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| Preliminary Datasheet LP78086 650mA High Efficiency Synchronous Buck with Dual Channel General Description The LP78086 are PMU, and contain a 650mA Buck DC/DC and dual channel 350mA Linear Regulator, Buck DC/DC is a constant frequency, current mode, PWM step-down converter. The device integrates a main switch and a synchronous rectifier for high efficiency. The 2.1V to 6.5V input voltage range makes the LP78086 is ideally suited for portable electronic devices that are powered from 1-cell Li-ion battery or from other power sources within the range such as cellular phones, PDAs and handy-terminals. Internal synchronous rectifier with low RDS(ON) dramatically reduces conduction loss at PWM mode. The internal synchronous switch increases efficiency while eliminate the need for an external Schottky diode.The switching ripple is easily smoothed-out by small package filtering elements due to a fixed operation frequency of 1.5MHz. This along with small TDFN-10 package provide small PCB area application. Other features include soft start, lower internal reference voltage with 2% accuracy, over temperature protection, and over current protection LDO Features 650mA Buck High Efficiency: 93% 1.5MHz Fixed-Frequency PWM Operation Adjustable Output From 0.6V to VIN Dual Channel 350mA LDO 100% Duty Cycle Low Dropout Operation Available in TDFN-10 Package Low than 1A Shutdown Current Pin Configurations Ordering Information LP78086 - F: Pb-Free Package Type QV: TDFN-10 Applications Portable Media Players/MP3 players Cellular and Smart mobile phone PDA DSC Wireless Card PIN NO 1 2 3 4 5 6 7 8 9 10 Function VDD SW GND OUT2 FB2 VINL OUT3 FB3 EN FB1 Marking Information Please see website. LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 1 of 9 Preliminary Datasheet Typical Application Circuit VIN + CIN 10uF 1 6 9 LP78086 VINL VIN EN OUT1-1.8V + Cout1 + Cout1 1uF 10uF 4.7uH R2 200K OUT2 4 R3 450K OUT2-3.3V + Cout2 10uF 2 SW LP78086 FB2 10 FB1 5 R4 100K R1 100K 3 11 OUT3 7 R5 316K 8 R5 100K OUT3-2.5V + Cout3 10uF GND PGND FB3 Figure 1. LP78086 High Efficiency Step-Down Converter Functional Pin Description Pin Number 1 2 3 4 5 6 7 8 9 10 11 Pin Name VDD SW GND OUT2 FB2 VINL OUT3 FB3 EN FB1 PGND Pin Function Chip Power Input. Pin For Switching. Ground. Output2,LDO output. Feedback2(OUT2) Input Pin, Reference voltage is 1.21V. LDO2,LDO3 Power Input. Output3,LDO output LDO Chip Enable(Active High). Feedback3(OUT3) Input Pin, Reference voltage is 1.21V. Chip Enable(Active High). Feedback1(OUT1) Input Pin, Reference voltage is 0.6V. Power Ground. LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 2 of 9 Preliminary Datasheet Function Block Diagram LP78086 Absolute Maximum Ratings Input Supply Voltage LDO Current P-Channel Switch Source Current(DC) N-Channel Switch Current(DC) Peak SW Sink and Source Current Operation Temperature Range Junction Temperature Storage Temperature Lead Temp(Soldering,10sec) ESD Rating(HBM) -0.3V to 6V 400mA 800mA 800mA 1100mA --40 to 85 125 --65 to 150 260 2KV LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 3 of 9 Preliminary Datasheet Electrical Characteristics LP78086 (VIN = 3.6V, VOUT1 = 2.5V, VREF = 0.6V, L = 2.2H, CIN= 4.7F, COUT= 10F, TA= 25C, IMAX = 600mA unless otherwise specified) Parameter Input Voltage Range Quiescent Current Shutdown Current Reference Voltage Symbol VIN IQ ISHDN VFB1 VFB2 VFB3 Adjustable Output Range VOUT VOUT VOUT Output Voltage Accuracy Fixed VOUT VOUT VOUT VOUT VOUT VIN = 2.2 to 5.5V, VOUT = 1.2V 0A < IOUT < 650mA VIN = 2.2 to 5.5V, VOUT = 1.5V 0A < IOUT < 650mA VIN = 2.2 to 5.5V, VOUT = 1.8V 0A < IOUT < 650mA VIN = 2.8 to 5.5V, VOUT = 2.5V 0A < IOUT < 650mA VIN = 3.5 to 5.5V, VOUT = 3.3V 0A < IOUT < 650mA VIN = VOUT + 0.2V to 5.5V, VIN 3.5V 0A < IOUT < 650mA VIN = VOUT + 0.4V to 5.5V, VIN 2.2V 0A < IOUT < 650mA IOUT = 0mA, VFB =0.5V IOUT = 0mA, VFB =0.7V EN = GND For DC/DC adjustable output voltage For LDO2 adjustable output voltage For LDO3 adjustable output voltage 0.588 1.18 1.18 VREF -3 -3 -3 -3 -3 -3 -3 300 IOUT = 200mA IOUT = 200mA VIN =2.2 to 5.5V VIN = 3.6V VIN = 3.6V Test Conditions Min 2.5 270 25 0.1 0.60 1.21 1.21 Typ Max 5.5 350 35 1 0.612 1.24 1.24 VIN - 0.2 +3 +3 +3 +3 +3 +3 +3 Units V uA uA V V V V % % % % % % % mA Adjustable LDO Output current PMOSFET RON NMOSFET RON P-Channel Current Limit EN Threshold EN Leakage Current ILDO 350 0.45 0.45 400 0.53 PRDS(ON) NRDS(ON) IP(LM) VEN VENL 600 0.8 -- 800 1.2 2 1000 1.5 mA V uA LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 4 of 9 Preliminary Datasheet Typical Operating Characteristics LP78086 LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 5 of 9 Preliminary Datasheet series resistance(ESR) Applications Information The basic LP78086 applicaton circuit is shown inTypical Application Circuit. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Inductor selection The output inductor is selected to limit the ripple current to some predetermined value. typically 20%~40% of the full load current at the maximum input voltage. Large value inductors lower ripple currents. Higher Vin or VOUT also increases the ripple current as shown in equation. A reasonable starting point for setting ripple current is IL=180mA(40% of 650mA). LP78086 that is required to minimize voltage ripple and load step transients, an well as the amount or bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viesing the load transient response as described in later section.the output ripple, VOUT, is determined by: 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. however care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is use at input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input ,VIN, 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. 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 (650mA+120mA). For better efficiency, choose a low DC-resistance inductor. CIN and COUT Selection The input capacitance, CIN ,is needed to filter the trapezoidal current at the source of the top MOSFET. To prevent large ripple voltage, a low ESR input capacitor Sized for the maximum RMS current should be used. RMS current is given by: Output voltage programming The output voltage is set by a resistive divider according to the Following formula: Vout=VFB X (1+R2/R1) The external resistive divider is connected to the output, allowing Remote voltage sensing as shown in figure3. 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 ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which 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 size or height requirements in the design. Efficiency considerations The selection of COUT is determined by the effective 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 Page 6 of 9 LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Preliminary Datasheet efficiency and which change would produce the most improvement efficiency can be expressed as : Efficiency= 100%- (L1+L2+L3...) Where L1L2, etc. are the individual losses as a percentage of Input power .although all dissipative elements in the for most of losses: VIN quiescent current and 12R 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. LP78086 2. 12Rlosses tae 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 LX pin is a function of both top and bottom MOSFER RDS(ON) and the duty cycle (DC) as follows: RSW=RDS(ON)TOPxDC+RDS(ON)BOTx(1-DC) The RDS(ON) for both the top and bottom MOSFETS can be obtained from the typical performance characteristics curves. thus, to obtain 12R losses, simply add RSW to RL and multiply the square of the average output 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 charge current.results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switches from high to low to high again, a packet of charge Q moves from VIN to ground. The resulting Q/t is the current out of VIN that is typically larger than the DC bias current. In continuous mode. LGATCHG=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. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% of the total loss. LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 7 of 9 Preliminary Datasheet Checking Transient Response LP78086 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. Layout Considerations Follow the PCB layout guidelines for optimal performance of LP78086. For the main current paths as indicated in bold lines, keep their traces short and wide. Put the input capacitor as close as possible to the device pins (VIN and GND). LX node is with high frequency voltage swing and should be kept small area. Keep analog components away from LX node to prevent stray capacitive noise pick-up. Connect feedback network behind the output capacitors. Keep the loop area small. Place the feedback components near the LP78086. Connect all analog grounds to a command node and then connect the command node to the power ground behind the output capacitors. LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 8 of 9 Preliminary Datasheet Packaging Information LP78086 LP78086 -00 Ver. 1.0 Datasheet Feb.-2008 Page 9 of 9 |
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