www.fairchildsemi.com ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 7/30/09 an-8023 negative voltage management using a FAN8303 buck regulator abstract FAN8303 is a 2a, 370khz monolithic integrated buck regulator with internal power mosfets. it is simple to use and needs minimal external components. this application note describes how to generate negative voltage using FAN8303. it introduces application examples and discusses optimized designs for a buck-boost circuit. introduction buck regulators are widely used for higher voltage to lower voltage dc conversion. likewise, FAN8303 was originally designed for application needing regulated dc voltage, such as set-top box microcontrollers and efficient pre-regulators for linear regulators in pc monitor and tv applications. in some cases, a non-synchronous buck regulator also can be utilized for buck-boost circuit to generate negative voltage with respect to ground. these applications include audio amplifier, timing control circuit for lcd panel, and so on. figure 1 shows a practical application of an lcd panel; it needs negative voltage for contrast control. in this block diagram, a charge pump is usually adopted due to the simple design and low cost. however, it has an amount of power dissipation and poor output voltage regulation relative to input voltage variation. FAN8303 with negative output would be a solution to overcome these problems. av board soc row drivers column drivers tft lcd panel p-gamma ic ddr2 eeprom 12v -5v , 15v ldo dc/dc (buck) dc/dc (boost) charge pump 2.5v, 3.3v 2.5v 16v figure 1. example of timing control block
an-8023 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 7/30/09 2 principle of operation to understand buck-boost topology, buck topolgy is briefly compared below. when the mosfet switch (q1 in figure 3) is turned on, the voltage across inductor (v l ) is v in -v out . during q1 off-time, v l is equal to -v out in buck topology. so the inductor current (i l ) ramps up with (v in -v out )/l and ramps down with v out /l slope. thus, the energy can be transferred to the load with positive output voltage. meanwhile, in buck-boost topology, the inductor and freewheeling diode switch positions. when the mosfet switch q1 (figure 2) is turned on, v l is same as v in , so i l ramps up with v in /l. during the q1 off-time, v l has reverse polarity to maintain continuous inductor current with -v out . therefore, it can generate negative output voltage. buck-boost circuit with buck regulators require several design considerations. table 1 summarizes the design parameter comparison between buck and buck-boost circuit. v in c in c out load v o pwm l out q1 d1 v in c in c out load vo pwm l out d1 q1 1 q i 1 d i l i ? l i l v in v out v t out v ? q1 on q1 off 1 q i 1 d i l i ? l i l v in ou t vv ? ou t v t out v ? q1 on q1 off figure 2. buck-boost topology figure 3. buck topology table 1. buck and buck-boost design parameters first of all, inductor current is limited by (1? d ); so attention is needed to see that the maximum output current of buck regulator is be always lower than the maximum current in buck-boost circuit. second, the switch node is a sum of input voltage and output voltage in buck-boost. it also needs to be limited to the maximum switch node voltage of buck regulator. since buck-boost is very noisy on input and output compared to buck circuit, it requires good-quality mlcc as input and output filters. topology i l (average) maximum v sw duty cycles buck-boost 1 out i d ? in out vv + out in out v vv + buck out i in v out in v v
an-8023 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 7/30/09 3 design considerations inductor selection when choosing inductor, the main concerns are inductance value, rms current rating, and dcr. inductance value is usually adopted higher than the minimum inductance to operate continuous current mode (ccm). rms current should be higher than the inductor current to prevent inductor saturation without core loss. a low-dcr inductor is usually adopted when a power system needs high efficiency. to operate in continuous current mode, critical minimum inductance is calculated by: l sw in i f d v l = (1) where: in out out v v v d + = = duty cycle; f sw = switching frequency; and i l = ripple current to maintain continuous current mode (typically 20%~30% of i l ). output capacitor an output capacitor is needed to satisfy the output voltage ripple requirement and to maintain constant output voltage during dynamic load condition. ripple voltage depends on esr, output capacitance, and esl. to obtain the desired output ripple, the below equation for required minimum capacitance is useful: out sw max outmax min v f d i c = (2) where: d max = maximum duty cycle; i outmax = maximum output current; and v out = desired output voltage ripple. the equation for required esr is: lmax out i v esr = (3) input capacitor the input capacitor should handle the maximum input rms current, so use the equations below for calculation. good estimation is given by 10f or 22f per amp with mlcc. maximum rms input current: () ( ) d 1 d i i outmax max _ rms ? = (4) required minimum capacitance: ( ) ( ) in sw rms min v f / d i c = (5) where v in is desired input voltage ripple. freewheeling diode the freewheeling diode acts as a inductor current path when the switch is turned off. breakdown voltage, lower forward drop voltage, and the maximum current rating are considered for low power dissipation. a schottky diode is preferred, which has low forward voltage drop. required diode current rating: lmax i > (6) where i lmax is maximum inductor current. required breakdown voltage: out in v v + > (7)
an-8023 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 7/30/09 4 design example a design example with test conditions v in =12v, v out = -5v, i out = 1a, and f sw =370 khz (fixed) is shown below. the first step is to set the cr itical design parameters, such as inductor ripple current ( ? i l ) and desired output ripple voltage ( ? v out ). the second step is calculation of duty cycle. to achieve accurate value, consider the forward voltage drop of diode and mosfet switch on drop voltage. fairchild FAN8303, non-synchronous buck regulator has integrated 0.22 ? n-channel mosfet, so on drop voltage is about 0.4v. forward voltage of the schottky diode (40v rrm / 2a i out ) is 0.45v. when it comes to the inductor, a higher value than calculated is recommended and a low dcr inductor is preferred: table 2. design example calculations duty cycle: = (|v out |+ v f ) / (v in +|v out |+v f -v q1 ) 0.33 inductance: = (v in d)/ (f sw ? i l ) 35.6h (desired ? i l = 20%) output capacitance: = (i out d) / (f sw ? v out ) 86.8f (desired ? v out = 10mv) input capacitance: i rms = () d 1 d i out ? 0.47a c in = i rms d/ ( ? v in f sw ) 4.05f diode current rating: i diode_max = i avg + ? i l /2 where i avg = average inductor current 1.77a input 12v c in2 10f c out 22f x 4ea load output -5v / 1a pwm l 1 39h d1 q1 c ss 10nf c c 1nf r c 40k ss comp vin vsw boot gnd c bs 10nf gnd c in 10f fb r 2 2.45k r 3 18k figure 4. buck-boost schematic using FAN8303
an-8023 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 7/30/09 5 typical waveforms & graphs figure 5 and figure 6 show the typical waveforms of the FAN8303 output ripple voltage. to achieve low ripple voltage, lower than 10m ? mlcc is used. figure 7 shows FAN8303 efficiency and power-loss graph. it indicates a maximum of 87% efficiency with 0.31w at 400ma load condition. figure 5. v out ripple (1s/div), 33mv at 100ma figure 6. v out ripple (1s/div), 89mv at 1a note: 1. test conditions: v in =12v, v out = -5v, f sw = fixed 370 khz, and i out = 0~1a. efficiency and power loss 70 72 74 76 78 80 82 84 86 88 0 200 400 600 800 1000 load (ma) efficiency (%) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 power loss (w) efficiency power loss figure 7. efficiency and power loss v out , 50mv/div i out , 500ma/div v s w , 5v/div v out , 50mv/div i out , 500ma/div v s w , 5v/div
an-8023 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 7/30/09 6 conclusion fairchild 2a monolithic and non-synchronous buck regulator, FAN8303, has wide input range (~23v) with excellent load and line regulation. in spite of buck regulator, FAN8303 also can be utilized for buck-boost circuit to generate negative output voltage with simple changes of passive element. author dseom application engineer, sgyoon application engineer related datasheets FAN8303 ? 2a 23v non-synchronous step-down dc/dc regulator 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. life support policy fairchild?s 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, or (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 significant injury to the user. 2. a critical component is 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.
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