Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 1.5MHz, 800mA Synchronous Step-Down Regulator in SOT23-5 DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ High Efficiency: Up to 96% Very Low Quiescent Current: Only 20µA During Operation 800mA Output Current 2.5V to 6.5V 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) SOT23-5 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 KB3426 is available in a low profile (1mm) SOT23-5 package. APPLICATIONS ■ The KB3426 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 1.8V and 3.3V. Supply current during operation is only 20µA and drops to )1µA in shutdown. The 2.5V to 5.5V input voltage range makes the KB3426 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. Cellular Telephones Personal Information Appliances Wireless and DSL Modems Digital Still Cameras MP3 Players Portable Instruments TYPICAL APPLICATION 95 90 4 + CIN 4.7µF Tan SW VIN 3 COUT 10µF CER KB3426-3.3 1 RUN VOUT 4.7µH 5 GND 2 VOUT 3.3V 800mA 85 EFFICIENCY (%) VIN 3.6V TO 6.5V VIN = 3.8V 80 VIN = 4.2V 75 70 65 60 0.1 Figure 1a. High Efficiency Step-Down Converter VIN = 3.6V VOUT = 3.3V 1 10 100 OUTPUT CURRENT (mA) 1000 Figure 1b. Efficiency vs Load Current 1 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage .................................. – 0.3V to 6.5V 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 PACkAGE/ORDER INFORMATION TOP VIEW SW 3 5 VFB RUN 1 GND 2 4 VIN S5 PACKAGE 5-LEAD PLASTIC SOT-23 TJMAX = 125°C, eJA = 250°C/ W, eJC = 90°C/ W Top Marking A17x A16x ORDER PART NUMBER KB3426B-3.3 TOP VIEW SW 3 Marking GND 2 Marking RUN 1 ORDER PART NUMBER KB3426-ADJ 5 VOUT Top Marking A33x KB3426B-1.8 4 VIN S5 PACKAGE 5-LEAD PLASTIC SOT-23 Top Marking A37x x: date code TJMAX = 125°C, eJA = 250°C/ W, eJC = 90°C/ W x: date code 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 KB3426 (Note 4) T A = 25°C KB3426 (Note 4) 0 °C TA ) 85°C KB3426 (Note 4) –40 °C ) TA ) 85°C ● 6VFB Reference Voltage Line Regulation VIN = 2.5V to 5.5V (Note 4) ● VOUT Regulated Output Voltage KB3426-1.8, I OUT = 100mA KB3426-3.3, I OUT = 100mA ● ● 6VOUT 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 ISW = 100mA RNFET RDS(ON) of N-Channel FET ILSW SW Leakage 2 TYP MAX UNITS ±30 nA 0.6 0.6 0.6 0.6120 0.6135 0.6150 V V V 0.04 0.4 1.800 3.300 1.854 3.366 0.04 0.4 %/V 1 1.25 A ● 0.5880 0.5865 0.5850 1.746 3.234 0.75 0.5 ● V V % 6.5 V 300 20 0.1 400 35 1 µA µA µA 1.5 210 1.8 MHz kHz 0.4 0.5 1 ISW = –100mA 0.35 0.45 1 VRUN = 0V, VSW = 0V or 5V, VIN = 5V ±0.01 ±1 µA ● 2.5 %/V 1.2 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 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 MIN 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 KB3426E 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. 0.3 TYP MAX UNITS 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: KB3426: T J = TA + (PD)(250°C/W) Note 4: The KB3426 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. TYPICAL PERFORMANCE 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 = 650mA 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 2 3 4 5 INPUT VOLTAGE (V) 60 0.1 6 EFFICIENCY (%) VIN = 3.6V 85 80 VIN = 4.2V 75 70 1.60 0.604 0.599 0.594 1 10 100 OUTPUT CURRENT (mA) 1000 1.55 1.50 1.45 1.40 0.589 65 60 0.1 VIN = 3.6V 1.65 0.609 VIN = 2.7V 1000 1.70 VIN = 3.6V 90 1 10 100 OUTPUT CURRENT (mA) Oscillator Frequency vs Temperature 0.614 VOUT = 2.5V REFERENCE VOLTAGE (V) 95 60 0.1 1000 Reference Voltage vs Temperature Efficiency vs Output Current 100 1 10 100 OUTPUT CURRENT (mA) FREQUENCY (MHz) 50 VOUT = 1.8V 0.584 –50 –25 1.35 50 25 75 0 TEMPERATURE (°C) 100 125 1.30 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 TYPICAL PERFORMANCE CHARACTERISTICS (From Figure1a Except for the Resistive Divider Resistor Values) Oscillator Frequency vs Supply Voltage 1.6 1.5 1.4 1.3 1.834 0.6 1.824 0.5 1.814 1.804 3 4 5 SUPPLY VOLTAGE (V) 1.784 0.1 0 6 0.7 VIN = 2.7V VIN = 3.6V 0.3 0.2 35 30 25 20 15 MAIN SWITCH SYNCHRONOUS SWITCH 50 25 75 0 TEMPERATURE (°C) 100 Switch Leakage vs Temperature SWITCH LEAKAGE (pA) SWITCH LEAKAGE (nA) 200 150 100 6 4 3 5 SUPPLY VOLTAGE (V) MAIN SWITCH RUN = 0V SW 5V/DIV SYNCHRONOUS SWITCH 80 60 VOUT 100mV/DIV AC COUPLED MAIN SWITCH IL 200mA/DIV 40 20 4 0 125 50 25 0 75 TEMPERATURE (°C) Burst Mode Operation VIN = 3.6V VOUT = 1.8V ILOAD = 50mA SYNCHRONOUS SWITCH 100 15 5 100 250 50 25 75 0 TEMPERATURE (°C) 20 Switch Leakage vs Input Voltage VIN = 5.5V RUN = 0V 0 –50 –25 25 0 –50 –25 120 50 30 0 2 300 0 7 35 5 125 6 10 10 0.1 5 4 2 3 INPUT VOLTAGE (V) VIN = 3.6V 45 VOUT = 1.8V = 0A I 40 LOAD VOUT = 1.8V ILOAD = 0A SUPPLY CURRENT (µA) 0.4 1 Supply Current vs Temperature 40 0.5 0 –50 –25 0 50 45 SUPPLY CURRENT (µA) VIN = 4.2V SYNCHRONOUS SWITCH 0 100 200 300 400 500 600 700 800 900 LOAD CURRENT (mA) Supply Current vs Supply Voltage 50 0.6 0.3 0.2 1.774 2 MAIN SWITCH 0.4 1.794 RDS(ON) vs Temperature RDS(ON) (1) 0.7 VIN = 3.6V RDS(ON) (1) OUTPUT VOLTAGE (V) OSCILLATOR FREQUENCY (MHz) 1.7 1.2 RDS(ON) vs Input Voltage Output Voltage vs Load Current 1.844 1.8 1 2 3 4 INPUT VOLTAGE (V) 5 6 4µs/DIV 100 125 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 TYPICAL PERFORMANCE CHARACTERISTICS (From Figure 1a Except for the Resistive Divider Resistor Values) Start-Up from Shutdown RUN 2V/DIV VOUT 2V/DIV Load Step Load Step VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED ILOAD 500mA/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 800mA 40µs/DIV IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 50mA TO 800mA VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 0mA TO 800mA Load Step 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 800mA VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 200mA TO 800mA PIN FUNCTIONS 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) (kB3426): Feedback Pin. Receives the feedback voltage from an external resistive divider across the output. VOUT (Pin 5) (kB3426-1.8/kB3426-3.3): Output Voltage Feedback Pin. An internal resistive divider divides the output voltage down for comparison to the internal reference voltage. 5 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 SIMPLIFIED BLOC DIAGRAM SLOPE COMP 0.65V OSC OSC 4 VIN FREQ SHIFT – VFB /VOUT + 5 0.6V R1 FB – + – 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 51 + ICOMP SHUTDOWN IRCMP – OPERATION 2 GND (Refer to Functional Diagram) Main Control Loop Burst Mode Operation The KB3426 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 KB3426 is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. 6 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. Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 (Refer to Functional Diagram) Short-Circuit Protection 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 KB3426 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). Low Supply Operation The KB3426 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 KB3426 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) OPERATION 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 Figure 2. Maximum Output Current vs Input Voltage 7 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION The basic KB3426 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 KB3426 requires to operate. Table 1 shows some typical surface mount inductors that work well in KB3426 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 6IL = 240mA (40% of 800mA). 6IL = £ V ¥ 1 VOUT ² 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 820mA rated inductor should be enough for most applications (700mA + 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 6IL) 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 8 Table 1. Representative Surface Mount Inductors PART NUMBER VALUE (µH) DCR (1 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. Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION 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 6VOUT is determined by: £ 1 ¥ 6VOUT 6IL ² ESR + ´ ¤ 8fC OUT ¦ where f = operating frequency, COUT = output capacitance and 6IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since 6IL 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 (kB3426 Only) In the adjustable version, the output voltage is set by a resistive divider according to the following formula: £ R2 ¥ VOUT = 0.6 V ² 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 KB3426 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 KB3426’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 Figure 3. Setting the kB3426 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. 9 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in KB3426 circuits: V IN 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 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 Thermal Considerations 1 10 100 LOAD CURRENT (mA) 1000 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. 10 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 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: In most applications the KB3426 does not dissipate much heat due to its high efficiency. But, in applications where the KB3426 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 KB3426 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)(eJA) where PD is the power dissipated by the regulator and eJA is the thermal resistance from the junction of the die to the ambient temperature. Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION The junction temperature, TJ, is given by: T J = TA + TR where TA is the ambient temperature. As an example, consider the KB3426 in dropout at an input voltage of 2.7V, a load current of 800mA 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 0.521. Therefore, power dissipated by the part is: PD = ILOAD2 • RDS(ON) = 187.2mW For the SOT-23 package, the eJA 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 (6ILOAD • ESR), where ESR is the effective series resistance of COUT. 6ILOAD 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 KB3426. 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. 11 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION 1 RUN 5 VFB KB3426 2 – GND 3 + L1 SW RUN KB3426B-1.8 COUT VOUT 1 R1 R2 4 VIN CIN 2 CFWD COUT VOUT 3 + + L1 VIN SW 4 VIN CIN BOLD LINES INDICATE HIGH CURRENT PATHS 5 GND VOUT – + VIN BOLD LINES INDICATE HIGH CURRENT PATHS Figure 5a. kB3426 Layout Diagram Figure 5b. kB3426B-1.8 Layout Diagram VIA TO GND R1 L1 PIN 1 CFWD KB3426 L1 Figure 6b. kB3426-1.8 Suggested Layout Design Example As a design example, assume the KB3426 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), 12 CIN GND Figure 6a. kB3426 Suggested Layout £ V ¥ 1 VOUT ² 1 < OUT ´ ( f)(6IL ) ¤ VIN ¦ SW COUT CIN GND L= KB3426B-1.8 VOUT SW COUT 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, 6IL = 240mA and f = 1.5MHz in equation (3) gives: L= 2.5V £ 2.5V ¥ ²1 < ´ = 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 0.21 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 0.251. In most cases, a ceramic capacitor will satisfy this requirement. Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION 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 = ² OUT < 1´ R1 = 1000k ¤ 0.6 ¦ 100 95 VOUT = 2.5V VIN = 2.7V VIN 2.7V TO 6.5V 4 + CIN 10µF Tan SW VIN 3 4.7µH VOUT 2.5V 22pF COUT** 10µF CER KB3426 1 VFB RUN GND 2 EFFICIENCY (%) 90 5 VIN = 3.6V 85 80 VIN = 4.2V 75 70 1M 65 316k 60 0.1 ** TAIYO YUDEN JMK316BJ106ML 1 10 100 OUTPUT CURRENT (mA) 1000 Figure 7b Figure 7a TYPICAL APPLICATION Single Li-Ion 1.5V/800mA Regulator for High Efficiency and Small Footprint VIN 2.7V TO 6.5V 4 + CIN 10 µF Tan SW VIN 3 KB3426B-1.5 1 RUN VOUT 5 4.7µH COUT1† 10µF CER VOUT 1.5V GND 2 † 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 800mA 65 60 0.1 1 10 100 OUTPUT CURRENT (mA) VIN = 3.6V 20µs/DIV VOUT = 1.5V ILOAD = 200mA TO 800mA 1000 13 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 TYPICAL APPLICATION Single Li-Ion 1.2V/800mA Regulator for High Efficiency and Small Footprint VIN 2.7V TO 6.5V 4 + CIN 10 µF Tan SW VIN 3 VFB RUN 5 300k 300k 2 90 COUT** 10µF CER KB3426 GND VOUT = 1.2V VOUT 1.2V 22pF 1 95 4.7µH ** TAIYO YUDEN JMK316BJ106ML 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 = 1.2V ILOAD = 0mA TO 800mA 65 60 0.1 1 10 100 OUTPUT CURRENT (mA) VIN = 3.6V 20µs/DIV VOUT = 1.2V ILOAD = 200mA TO 800mA 1000 Tiny 3.3V/800mA Buck Regulator VIN 3.6V to 6.5V 4 + CIN 4.7µF Tan SW VIN 3 4.7µH NC KB3426B-3.3 1 VFB RUN GND 2 5 VOUT 3.3V 800mA COUT** 10µF CER 0 NC ** TAIYO YUDEN JMK316BJ106ML 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 800mA 65 60 0.1 14 1 10 100 OUTPUT CURRENT (mA) 1000 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 PACAGE DESCRIPTION SOT23-5 2.9±0.2 +0.2 1.1 –0.1 1.9±0.2 (0.95) 4 1 2 0 to 0.1 0.2 MIN. +0.2 1.6 –0.1 5 0.8±0.1 2.8±0.3 (0.95) 3 0.4±0.1 +0.1 0.15 –0.05 15 Kingbor Technology Co.,Ltd KB3426 TEL:(86)0755-26508846 FAX:(86)0755-26509052 TYPICAL APPLICATION Single Li-Ion 1.8V/800mA Regulator for Low Output Ripple and Small Footprint VIN 2.7V TO 6.5V 4 + CIN 10 µF Tan SW VIN 3 KB3426-1.8 1 RUN VOUT 5 4.7µH VOUT 1.8V COUT1 47 µF GND 2 95 VOUT = 1.8V 90 VIN = 2.7V 85 EFFICIENCY (%) VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED VIN = 3.6V 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 800mA 65 60 0.1 16 1 10 100 OUTPUT CURRENT (mA) 1000 VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 200mA TO 800mA