Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 High Efficiency, Low Noise, Fast Transient Dual 800mA, 2.2MHz Step-Down DC/DC Converter System-Power DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Up to 97% Efficiency 25uA No Load Current Per Channel 800mA Output Current 2.5V to 6.5V Input Voltage Range 2.2MHz 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 DFN33-10 Package ■ ■ ■ ■ ■ Switching frequency is internally set at 2.2MHz, 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 KB3511 is available in a low profile (0.8mm) DFN33-10 package. APPLICATIONS ■ The KB3511 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 25µA and drops to )1µA in shutdown. The 2.5V to 6.5V input voltage range makes the KB3511 ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. Automatic Trickle 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 KB3511 Efficiency Curve VIN = 3.6V TO 6.0V Vin>4.5V Used R1 RESET *1 VOUT2 = 3.3V AT 800mA 2 C1 4.7µF L2 2.2µH RUN1 6 R3 150k C1, C2, C3: 4.7uF 6.3V 0603 POR 8 KB3511A 4 SW1 SW2 7 VFB2 VFB1 GND 11 5 95 L1 2.2µH 3.3V 90 RESET C4, 22pF DFN3x3mm 1 C3 4.7µF R5 100k RUN2 MODE/SYNC C5, 22pF R4 680k 9 3 VIN VOUT1 = 1.8V AT 800mA EFFICIENCY (%) C6 *10µ F * VIN=3.6V 100 1.8V 85 80 75 70 10 R1 150k R2 300k C2 4.7µF 65 60 1 10 100 LOAD CURRENT (mA) 1000 L1, L2: EVERCOM SD11-2R2 (3x3x1.2mm) Figure 1. 3.3V/1.8V at 800mA Step-Down Regulators 1.3mm Height Core Supply 1 Kingbor Technology Co.,Ltd KB3511 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 ORDER PART NUMBER TOP VIEW VFB1 1 10 VFB2 RUN1 2 9 RUN2 VIN 3 SW1 4 7 SW2 GND 5 6 MODE/ SYNC 11 RUN1 2 VIN 3 SW1 4 7 SW2 GND 5 6 MODE/ SYNC 8 POR DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN 10 VOUT2 VOUT1 1 KB3511A ORDER PART NUMBER TOP VIEW VFB1 = VFB2 = 0.6V DD PART MARKING KB3511B 9 RUN2 11 8 NC VOUT1 = 1.5V VOUT2 = 2.2V DD PART MARKING DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN EXPOSED PAD IS PGND (PIN 11) MUST BE CONNECTED TO GND EXPOSED PAD IS PGND (PIN 11) MUST BE CONNECTED TO GND TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W 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 VIN Operating Voltage Range CONDITIONS ● IFB Feedback Pin Input Current ● VFB Feedback Voltage (Note 3) 0°C ≤ TA ≤ 85°C –40°C ≤ TA ≤ 85°C MIN ● TYP 2.5 0.588 0.585 V 30 nA 0.6 0.6 0.612 0.612 V V 0.5 Reference Voltage Line Regulation VIN = 2.5V to 5.5V (Note 3) 0.3 ∆VLOAD REG Output Voltage Load Regulation (Note 3) 0.5 IS Input DC Supply Current VFB1 = VFB2 = 0.6V RUN = 0V, VIN = 5.5V, MODE/SYNC = 0V 50 0.1 fOSC Oscillator Frequency fSYNC Synchronization Frequency ILIM Peak Switch Current Limit VIN = 3V, FBK = 0.5V, Duty Cycle <35% RDS(ON) Top Switch On-Resistance Bottom Switch On-Resistance ISW(LKG) Switch Leakage Current 2 VFB = 0.6V ● 1.6 2.2 %/V % 100 1 2.8 2.2 0.85 UNITS 6.5 ∆VLINE REG Active Mode Shutdown MAX µA µA MHz MHz 1 1.25 A VIN = 3.6V (Note 6) VIN = 3.6V (Note 6) 0.35 0.30 0.45 0.45 Ω Ω VIN = 5V, VRUN = 0V, VFB = 0V 0.01 1 µA Kingbor Technology Co.,Ltd KB3511 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 POR Power-On Reset Threshold VFB Ramping Up, MODE/SYNC = 0V VFB Ramping Down, MODE/SYNC = 0V MIN TYP MAX UNITS 8.5 –8.5 Power-On Reset On-Resistance % % 100 Power-On Reset Delay Ω 200 262,144 VRUN RUN Threshold High ● 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 KB3511 is guaranteed to meet specified performance from 0°C to 70°C. Specifications over the – 40°C and 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: The KB3511 is tested in a proprietary test mode that connects 0.3 Cycles 1 1.5 V 0.01 1 µA VFB to the output of the error amplifier. Note 4: Dynamic supply current is higher due to the internal gate charge being delivered at the switching frequency. Note 5: TJ is calculated from the ambient TA and power dissipation PD according to the following formula: TJ = TA + (PD • θJA). Note 6: The DFN switch on-resistance is guaranteed by correlation to wafer level measurements. TYPICAL PERFORMANCE CHARACTERISTICS Trickle Mode Operation Load Step PWM Mode SW 5V/DIV SW 5V/DIV VOUT 200mV/DIV VOUT 100mV/DIV VOUT 10mV/DIV IL 500mA/DIV IL 200mA/DIV IL 200mA/DIV ILOAD 500mA/DIV VIN = 3.6V 1µs/DIV VOUT = 1.8V ILOAD = 50mA CIRCUIT OF FIGURE 1 VIN = 3.6V 4µs/DIV VOUT = 1.8V ILOAD = 50mA CIRCUIT OF FIGURE 1 VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 50mA TO 600mA CIRCUIT OF FIGURE 1 Oscillator Frequency vs Supply Voltage Oscillator Frequency vs Temperature Efficiency vs Input Voltage 100 1.70 1.8 TA = 25°C 100mA 1mA 1.60 10mA FREQUENCY (MHz) EFFICIENCY (%) 90 85 TA = 25°C 1.65 OSCILLATOR FREQUENCY (MHz) 95 600mA 80 75 1.55 1.50 1.45 70 1.40 65 VOUT = 1.8V CIRCUIT OF FIGURE 1 60 4 5 2 3 INPUT VOLTAGE (V) 1.35 6 1.30 –50 –25 1.7 1.6 1.5 1.4 1.3 1.2 50 25 75 0 TEMPERATURE (°C) 100 125 2 3 4 5 6 SUPPLY VOLTAGE (V) 3 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature RDS(ON) vs Input Voltage 0.615 VIN = 3.6V 550 TA = 25°C 0.610 VIN = 2.7V 500 450 VIN = 4.2V 450 0.600 0.595 400 350 300 0.590 250 0.585 –50 –25 200 50 25 75 0 TEMPERATURE (°C) 100 MAIN SWITCH RDS(ON) (mΩ) 0.605 RDS(ON) (mΩ) REFERENCE VOLTAGE (V) RDS(ON) vs Temperature 500 125 SYNCHRONOUS SWITCH Efficiency vs Load Current 2 3 4 VIN (V) 5 7 6 2 VOUT ERROR (%) EFFICIENCY (%) 85 80 PWM MODE 75 70 65 VOUT = 3.3V Trickle Mode OPERATION CIRCUIT OF FIGURE 1 1 10 100 LOAD CURRENT (mA) 1 Efficiency vs Load Current 100 95 95 2.7V 10 100 LOAD CURRENT (mA) –4 1000 1 EFFICIENCY (%) 80 75 70 10 100 LOAD CURRENT (mA) 1000 Line Regulation VOUT = 1.8V IOUT = 200mA TA = 25°C 0.4 2.7V 0.3 90 4.2V VIN = 3.6V, VOUT = 1.8V NO LOAD ON OTHER CHANNEL 0.5 3.3V 90 85 PWM MODE –1 Efficiency vs Load Current 100 3.3V 0 –3 VIN = 3.6V, VOUT = 1.8V NO LOAD ON OTHER CHANNEL 60 1000 1 –2 VOUT ERROR (%) EFFICIENCY (%) 75 70 EFFICIENCY (%) 3 Trickle Mode OPERATION 90 80 25 50 75 100 125 150 TEMPERATURE (°C) 0 Load Regulation Trickle Mode OPERATION 85 MAIN SWITCH SYNCHRONOUS SWITCH 4 95 4.2V 60 250 100 –50 –25 3.6V 3.8V 65 300 200 100 90 350 Efficiency vs Load Current 100 95 400 150 1 VIN = 3.6V 4.2V 85 80 75 70 0.2 0.1 0 –0.1 –0.2 –0.3 65 VOUT = 1.2V Trickle Mode OPERATION CIRCUIT OF FIGURE 1 60 1 4 10 100 LOAD CURRENT (mA) 1000 65 VOUT = 1.5V Trickle Mode OPERATION CIRCUIT OF FIGURE 1 60 1 10 100 LOAD CURRENT (mA) 1000 –0.4 –0.5 2 3 4 VIN (V) 5 6 Kingbor Technology Co.,Ltd TEL:(86)0755-26508846 FAX:(86)0755-26509052 KB3511 PIN FUNCTIONS VFB1 (Pin 1): Output Feedback. Receives the feedback voltage from the external resistive divider across the output. Nominal voltage for this pin is 0.6V. RUN1 (Pin 2): Regulator 1 Enable. Forcing this pin to VIN enables regulator 1, while forcing it to GND causes regulator 1 to shut down. VIN (Pin 3): Main Power Supply. Must be closely decoupled to GND. SW1 (Pin 4): Regulator 1 Switch Node Connection to the Inductor. This pin swings from VIN to GND. GND (Pin 5): Main Ground. Connect to the (–) terminal of COUT, and (–) terminal of CIN. MODE/SYNC (Pin 6): Combination Mode Selection and Oscillator Synchronization. This pin controls the operation of the device. When tied to VIN or GND, Trickle Mode operation or PWM mode is selected, respectively. Do not float this pin. The oscillation frequency can be syncronized to an external oscillator applied to this pin and pulse skipping mode is automatically selected. SW2 (Pin 7): Regulator 2 Switch Node Connection to the Inductor. This pin swings from VIN to GND. POR (Pin 8): Power-On Reset . This common-drain logic output is pulled to GND when the output voltage is not within ±8.5% of regulation and goes high after 175ms when both channels are within regulation. RUN2 (Pin 9): Output Feedback. Forcing this pin to VIN enables regulator 2, while forcing it to GND causes regulator 2 to shut down. VFB2 (Pin 10): Output Feedback. Receives the feedback voltage from the external resistive divider across the output. Nominal voltage for this pin is 0.6V. Exposed Pad (GND) (Pin 11): Power Ground. Connect to the (–) terminal of COUT, and (–) terminal of CIN. Must be soldered to electrical ground on PCB. 5 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 SIMPLIFIED BLOC DIAGRAM REGULATOR 1 MODE/SYNC 6 Trickle Clamp VIN SLOPE COMP 0.6V EA VFB1 SLEEP ITH – + 5Ω ICOMP + 0.35V – 1 EN – + Trickle S Q RS LATCH R Q 0.55V – UVDET SWITCHING LOGIC AND BLANKING CIRCUIT UV + ANTI SHOOTTHRU 4 SW1 + OVDET – + 0.65V OV IRCMP SHUTDOWN – 11 GND VIN PGOOD1 8 POR 2 RUN1 0.6V REF POR COUNTER OSC,2.2MHz 9 RUN2 3 VIN OSC 5 GND PGOOD2 REGULATOR 2 (IDENTICAL TO REGULATOR 1) VFB2 10 7 SW2 OPERATION The KB3511 uses a constant frequency, current mode architecture. The operating frequency is set at 2.2MHz and can be synchronized to an external oscillator. Both channels share the same clock and run in-phase. To suit a variety of applications, the selectable Mode pin allows the user to trade-off noise for efficiency. The output voltage is set by an external divider returned to the VFB pins. An error amplfier compares the divided output voltage with a reference voltage of 0.6V and adjusts the peak inductor current accordingly. Overvoltage and undervoltage comparators will pull the POR output low if the output voltage is not within ±8.5%. The POR output will go high after 262,144 clock cycles (about 120ms) of achieving regulation. 6 Main Control Loop During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle when the VFB voltage is below the the reference voltage. The current into the inductor and the load increases until the current limit is reached. The switch turns off and energy stored in the inductor flows through the bottom switch (N-channel MOSFET) into the load until the next clock cycle. The peak inductor current is controlled by the internally compensated ITH voltage, which is the output of the error amplifier.This amplifier compares the VFB pin to the 0.6V reference. When the load current increases, the VFB voltage decreases slightly below the reference. This Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 OPERATION decrease causes the error amplifier to increase the ITH voltage until the average inductor current matches the new load current. The main control loop is shut down by pulling the RUN pin to ground. Low Current Operation Two modes are available to control the operation of the KB3511 at low currents. Both modes automatically switch from continuous operation to to the selected mode when the load current is low. To optimize efficiency, the Trickle Mode operation can be selected. When the load is relatively light, the KB3511 automatically switches into Trickle Mode operation in which the PMOS switch operates intermittently based on load demand with a fixed peak inductor current. By running cycles periodically, the switching losses which are dominated by the gate charge losses of the power MOSFETs are minimized. The main control loop is interrupted when the output voltage reaches the desired regulated value. A hysteretic voltage comparator trips when ITH is below 0.28V, shutting off the switch and reducing the power. The output capacitor and the inductor supply the power to the load until ITH/RUN exceeds 0.6V, turning on the switch and the main control loop which starts another cycle. For lower ripple noise at low currents, the pulse skipping mode can be used. In this mode, the KB3511 continues to switch at a constant frequency down to very low currents, where it will begin skipping pulses. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases to 100% which is the dropout condition. In dropout, the PMOS switch is turned on continuously with the output voltage being equal to the input voltage minus the voltage drops across the internal p-channel MOSFET and the inductor. An important design consideration is that the RDS(ON) of the P-channel switch increases with decreasing input supply voltage (See Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the KB3511 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information Section). Low Supply Operation The KB3511 incorporates an Under-Voltage Lockout circuit which shuts down the part when the input voltage drops below about 1.65V to prevent unstable operation. APPLICATIONS INFORMATION A general KB3511 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 ∆IL decreases with higher inductance and increases with higher VIN or VOUT: ⎛ V ⎞ V ∆IL = OUT ⎜ 1 OUT ⎟ fO L ⎝ VIN ⎠ Accepting larger values of ∆IL 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 ∆IL = 0.3 • ILIM, where ILIM is the peak switch current limit. The largest ripple current ∆IL occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L= VOUT ⎛ VOUT ⎞ ⎜1 ⎟ fO ∆IL ⎝ VIN(MAX) ⎠ The inductor value will also have an effect on Trickle Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the Trickle clamp. Lower inductor values result in higher ripple current which causes this to occur at lower load 7 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION currents. This causes a dip in efficiency in the upper range Table 1. Representative Surface Mount Inductors VALUE DCR MAX DC SIZE of low current operation. In Trickle Mode operation, lower PART NUMBER (µH) (Ω MAX) CURRENT (A) W × L × H (mm3) inductance values will cause the Trickle frequency to inEVERCOM 1.5 0.043 0.95 3.0 × 3.0 × 1.2 crease. 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 characterisitics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the KB3511 requires to operate. Table 1 shows some typical surface mount inductors that work well in KB3511 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: 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 – ∆IL/2. IRMS ≈ IMAX 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 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. 8 SD11 2.2 3.3 4.7 0.075 0.110 0.162 0.80 0.70 0.60 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 Output Capacitor (COUT) Selection The selection of COUT is driven by the required ESR to minimize voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple (∆VOUT) is determined by: ⎛ ⎞ 1 ∆VOUT ≈ ∆IL ⎜ ESR + 8fO C OUT ⎟⎠ ⎝ where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ∆IL increases with input voltage. With ∆IL = 0.3 • ILIM the output ripple will be less than 100mV at maximum VIN and fO = 2.2MHz with: ESRCOUT < 150mΩ Once the ESR requirements for COUT have been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement, except for an all ceramic solution. In surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirement of the application. Aluminum electrolytic, special polymer, ceramic and dry tantulum capacitors are all available in surface mount packages. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR(size) product of any aluminum electrolytic at a somewhat higher price. Special polymer Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION capacitors, such as Sanyo POSCAP, offer very low ESR, but have a lower capacitance density than other types. Tantalum capacitors have the highest capacitance density, but it has a larger ESR and 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 tantalums, available in case heights ranging from 2mm to 4mm. Aluminum electrolytic capacitors have a significantly larger ESR, and are often used in extremely costsensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have the lowest ESR and cost, but also have the lowest capacitance density, a high voltage and temperature coefficient, and exhibit audible piezoelectric effects. In addition, the high Q of ceramic capacitors along with trace inductance can lead to significant ringing. Other capacitor types include the Panasonic Special Polymer (SP) capacitors. In most cases, 0.1µF to 1µF of ceramic capacitors should also be placed close to the KB3511 in parallel with the main capacitors for high frequency decoupling. VIN = 2.5V TO 5.5V CIN Trickle* PWM* RUN2 VIN R5 RUN1 POWER-ON RESET POR MODE/SYNC KB3511ADJ L1 L2 VOUT1 SW2 SW1 C5 C4 COUT2 VFB2 VFB2 R4 VOUT2 GND R3 Also, ceramic caps are prone to temperature effects which requires the designer to check loop stability over the operating temperature range. To minimize their large temperature and voltage coefficients, only X5R or X7R ceramic capacitors should be used. A good selection of ceramic capacitors is available from Taiyo Yuden, TDK, and Murata. Great care must be taken when using only ceramic input and output capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a load step at the output can induce ringing at the VIN pin. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, the ringing at the input can be large enough to damage the part. Since the ESR of a ceramic capacitor is so low, the input and output capacitor must instead fulfill a charge storage requirement. During a load step, the output capacitor must instantaneously supply the current to support the load until the feedback loop raises the switch current enough to support the load. The time required for the feedback loop to respond is dependent on the compensation and the output capacitor size. Typically, 3-4 cycles are required to respond to a load step, but only in the first cycle does the output drop linearly. The output droop, VDROOP, is usually about 3 times the linear drop of the first cycle. Thus, a good place to start is with the output capacitor size of approximately: R2 R1 COUT1 *MODE/SYNC = 0V: PWM MODE/SYNC = VIN: Trickle Mode Figure 2. kB3511 General Schematic Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempting for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumental in giving acceptable loop phase margin. Ceramic capacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before ESR becomes effective. COUT ≈ 3 ∆IOUT fO VDROOP More capacitance may be required depending on the duty cycle and load step requirements. In most applications, the input capacitor is merely required to supply high frequency bypassing, since the impedance to the supply is very low. A 10µF ceramic capacitor is usually enough for these conditions. Setting the Output Voltage The KB3511 develops a 0.6V reference voltage between the feedback pin, VFB, and the ground as shown in Figure 2. The output voltage is set by a resistive divider according to the following formula: 9 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION VOUT ⎛ R2⎞ = 0.6V⎜ 1 + ⎟ ⎝ R1⎠ Keeping the current small (<5µA) in these resistors maximizes efficiency, but making them too small may allow stray capacitance to cause noise problems and reduce the phase margin of the error amp loop. To improve the frequency response, a feed-forward 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. 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 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. The initial output voltage step may not be within the bandwidth of the feedback loop, so the standard secondThe POR pin is an open-drain output which pulls low when order overshoot/DC ratio cannot be used to determine either regulator is out of regulation. When both output phase margin. In addition, a feed-forward capacitor, C F, voltages are within ±8.5% of regulation, a timer is started can be added to improve the high frequency response, as which releases POR after 218 clock cycles (about 120ms). shown in Figure 2. Capacitor CF provides phase lead by This delay can be significantly longer in Trickle Mode creating a high frequency zero with R2 which improves the operation with low load currents, since the clock cycles phase margin. only occur during a Trickle and there could be milliseconds of time between Trickles. This can be bypassed by tying the The output voltage settling behavior is related to the POR output to the MODE/SYNC input, to force pulse stability of the closed-loop system and will demonstrate the actual overall supply performance. For a detailed PWM mode during a reset. In addition, if the output voltage faults during Trickle Mode sleep, POR could have a explanation of optimizing the compensation components, slight delay for an undervoltage output condition and may including a review of control loop theory, refer to Applicanot respond to an overvoltage output. This can be avoided tion Note 76. by using pulse skipping mode instead. When either chan- In some applications, a more severe transient can be nel is shut down, the POR output is pulled low, since one caused by switching in loads with large (>1µF) input or both of the channels are not in regulation. capacitors. The discharged input capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No Mode Selection & Frequency Synchronization regulator can deliver enough current to prevent this probThe MODE/SYNC pin is a multipurpose pin which provides lem, if the switch connecting the load has low resistance mode selection and frequency synchronization. Connect- and is driven quickly. The solution is to limit the turn-on ing this pin to VIN enables Trickle Mode operation, which speed of the load switch driver. A Hot Swap controller is provides the best low current efficiency at the cost of a designed specifically for this purpose and usually incorpohigher output voltage ripple. When this pin is connected to rates current limiting, short-circuit protection, and softstarting. ground, PWM operation is selected which provides the lowest output ripple, at the cost of low current Efficiency Considerations efficiency. The percent efficiency of a switching regulator is equal to The KB3511 can also be synchronized to an external 2.2MHz clock signal by the MODE/SYNC pin. During the output power divided by the input power times 100%. synchronization, the mode is set to pulse skipping and the It is often useful to analyze individual losses to determine top switch turn-on is synchronized to the rising edge of the what is limiting the efficiency and which change would external clock. Power-On Reset 10 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION produce the most improvement. Percent efficiency can be expressed as: %Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, 4 main sources usually account for most of the losses in KB3511 circuits: 1)V IN quiescent current, 2) switching losses, 3) I2R losses, 4) other 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 (<0.1%) loss that increases with VIN, even at no load. 2) The switching current is the sum of the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is a current out of VIN that is typically much larger than the DC bias current. In continuous mode, IGATECHG = fO(QT + QB), where QT and QB are the gate charges of the internal top and bottom MOSFET switches. The gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 3) I2R losses are calculated from the DC resistances of the internal switches, RSW, and external inductor, RL. In continuous mode, the average output current flowing through inductor L, but is “chopped” between the internal top and bottom switches. 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: 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 Characteristics curves. Thus, to obtain I2R losses: I2R losses = IOUT2(RSW + RL) 4) Other ‘hidden’ losses such as copper trace and internal battery resistances can account for additional efficiency degradations in portable systems. It is very important to include these “system” level losses in the design of a system. The internal battery and fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency. Other losses including diode conduction losses during dead-time and inductor core losses generally account for less than 2% total additional loss. Thermal Considerations In a majority of applications, the KB3511 does not dissipate much heat due to its high efficiency. However, in applications where the KB3511 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 prevent the KB3511 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: TRISE = 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. The junction temperature, TJ, is given by: TJ = TRISE + TAMBIENT As an example, consider the case when the KB3511 is in dropout on both channels at an input voltage of 2.7V with a load current of 800mA and an ambient temperature of 70°C. From the Typical Performance Characteristics graph of Switch Resistance, the RDS(ON) resistance of the main switch is 0.425Ω. Therefore, power dissipated by each channel is: PD = I2 • RDS(ON) = 272mW The MS package junction-to-ambient thermal resistance, θJA, is 45°C/W. Therefore, the junction temperature of the 11 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION regulator operating in a 70°C ambient temperature is approximately: TJ = 2 • 0.272 • 45 + 70 = 95°C which is below the absolute maximum junction temperature of 125°C. Design Example As a design example, consider using the KB3511 in an portable application with a Li-Ion battery. The battery provides a VIN = 3.6V to 4.2V. The load requires a maximum of 600mA in active mode and 2mA in standby mode. The output voltage is VOUT = 3.3V. Since the load still needs power in standby, Trickle Mode operation is selected for good low load efficiency. First, calculate the inductor value for about 30% ripple current at maximum VIN: L= 3.3V 3.3V ⎜1 − ⎟ = 1.07µH 2.2MHz 300mA ⎝ 4.2V ⎠ Choosing the closest inductor from a vendor of 1.0µH inductor, results in a maximum ripple current of: ∆IL = ⎛ 3.3V ⎞ 3.3V ⎜ 1− ⎟ = 330mA 2.2MHz 1.0µ ⎝ 4.2V ⎠ For cost reasons, a ceramic capacitor will be used. COUT selection is then based on load step droop instead of ESR requirements. For a 5% output droop: COUT ≈ 3 600mA = 4.9µF 2.2MHz (5% 3.3V) The closest standard value is 10µF. Since the output impedance of a Li-Ion battery is very low, CIN is typically 10µF. The output voltage can now be programmed by choosing the values of R1 and R2. To maintain high efficiency, the current in these resistors should be kept small. Choosing 2µA with the 0.6V feedback voltage makes R1~300k. A close standard 1% resistor is 280k, and R2 is then 887k. The PGOOD pin is a common drain output and requires a pull-up resistor. A 100k resistor is used for adequate speed. Figure 1 shows the complete schematic for this design example. 12 Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the KB3511. These items are also illustrated graphically in the layout diagram of Figure 3. Check the following in your layout: 1. Does the capacitor CIN connect to the power VIN (Pin 3) and GND (exposed pad) as close as possible? This capacitor provides the AC current to the internal power MOSFETs and their drivers. 2. Are the COUT and L1 closely connected? The (–) plate of COUT returns current to GND and the (–) plate of CIN. 3. The resistor divider, R1 and R2, must be connected between the (+) plate of COUT and a ground sense line terminated near GND (exposed pad). The feedback signals VFB should be routed away from noisy components and traces, such as the SW line (Pins 4 and 7), and its trace should be minimized. 4. Keep sensitive components away from the SW pins. The input capacitor CIN and the resistors R1 to R4 should be routed away from the SW traces and the inductors. 5. A ground plane is preferred, but if not available, keep the signal and power grounds segregated with small signal components returning to the GND pin at one point and should not share the high current path of CIN or COUT. 6. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. These copper areas should be connected to VIN or GND. VIN CIN RUN2 VIN RUN1 MODE/SYNC POR KB3511ADJ L1 L2 VOUT1 SW1 SW2 C5 C4 VFB2 VFB1 R4 COUT2 GND R3 VOUT2 R2 R1 COUT1 BOLD LINES INDICATE HIGH CURRENT PATHS Figure 3. kB3511 Layout Diagram (See Board Layout Checklist) Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 APPLICATIONS INFORMATION TYPICAL APPLICATION Low Ripple Buck Regulators Using Ceramic Capacitors VIN = 3.6V TO 6.0V 2 C1 4.7µF 6 RESET VOUT2 = 3.3V AT 800mA RUN1 L2 2.2µH POR 8 KB3511A 4 SW1 SW2 7 RESET L1 2.2µH C4, 22pF DFN3x3mm 1 C3 4.7µF R5 100k RUN2 MODE/SYNC C5, 22pF R4 680k 9 3 VIN VFB2 VFB1 10 R2 R1 300k 150k GND R3 150k 11 5 C1, C2, C3: 4.7uF 6.3V 0603 VOUT1 = 1.8V AT 800mA C2 4.7µF L1, L2: EVERCOM SD11-2R2 (3x3x1.2mm) KB3511 Efficiency Curve VIN=3.6V 100 95 3.3V EFFICIENCY (%) 90 1.8V 85 80 75 70 65 60 VOUT 10 100 LOAD CURRENT (mA) 1 91.5% 1000 91.5% 120ms 120ms RESET Reset Timing Diagram 13 Kingbor Technology Co.,Ltd KB3511 TEL:(86)0755-26508846 FAX:(86)0755-26509052 PACAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) R = 0.115 TYP 0.38 ± 0.10 6 10 5 1 0.675 ±0.05 3.50 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 3.00 ±0.10 (4 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 5) 0.200 REF 0.50 BSC 2.38 ±0.05 (2 SIDES) (DD10) DFN 0403 0.75 ±0.05 0.00 – 0.05 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE KB WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. ALL DIMENSIONS ARE IN MILLIMETERS 3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE Kingbor Technology TEL:(86)0755-26508846 FAX:(86)0755-26509052 www.kingbor.com 4. EXPOSED PAD SHALL BE SOLDER PLATED 5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 14