85 EFFICIENCY (%) VIN 2.7V TO 5.5V VIN = 2.7V VIN = 3.6V 80 VIN = 4.2V 75 70 65 60 0.1 VOUT = 1.8V 1 10 100 OUTPUT CURRENT (mA) 1000 3406 F01b Figure 1a. High Efficiency Step-Down Converter Figure 1b. Efficiency vs Load Current 3406fa 1 LTC3406 LTC3406-1.5/LTC3406-1.8 U W W W ABSOLUTE AXI U RATI GS (Note 1) Input Supply Voltage .................................. – 0.3V to 6V RUN, VFB Voltages ..................................... – 0.3V to VIN SW Voltage .................................. – 0.3V to (VIN + 0.3V) P-Channel Switch Source Current (DC) ............. 800mA N-Channel Switch Sink Current (DC) ................. 800mA Peak SW Sink and Source Current ........................ 1.3A Operating Temperature Range (Note 2) .. – 40°C to 85°C Junction Temperature (Note 3) ............................ 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C U U W PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW RUN 1 5 VFB LTC3406ES5 GND 2 SW 3 4 VIN S5 PACKAGE 5-LEAD PLASTIC TSOT-23 RUN 1 TJMAX = 125°C, θJA = 250°C/ W, θJC = 90°C/ W LTA5 5 VOUT LTC3406ES5-1.5 LTC3406ES5-1.8 GND 2 SW 3 S5 PART MARKING ORDER PART NUMBER TOP VIEW 4 VIN S5 PART MARKING S5 PACKAGE 5-LEAD PLASTIC TSOT-23 LTD6 LTC4 TJMAX = 125°C, θJA = 250°C/ W, θJC = 90°C/ W Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified. SYMBOL PARAMETER IVFB Feedback Current VFB Regulated Feedback Voltage CONDITIONS MIN TYP MAX UNITS ±30 nA 0.5880 0.5865 0.5850 0.6 0.6 0.6 0.6120 0.6135 0.6150 V V V 0.04 0.4 1.455 1.746 1.500 1.800 1.545 1.854 ● LTC3406 (Note 4) TA = 25°C LTC3406 (Note 4) 0°C TA ≤ 85°C LTC3406 (Note 4) –40°C ≤ TA ≤ 85°C ● ∆VFB Reference Voltage Line Regulation VIN = 2.5V to 5.5V (Note 4) ● VOUT Regulated Output Voltage LTC3406-1.5, IOUT = 100mA LTC3406-1.8, IOUT = 100mA ● ● ∆VOUT Output Voltage Line Regulation VIN = 2.5V to 5.5V ● IPK Peak Inductor Current VIN = 3V, VFB = 0.5V or VOUT = 90%, Duty Cycle < 35% VLOADREG Output Voltage Load Regulation VIN Input Voltage Range IS Input DC Bias Current Active Mode Sleep Mode Shutdown (Note 5) VFB = 0.5V or VOUT = 90%, ILOAD = 0A VFB = 0.62V or VOUT = 103%, ILOAD = 0A VRUN = 0V, VIN = 4.2V fOSC Oscillator Frequency VFB = 0.6V or VOUT = 100% VFB = 0V or VOUT = 0V RPFET RDS(ON) of P-Channel FET RNFET ILSW 0.75 V V 0.04 0.4 %/V 1 1.25 A 0.5 ● %/V % 5.5 V 300 20 0.1 400 35 1 µA µA µA 1.5 210 1.8 MHz kHz ISW = 100mA 0.4 0.5 Ω RDS(ON) of N-Channel FET ISW = –100mA 0.35 0.45 Ω SW Leakage VRUN = 0V, VSW = 0V or 5V, VIN = 5V ±0.01 ±1 ● 2.5 1.2 µA 3406fa 2 LTC3406 LTC3406-1.5/LTC3406-1.8 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified. SYMBOL PARAMETER CONDITIONS VRUN RUN Threshold ● IRUN RUN Leakage Current ● Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3406E is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. MIN TYP MAX UNITS 0.3 1 1.5 V ±0.01 ±1 µA Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC3406: TJ = TA + (PD)(250°C/W) Note 4: The LTC3406 is tested in a proprietary test mode that connects VFB to the output of the error amplifier. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. U W TYPICAL PERFOR A CE CHARACTERISTICS (From Figure1a Except for the Resistive Divider Resistor Values) Efficiency vs Input Voltage 95 IOUT = 100mA 95 Efficiency vs Output Current Efficiency vs Output Current 100 IOUT = 10mA 95 VOUT = 1.2V VIN = 2.7V 90 85 IOUT = 600mA 80 75 70 IOUT = 0.1mA 65 VIN = 2.7V 85 EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%) 90 85 IOUT = 1mA VOUT = 1.5V 90 VIN = 4.2V 80 VIN = 3.6V 75 VIN = 4.2V 80 VIN = 3.6V 75 70 70 65 65 60 55 50 VOUT = 1.8V 3 2 4 5 INPUT VOLTAGE (V) 60 0.1 6 1 10 100 OUTPUT CURRENT (mA) VIN = 3.6V 85 VIN = 4.2V 75 70 1.60 FREQUENCY (MHz) REFERENCE VOLTAGE (V) EFFICIENCY (%) VIN = 3.6V 1.65 0.609 90 0.604 0.599 0.594 10 100 1 OUTPUT CURRENT (mA) 1000 3406 G04 1.55 1.50 1.45 1.40 0.589 65 60 0.1 1.70 VIN = 3.6V VIN = 2.7V 80 Oscillator Frequency vs Temperature 0.614 VOUT = 2.5V 1000 3406 G03 Reference Voltage vs Temperature Efficiency vs Output Current 95 1 10 100 OUTPUT CURRENT (mA) 3406 G02 3406 G01 100 60 0.1 1000 0.584 –50 –25 1.35 50 25 75 0 TEMPERATURE (°C) 100 125 3406 G05 1.30 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3406 G06 3406fa 3 LTC3406 LTC3406-1.5/LTC3406-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Figure1a Except for the Resistive Divider Resistor Values) Oscillator Frequency vs Supply Voltage 1.6 1.5 1.4 1.3 0.7 VIN = 3.6V 1.834 0.6 1.824 0.5 RDS(ON) (Ω) 1.7 OUTPUT VOLTAGE (V) OSCILLATOR FREQUENCY (MHz) 1.8 1.2 1.814 1.804 3 4 5 SUPPLY VOLTAGE (V) 1.784 0.1 0 6 RDS(ON) vs Temperature VIN = 2.7V VIN = 3.6V 0.2 30 25 20 15 0 6 4 3 5 SUPPLY VOLTAGE (V) 2 15 0 –50 –25 VIN = 5.5V RUN = 0V SWITCH LEAKAGE (pA) 150 100 MAIN SWITCH RUN = 0V SW 5V/DIV SYNCHRONOUS SWITCH 80 60 VOUT 100mV/DIV AC COUPLED MAIN SWITCH IL 200mA/DIV 40 20 VIN = 3.6V VOUT = 1.8V ILOAD = 50mA SYNCHRONOUS SWITCH 0 125 3406 G13 125 Burst Mode Operation 100 250 100 3406 G12 Switch Leakage vs Input Voltage 200 50 25 0 75 TEMPERATURE (°C) 3406 G11 120 100 20 5 Switch Leakage vs Temperature 50 25 75 0 TEMPERATURE (°C) 25 10 125 300 0 –50 –25 30 5 3406 G10 50 35 10 MAIN SWITCH SYNCHRONOUS SWITCH 7 6 VIN = 3.6V 45 VOUT = 1.8V = 0A I 40 LOAD VOUT = 1.8V ILOAD = 0A 35 0.1 100 5 4 2 3 INPUT VOLTAGE (V) Supply Current vs Temperature SUPPLY CURRENT (µA) SUPPLY CURRENT (µA) RDS(ON) (Ω) 0.3 1 3406 G09 40 0.4 50 25 75 0 TEMPERATURE (°C) 0 50 45 0.5 0 –50 –25 0 100 200 300 400 500 600 700 800 900 LOAD CURRENT (mA) Supply Current vs Supply Voltage 50 VIN = 4.2V SYNCHRONOUS SWITCH 3406 G08 0.7 0.6 0.3 0.2 1.774 2 MAIN SWITCH 0.4 1.794 3406 G07 SWITCH LEAKAGE (nA) RDS(ON) vs Input Voltage Output Voltage vs Load Current 1.844 0 1 2 3 4 INPUT VOLTAGE (V) 5 4µs/DIV 3406 G15 6 3406 G14 3406fa 4 LTC3406 LTC3406-1.5/LTC3406-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Figure 1a Except for the Resistive Divider Resistor Values) Start-Up from Shutdown Load Step RUN 2V/DIV VOUT 2V/DIV Load Step VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED ILOAD 500mA/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 600mA 40µs/DIV 3406 G16 IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 0mA TO 600mA Load Step VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 50mA TO 600mA 3406 G18 Load Step VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 100mA TO 600mA 3406 G17 3406 G19 VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 200mA TO 600mA 3406 G20 U U U PI FU CTIO S RUN (Pin 1): Run Control Input. Forcing this pin above 1.5V enables the part. Forcing this pin below 0.3V shuts down the device. In shutdown, all functions are disabled drawing <1µA supply current. Do not leave RUN floating. GND (Pin 2): Ground Pin. SW (Pin 3): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. VIN (Pin 4): Main Supply Pin. Must be closely decoupled to GND, Pin 2, with a 2.2µF or greater ceramic capacitor. VFB (Pin 5) (LTC3406): Feedback Pin. Receives the feedback voltage from an external resistive divider across the output. VOUT (Pin 5) (LTC3406-1.5/LTC3406-1.8): Output Voltage Feedback Pin. An internal resistive divider divides the output voltage down for comparison to the internal reference voltage. 3406fa 5 LTC3406 LTC3406-1.5/LTC3406-1.8 W FU CTIO AL DIAGRA U U SLOPE COMP 0.65V OSC OSC 4 VIN FREQ SHIFT – VFB /VOUT + 5 LTC3406-1.5 R1 + R2 = 550k 0.6V R1 FB LTC3406-1.8 R1 + R2 = 540k – + – EA 0.4V R2 SLEEP – + BURST S Q R Q RS LATCH VIN RUN SWITCHING LOGIC AND BLANKING CIRCUIT ANTISHOOTTHRU 3 SW 0.6V REF + 1 5Ω + ICOMP SHUTDOWN IRCMP 2 GND – 3406 BD U OPERATIO (Refer to Functional Diagram) Main Control Loop Burst Mode Operation The LTC3406 uses a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback voltage, FB, relative to the 0.6V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. The LTC3406 is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. In Burst Mode operation, the peak current of the inductor is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 20µA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier’s output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. 3406fa 6 LTC3406 LTC3406-1.5/LTC3406-1.8 U OPERATIO (Refer to Functional Diagram) When the output is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, 1/7 the nominal frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator’s frequency will progressively increase to 1.5MHz when VFB or VOUT rises above 0V. Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the LTC3406 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). Low Supply Operation The LTC3406 will operate with input supply voltages as low as 2.5V, but the maximum allowable output current is reduced at this low voltage. Figure 2 shows the reduction in the maximum output current as a function of input voltage for various output voltages. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current for duty cycles > 40%. However, the LTC3406 uses a patent-pending scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. 1200 MAXIMUM OUTPUT CURRENT (mA) Short-Circuit Protection 1000 800 600 VOUT = 1.8V VOUT = 2.5V VOUT = 1.5V 400 200 0 2.5 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 5.5 3406 F02 Figure 2. Maximum Output Current vs Input Voltage 3406fa 7 LTC3406 LTC3406-1.5/LTC3406-1.8 U W U U APPLICATIO S I FOR ATIO The basic LTC3406 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3406 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3406 applications. Inductor Selection For most applications, the value of the inductor will fall in the range of 1µH to 4.7µH. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in equation 1. A reasonable starting point for setting ripple current is ∆IL = 240mA (40% of 600mA). ∆IL = ⎛ V ⎞ 1 VOUT ⎜ 1 − OUT ⎟ ( f)(L) ⎝ VIN ⎠ (1) The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications (600mA + 120mA). For better efficiency, choose a low DC-resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 200mA. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. 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 don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style Table 1. Representative Surface Mount Inductors PART NUMBER VALUE (µH) DCR (Ω MAX) MAX DC SIZE CURRENT (A) W × L × H (mm3) Sumida CDRH3D16 1.5 2.2 3.3 4.7 0.043 0.075 0.110 0.162 1.55 1.20 1.10 0.90 3.8 × 3.8 × 1.8 Sumida CMD4D06 2.2 3.3 4.7 0.116 0.174 0.216 0.950 0.770 0.750 3.5 × 4.3 × 0.8 Panasonic ELT5KT 3.3 4.7 0.17 0.20 1.00 0.95 4.5 × 5.4 × 1.2 Murata LQH32CN 1.0 2.2 4.7 0.060 0.097 0.150 1.00 0.79 0.65 2.5 × 3.2 × 2.0 CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: 1/ 2 VOUT (VIN − VOUT )] [ CIN required IRMS ≅ IOMAX VIN This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. 3406fa 8 LTC3406 LTC3406-1.5/LTC3406-1.8 U W U U APPLICATIO S I FOR ATIO 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 ∆VOUT is determined by: ⎛ 1 ⎞ ∆VOUT ≅ ∆IL ⎜ ESR + ⎟ ⎝ 8fC OUT ⎠ where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since ∆IL increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, 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 T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. 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. 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. Output Voltage Programming (LTC3406 Only) In the adjustable version, the output voltage is set by a resistive divider according to the following formula: ⎛ R2 ⎞ VOUT = 0.6 V ⎜ 1 + ⎟ ⎝ R1⎠ (2) The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 3. 0.6V ≤ VOUT ≤ 5.5V R2 VFB LTC3406 R1 GND 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 LTC3406’s 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 and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can 3406 F03 Figure 3. Setting the LTC3406 Output Voltage Efficiency Considerations The 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. Efficiency can be expressed as: Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. 3406fa 9 LTC3406 LTC3406-1.5/LTC3406-1.8 U W U U APPLICATIO S I FOR ATIO Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3406 circuits: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 4. 1 POWER LOSS (W) 0.1 RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Charateristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. VOUT = 1.2V VOUT = 1.5V VOUT = 1.8V VOUT = 2.5V 0.01 Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. 0.001 0.0001 0.00001 0.1 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is “chopped” between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: Thermal Considerations 1 10 100 LOAD CURRENT (mA) 1000 3406 F04 Figure 4. Power Lost vs Load Current 1. The VIN quiescent current is due to two components: the DC bias current as given in the electrical characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. In continuous mode, IGATECHG = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. In most applications the LTC3406 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3406 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150°C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3406 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(θJA) where PD is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the ambient temperature. 3406fa 10 LTC3406 LTC3406-1.5/LTC3406-1.8 U W U U APPLICATIO S I FOR ATIO The junction temperature, TJ, is given by: T J = TA + TR where TA is the ambient temperature. As an example, consider the LTC3406 in dropout at an input voltage of 2.7V, a load current of 600mA and an ambient temperature of 70°C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70°C is approximately 0.52Ω. Therefore, power dissipated by the part is: PD = ILOAD2 • RDS(ON) = 187.2mW For the SOT-23 package, the θJA is 250°C/ W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.1872)(250) = 116.8°C which is below the maximum junction temperature of 125°C. Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). Checking Transient Response 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 (∆ILOAD • ESR), where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its steadystate value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76. A second, more severe transient is caused by switching in loads with large (>1µF) supply bypass capacitors. The discharged bypass 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 load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 • CLOAD). Thus, a 10µF capacitor charging to 3.3V would require a 250µs rise time, limiting the charging current to about 130mA. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3406. These items are also illustrated graphically in Figures 5 and 6. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide. 2. Does the VFB pin connect directly to the feedback resistors? The resistive divider R1/R2 must be connected between the (+) plate of COUT and ground. 3. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the switching node, SW, away from the sensitive VFB node. 5. Keep the (–) plates of CIN and COUT as close as possible. 3406fa 11 LTC3406 LTC3406-1.5/LTC3406-1.8 U W U U APPLICATIO S I FOR ATIO 1 RUN VFB 5 LTC3406 2 – 3 + L1 SW RUN LTC3406-1.8 COUT VOUT 1 R1 R2 GND VIN 2 CFWD 4 GND VOUT – COUT VOUT CIN 5 3 + L1 VIN SW VIN 4 CIN VIN 3406 F05b 3406 F05a BOLD LINES INDICATE HIGH CURRENT PATHS BOLD LINES INDICATE HIGH CURRENT PATHS Figure 5a. LTC3406 Layout Diagram Figure 5b. LTC3406-1.8 Layout Diagram VIA TO GND R1 L1 PIN 1 CFWD LTC3406 LTC3406-1.8 VOUT SW L1 COUT SW COUT CIN CIN GND GND 3406 F06b 3406 F06a Figure 6a. LTC3406 Suggested Layout Figure 6b. LTC3406-1.8 Suggested Layout Design Example As a design example, assume the LTC3406 is used in a single lithium-ion battery-powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 0.6A but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low and high load currents is important. Output voltage is 2.5V. With this information we can calculate L using equation (1), L= ⎛ V ⎞ 1 VOUT ⎜ 1 − OUT ⎟ ( f)(∆IL ) ⎝ VIN ⎠ VIN VIA TO VOUT R2 PIN 1 VOUT VIA TO VOUT VIA TO VIN VIN VIA TO VIN (3) Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 240mA and f = 1.5MHz in equation (3) gives: L= 2.5V ⎛ 2.5V ⎞ ⎜1 − ⎟ = 2.81µH 1.5MHz(240mA) ⎝ 4.2V ⎠ A 2.2µH inductor works well for this application. For best efficiency choose a 720mA or greater inductor with less than 0.2Ω series resistance. CIN will require an RMS current rating of at least 0.3A ≅ ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.25Ω. In most cases, a ceramic capacitor will satisfy this requirement. 3406fa 12 LTC3406 LTC3406-1.5/LTC3406-1.8 U W U U APPLICATIO S I FOR ATIO For the feedback resistors, choose R1 = 316k. R2 can then be calculated from equation (2) to be: Figure 7 shows the complete circuit along with its efficiency curve. ⎛V ⎞ R2 = ⎜ OUT − 1⎟ R1 = 1000k ⎝ 0.6 ⎠ 100 95 VOUT = 2.5V VIN = 2.7V VIN 2.7V TO 4.2V 4 CIN† 2.2µF CER VIN SW 2.2µH* 3 VOUT 2.5V 22pF COUT** 10µF CER LTC3406 1 VFB RUN EFFICIENCY (%) 90 5 VIN = 3.6V 85 80 VIN = 4.2V 75 70 1M GND 65 316k 2 60 0.1 3406 F07a 1 10 100 OUTPUT CURRENT (mA) *MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML † TAIYO YUDEN LMK212BJ225MG 1000 3406 F07b Figure 7b Figure 7a U TYPICAL APPLICATIO S Single Li-Ion 1.5V/600mA Regulator for High Efficiency and Small Footprint VIN 2.7V TO 4.2V 4 CIN** 4.7µF CER VIN SW 3 2.2µH* COUT1† LTC3406-1.5 1 10µF CER RUN VOUT GND VOUT 1.5V 5 3406 TA05 2 *MURATA LQH32CN2R2M33 **TAIYO YUDEN JMK212BJ475MG † TAIYO YUDEN JMK316BJ106ML 95 VOUT = 1.5V 90 VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED VIN = 2.7V EFFICIENCY (%) 85 VIN = 4.2V 80 IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 75 70 VIN = 3.6V 20µs/DIV VOUT = 1.5V ILOAD = 0A TO 600mA 65 60 0.1 1 10 100 OUTPUT CURRENT (mA) 3406 TA07 VIN = 3.6V 20µs/DIV VOUT = 1.5V ILOAD = 200mA TO 600mA 3406 TA08 1000 3406 TA06 3406fa 13 LTC3406 LTC3406-1.5/LTC3406-1.8 U TYPICAL APPLICATIO S Single Li-Ion 1.2V/600mA Regulator for High Efficiency and Small Footprint VIN 2.7V TO 4.2V 4 CIN† 2.2µF CER VIN 3 SW 2.2µH* COUT** 10µF CER LTC3406 1 5 VFB RUN 301k GND 301k 2 3406 TA09 95 VOUT = 1.2V 90 VOUT 1.2V 22pF *MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML † TAIYO YUDEN LMK212BJ225MG VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED VIN = 2.7V EFFICIENCY (%) 85 VIN = 4.2V 80 IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 75 70 60 0.1 3406 TA11 VIN = 3.6V 20µs/DIV VOUT = 1.2V ILOAD = 0mA TO 600mA 65 1 10 100 OUTPUT CURRENT (mA) VIN = 3.6V 20µs/DIV VOUT = 1.2V ILOAD = 200mA TO 600mA 3406 TA12 1000 3406 TA10 Tiny 3.3V/600mA Buck Regulator VIN 5V 4 † CIN 4.7µF CER VIN SW 3 2.2µH* VOUT 3.3V 600mA 22pF COUT** 10µF CER LTC3406 1 VFB RUN GND 2 5 301k 66.5k 3406 TA13 *MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML † TAIYO YUDEN JMK212BJ475MG 100 VIN = 5V 95 VOUT = 3.3V VOUT 100mV/DIV AC COUPLED EFFICIENCY (%) 90 IL 500mA/DIV 85 80 ILOAD 500mA/DIV 75 70 VIN = 5V 20µs/DIV VOUT = 3.3V ILOAD = 200mA TO 600mA 65 60 0.1 10 100 1 OUTPUT CURRENT (mA) 3406 TA15 1000 3406 TA14 3406fa 14 LTC3406 LTC3406-1.5/LTC3406-1.8 U PACKAGE DESCRIPTIO S5 Package 5-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1635) 0.62 MAX 0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 3.85 MAX 2.62 REF 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193 1.90 BSC S5 TSOT-23 0302 3406fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LTC3406 LTC3406-1.5/LTC3406-1.8 U TYPICAL APPLICATIO Single Li-Ion 1.8V/600mA Regulator for Low Output Ripple and Small Footprint VIN 2.7V TO 4.2V 4 CIN** 4.7µF CER VIN 3 SW VOUT 1.8V + LTC3406-1.8 1 4.7µH* RUN 5 VOUT GND COUT1† 100µF 3406 TA01 2 *MURATA LQH32CN4R7M34 **TAIYO YUDEN CERAMIC JMK212BJ475MG † SANYO POSCAP 4TPB100M 95 VOUT = 1.8V VIN = 2.7V VIN = 3.6V EFFICIENCY (%) 85 VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED 90 80 VIN = 4.2V IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV 75 70 VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 0mA TO 600mA 65 60 0.1 1 10 100 OUTPUT CURRENT (mA) 3406 TA03 3406 TA04 VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 200mA TO 600mA 1000 3406 TA02 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1474/LTC1475 250mA (IOUT) Low Quiescent Current Step-Down DC/DC Converters VIN: 3V to 18V, Constant Off-Time, IQ = 10µA, MS8 Package LT1616 1.4MHz, 600mA Step-Down DC/DC Converter VIN: 3.6V to 25V, IQ = 1.9mA, ThinSOT Package LTC1701 1MHz, 500mA (IOUT) Step-Down DC/DC Converter VIN: 2.5V to 5.5V, Constant Off-Time, IQ = 135µA, ThinSOT Package LTC1767 1.5A, 1.25MHz Step-Down Switching Regulator VIN: 3V to 25V, IQ = 1mA, MS8/E Packages LTC1779 550kHz, 250mA (IOUT) Step-Down Switching Regulator VIN: 2.5V to 9.8V, IQ = 135µA, ThinSOT Package LTC1875 550kHz, 1.2A (IOUT) Synchronous Step-Down Regulator VIN: 2.7V to 6V, IQ = 15µA, TSSOP-16 Package LTC1877 550kHz, 600mA (IOUT) Synchronous Step-Down Regulator VIN: 2.65V to 10V, IQ = 10µA, MS8 Package LTC1878 550kHz, 600mA (IOUT) Synchronous Step-Down Regulator VIN: 2.65V to 6V, IQ = 10µA, MS8 Package LTC1879 550kHz, 1.2A (IOUT) Synchronous Step-Down Regulator VIN: 2.7V to 10V, IQ = 15µA, TSSOP-16 Package LTC3404 1.4MHz, 600mA (IOUT) Synchronous Monolithic Step-Down Regulator Up to 95% Efficiency, VIN: 2.65V to 6V, IQ = 10µA, MS8 Package LTC3405/LTC3405A LTC3405A-1.5 LTC3405A-1.8 1.5MHz, 300mA (IOUT) Synchronous Monolithic Step-Down Regulators Up to 95% Efficiency, VIN: 2.5V to 5.5V, IQ = 20µA, Fixed Output Voltages Available, ThinSOT Package LTC3406B LTC3406B-1.5 LTC3406B-1.8 1.5MHz, 600mA (IOUT) Synchronous Monolithic Step-Down Regulators Up to 95% Efficiency, with Pulse Skipping Mode Fixed Output Voltages Available, ThinSOT Package LTC3411 4MHz, 1.25A (IOUT) Synchronous Monolithic Step-Down Regulator Up to 95% Efficiency, VIN: 2.5V to 5.5V, IQ = 60µA, MS Package LTC3412 4MHz, 2.5A (IOUT) Synchronous Monolithic Step-Down Regulator Up to 95% Efficiency, VIN: 2.5V to 5.5V, IQ = 60µA, TSSOP Package 3406fa 16 Linear Technology Corporation LT/TP 0604 1K REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2002