july 2000 ML4863 high efficiency flyback controller block diagram general description the ML4863 is a flyback controller designed for use in multi-cell battery powered systems such as pdas and notebook computers. the flyback topology is ideal for systems where the battery voltage can be either above or below the output voltage, and where multiple output voltages are required. the ML4863 uses the output voltage as the feedback control signal to the current mode variable frequency flyback controller. in addition, a synchronous rectifier control output is supplied to provide the highest possible conversion efficiency (greater than 85% efficiency over a 1ma to 1a load range). the ML4863 has been designed to operate with a minimum number of external components to optimize space and cost. features n variable frequency current mode control and synchronous rectification for high efficiency n minimum external components n guaranteed start-up and operation over a wide input voltage range (3.15v to 15v) n high frequency operation (>200khz) minimizes the size of the magnetics n flyback topology allows multiple outputs in addition to the regulated 5v n built-in overvoltage and current limit protection switching control blanking cross-conduction protection v fb i 4.5v ldo rectifier comparator current comparator r gm 18mv C + v ref 3 shdn 5 v cc 8 gnd 6 out 1 7 out 2 2 sense 1 v in v fb v cc 4 v fb bias & uvlo + C comp C + comp 18mv v cc v cc a2 a1 featuring extended commercial temperature range -20?c to 70?c for portable handheld equipment rev. 1.0 10/12/2000
ML4863 2 rev. 1.0 10/12/2000 pin configuration pin name function 1v in battery input voltage 2 sense secondary side current sense 3 shdn pulling this pin high initiates a shutdown mode to minimize battery drain 4v fb feedback input from transformer secondary, and supply voltage when v out > 4.5v pin description pin name function 5v cc internal power supply node for connection of a bypass capacitor 6 out 1 flyback primary switch mosfet driver output 7 out 2 flyback synchronous rectifier mosfet driver output 8 gnd analog signal ground ML4863 8-pin soic (s08) 1 2 3 4 8 7 6 5 v in sense shdn v fb gnd out 2 out 1 v cc top view
ML4863 rev. 1.0 10/12/2000 3 absolute maximum ratings absolute maximum ratings are those values beyond which the device could be permanently damaged. absolute maximum ratings are stress ratings only and functional device operation is not implied. v in ................................................................. gnd C 0.3v to 18v voltage on any other pin ........................... gnd C 0.3v to 7v source or sink current (out1 & out2) ...................... 1a junction temperature .............................................. 150oc storage temperature range...................... C65oc to 150oc electrical characteristics unless otherwise specified, v in = 12v, t a = operating temperature range (note 1) symbol parameter conditions min typ max units oscillator t on on time c suffix 2.1 2.5 2.8 s e/i suffix 2.1 2.5 2.95 s minimum off time v fb = 0v 450 650 850 ns v fb regulation total variation line, load, & temp 4.85 5 5.15 v output drivers out1 rise time c load = 3nf, 20% to 90% of v cc 60 70 ns out1 fall time c load = 3nf, 90% to 20% of v cc 60 70 ns out2 rise time c load = 3nf, 20% to 90% of v cc 60 70 ns out2 fall time continuous mode, c load = 3nf, 90% to 20% of v cc 60 70 ns discontinuous mode, c load = 3nf, 90% to 20% of v cc 125 150 ns shdn input high voltage 2.0 v input low voltage 0.8 v input bias current shdn = 5v 5 10 a sense sense threshold C full load v in = 5v, v fb = v fb (no load) C 100mv 130 150 160 mv sense threshold C short circuit v fb = 0v 235 mv circuit protection undervoltage lockout start-up threshold 3.0 3.15 v undervoltage lockout hysteresis 0.5 0.6 v lead temperature (soldering 10 sec.) ..................... 260oc thermal resistance ( q ja ) .................................... 160oc/w operating conditions temperature range ML4863cs ................................................. 0oc to 70oc ML4863es ............................................. C20oc to 70oc ML4863is .............................................. C40oc to 85oc v in operating range ................................... 3.15v to 15v
ML4863 4 rev. 1.0 10/12/2000 electrical characteristics (continued) symbol parameter conditions min typ max units supply i fb v fb quiescent current 100 150 a i in v in shutdown current shdn = 5v 20 25 a shdn = 5v, v in < 6v 5 10 a v cc v cc output voltage v fb = 0v, v in = 15v, c vcc = 0.1f 4.5 5.5 v v fb = 0v, v in = 6v, c vcc = 0.1f 4.0 5.0 v v fb = 0v, v in = 3.15v, c vcc = 0.1f 2.8 v v fb = 5v 4.5 5 5.15 v note 1: limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
ML4863 rev. 1.0 10/12/2000 5 functional description the ML4863 utilizes a flyback topology with constant on- time control. the circuit determines the length of the off- time by waiting for the inductor current to drop to a level determined by the feedback voltage (v fb ). consequently, the current programming is somewhat unconventional because the valley of the current ripple is programmed instead of the peak. the controller automatically enters burst mode when the programmed current falls below zero. constant on-time control therefore features a transition into and out of burst mode which does not require additional control circuitry. the control circuit is made up of four distinctive blocks; the constant on-time oscillator, the current programming comparator, the feedback transconductance amplifier, and the synchronous rectifier controller. a simplified circuit diagram is shown in figure 1. oscillator & comparator the oscillator has a constant on-time and a minimum off- time. the off-time is extended as long as the output of the current programming comparator is low. note that in constant on-time control, a discharge (off-time) cycle is needed for the inductor current to be sensed. the minimum off-time is required to account for the finite circuit delays in sensing the inductor output current. transconductance amplifier the feedback transconductance amplifier generates a current from the voltage difference between the output and the reference. this current produces a voltage across r gm that adds to the negative voltage on the current sense resistor, r sense . when the current level in the inductor drops low enough to cause the voltage at the non-inverting input of the current programming comparator to go positive, the comparator trips and the converter starts a new on-cycle. the current programming comparator controls the length of the off-time by waiting until the current in the secondary decreases to the value specified by the feedback transconductance amplifier. in this way, the feedback transconductance amplifiers output current steers the current level in the inductor. when the output voltage drops due to a load increase, it will increase the output current of the feedback amplifier and generate a larger voltage across r gm which in turn raises the secondary current trip level. however, when the output voltage is too high, the feedback amplifiers output current will eventually become negative. because the output current of the inductor can never go negative by virtue of the diode, the non-inverting input of the comparator will also stay negative. this causes the converter to stop operation until the output voltage drops enough to increase the output current of the feedback transconductance amplifier above zero. figure 1. schematic of the ML4863 controller and power stage load r p c p feedback transconductance amplifier r gm current programming comparator one shot t on 2.5s one shot constant on-time minimum off-time oscillator v in l p 1:1 c r esr v out r sense blanking a2 ML4863 rectifier comparator i s sense out 2 out 1 C + comp + C comp + C v ref v fb t off 500ns 4 7 6 2
ML4863 6 rev. 1.0 10/12/2000 synchronous rectifier control the control circuitry for the synchronous rectifier does not influence the operation of the main controller. the synchronous rectifier is turned on during the minimum off time, or whenever the sense pin is less than C18mv. during transitions where the primary switch is turned on before the voltage on the sense pin goes above C18mv, the gate of the synchronous rectifier is discharged softly to avoid accidently triggering the current-mode comparator with the gate discharge spike on the sense resistor. the part will also operate with a schottky diode in place of the synchronous rectifier, but the conversion efficiency will suffer. current limit and modes of operation the normal operating range and current limit point are determined by the current programming comparator. they are dependent on the value of the synchronous rectifier current sense resistor (r sense ), the nominal transformer primary inductance (l p ), and the input voltage. r sense can be calculated by: r v vv mv i v vi sense in min out in out max in min in max out max = + + f h g i k j af af af af af 150 20 h (1) functional description (continued) where h = converter efficiency. once r sense has been determined, l p can be found: lvr p in max sense = - () 25 10 6 af (2) three operational modes are defined by the voltage at the sense pin at the end of the off-time: discontinuous mode, continuous mode, and current limit. the following values can be used to determine the current levels of each mode: v sense < 0v: discontinuous mode 0v < v sense < 160mv: continuous mode 160mv < v sense < 235mv: current limit inserting the maximum value of v sense for each operational mode into the following equation will determine the maximum current levels for each operational mode: i v vv v r tv l out in out in sense sense on in p = + + f h g i k j 2 h (3)
ML4863 rev. 1.0 10/12/2000 7 design considerations design procedure a typical design can be implemented by using the following procedure. 1. specify the application by defining: the maximum input voltage (v in(max) ) the mainimum input voltage (v in(min) ) the maximum output current (i out(max) ) the maximum output ripple ( d v out ) as a general design rule, the output ripple should be kept below 100mv to ensure stability. 2. select a sense resistor, r sense , using equation 1. 3a. determine the inductance required for the optimum output ripple using equation 2. 3b. determine the minimum inductor current rating required. the peak inductor current is calculated using the following formula: i mv r v l lpeak sense in max p =+ - 235 25 10 6 () (. ) (4) 3c. specify the inductor's dc winding resistance. a good rule of thumb is to allow 5m w , or less, of resistance per h of inductance. for minimum core loss, choose a high frequency core material such as kool-mu, ferrite, or mpp. 3d. specify the coupled inductor's turns ratio: np : ns = 1:1 4a. calculate the minimum output capacitance required using: ci vv vv out max out in max out out = + f h g i k j - () () . 25 10 6 d (5) 4b. establish the maximum allowable esr for the ouput capacitor: r vr mv esr out sense < d 150 (6) 5. as a final design check, evaluate the system stability (no compensation, single pole response) by using the following equation: d v rvv l out sense out in min p + l n m o q p - () () () 610 6 (7) where r sense and l p are the actual values to be used. see table 1 for suggested component manufacturers. table 1. component suppliers part component manufacturer number phone sense dale lrc series (402) 563-6506 resistors irc wsl series (512) 992-7900 inductors coilcraft r4999 (847) 639-6400 coiltronics octa-pac series (561)241-7876 dale lpe-6562 series (605) 665-9301 lpt-4545 series capacitors avx tps series (207) 282-5111 sprague 593d series (207) 324-4140 mosfets national nds94xx (800) 272-9954 nds99xx motorola mmdf series (602) 897-5056 mmsf series siliconix littlefoot series (408) 988-8000 design example 1. specify the application by defining: v in(max) = 6v v in(min) = 4v i out(max) = 500ma d v out = 100mv 2. select the sense resistor, r sense , using equation 1: r mv ma v sense = + + f h g i k j 4 54 150 500 4 20605 085 . . (1a) r sense = 138m w @ 120m w 3a. determine the inductance required using equation 2. l p = - (). 25 10 6 0 12 6 = 18h (2a) 3b. determine the minimum inductor current rating required. i mv m a lpeak =+ = - 235 120 62510 18 10 279 6 w (. ) . C6 (4a)
ML4863 8 rev. 1.0 10/12/2000 figure 3. 5v, 2a circuit figure 2. 5v, 1a circuit 47f v in nds9955 1f coiltronics ctx20-4 100m w v out 5v, 1a ML4863 v in sense shdn v fb gnd out 2 out 1 v cc 400f 100f v in nds9410 nds9410 1f dale lpe6562 50m w v out 5v, 2a ML4863 v in sense shdn v fb gnd out 2 out 1 v cc 800f design considerations (continued) 3c. specify the inductors dc winding resistance: l dcr = 90m w 3d. specify the coupled inductor's turn ratio: np : ns = 1:1 4a. calculate the minimum output capacitance required using equation 5. c = + f h g i k j - 050 56 5 25 10 01 6 . . . = 55f (5a) 4b. establish the maximum esr for the output capacitor using equation 6. r mv esr < 01 012 150 .. = 80m w (6a) based on these calculations, the design should use two 100f capacitors, with an esr of 100m w each, in parallel to meet the capacitance and esr requirements. 5. as a final design check, evaluate the system stability using equation 7. 100 6 10 012 5 4 18 10 6 mv + l n m o q p - () .( ) C6 = 360mv (7a) since the inequality is met, the circuit should be stable. some t ypical application circuits are shown in figures 2, 3, and 4. layout good pc board layout practices will ensure the proper operation of the ML4863. important layout considerations follow: ? the connection from the current sense resistor to the sense pin of the ML4863 should be made by a separate trace and connected right at the sense resistor lead. ? the v cc bypass capacitor needs to be located close to the ML4863 for adequate filtering of the ic's internal bias voltage. ? trace lengths from the capacitors to the inductor, and from the inductor to the fet should be as short as possible to minimize noise and ground bounce. ? power and ground planes must be large enough to handle the current the converter has been designed for. see figure 5 for a sample pc board layout.
ML4863 rev. 1.0 10/12/2000 9 figure 4. 5w multiple output dc-dc converter shdn v in c1 33f 20v c2 33f 20v c3 1f 50v nds9955 r3 60m w 5v q2a q2b 1,5 6,10 423 798 mmdf3n03 3.3v c4 33f 20v c5 33f 20v 12v t1 dale lpe-6562-a145 ML4863 v in sense shdn v fb gnd out 2 out 1 v cc r2 30m w r1 120m w q1a q1b c8 100f 6.3v c9 100f 6.3v c7 100f 6.3v c6 100f 6.3v c12 100f 6.3v c13 100f 6.3v c11 100f 6.3v c10 100f 6.3v figure 5. typical pc board layout
ML4863 10 rev. 1.0 10/12/2000 ordering information part number temperature range package ML4863cs 0oc to 70oc 8-pin soic (s08) ML4863es C20oc to 70oc 8-pin soic (s08) ML4863is (obsolete) C40oc to 85oc 8-pin soic (s08) physical dimensions inches (millimeters) seating plane 0.148 - 0.158 (3.76 - 4.01) pin 1 id 0.228 - 0.244 (5.79 - 6.20) 0.189 - 0.199 (4.80 - 5.06) 0.012 - 0.020 (0.30 - 0.51) 0.050 bsc (1.27 bsc) 0.015 - 0.035 (0.38 - 0.89) 0.059 - 0.069 (1.49 - 1.75) 0.004 - 0.010 (0.10 - 0.26) 0.055 - 0.061 (1.40 - 1.55) 8 0.006 - 0.010 (0.15 - 0.26) 0o - 8o 1 0.017 - 0.027 (0.43 - 0.69) (4 places) package: s08 8-pin soic life support policy fairchild? products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of fairchild semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. 2. a critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com ?2000 fairchild semiconductor corporation disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function or design. fairchild does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights, nor the rights of others.
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