? semiconductor components industries, llc, 2003 august, 2003 ? rev. 0 1 publication order number: and8126/d and8126/d NCP5008/ncp5009 compensation methods prepared by: pierre genest on semiconductor applications engineer abstract the NCP5008 and ncp5009 are high efficiency boost converters operating in current loop control mode to drive light emitting diode (led). these products are dedicated to supply any leds for any portable application. a digital programming current level allows the user to set 8 different levels of brightness intensity. the photo sense input is an innovative function: it adapts the brightness backlight in function of the ambient light in order to save the battery lifetime duration. the devices are designed to operate in open?loop configuration to enable the digital and the photo?sense brightness control. this application note describes alternate application schematics to increase the output current accuracy. introduction this manual describes several possible applications built around the NCP5008 and ncp5009. we thank you for choosing an on semiconductor part, and we hope this guide will help you to benefit from these devices' advanced features. please use the ncp5009 data sheet as a reference source. for an up?to?date document, refer to the ncp5009 data sheet which is available on our web page: http://www.onsemi.com/home. among the electrical parameters, the power supply must be capable to yield 200 ma minimum continuous dc current to operate the system properly. peak current up to 300 ma can be developed when the driver operates at full power. general description as opposed to classical boost dc?dc converter, which operates in closed loop mode, the ncp5009 is operating in open?loop mode to avoid the need for a sense resistor in series with the white leds. this feature significantly simplifies the implementation when the led network and the driver are located on separate boards because only one connection is required. the second advantage is that the current flowing through the photo sensor can be easily subtracted from the dc reference current. the external reference current is generated by connecting a resistor between the iref pin and the ground. it can be internally modified by a digital programming word or by the current flowing through the photo sensor. the modified current is used to control the inductor's current peak value. the consequence of open?loop operation, is that the current actually flowing through the led is indirectly controlled, leading to lower accuracy as opposed to closed loop operation. the main cause of variation is the power supply, because the amount of energy stored in the inductor is proportional to it. this may be an issue when the device is supplied from a nonconstant dc source, such as a battery. application note http://onsemi.com
and8126/d http://onsemi.com 2 bandgap serial to parallel latches 1:8 v ref selection por por v bat v bat vbias clk cs 50 k v bat v bat 50 k q2 por local i ref photo (see note) controller v ref v bat + a=10 iout reference 1r8 r1 isense v bat gnd + - v bat_ ok v bat gnd q1 bandgap reference bandgap v bat 4 5 3 6 1 2 10 9 8 7 v bat l1 l2 gnd figure 1.
and8126/d http://onsemi.com 3 this behavior can be explained in detail by reviewing the boost converter operation. the NCP5008 has been modelized in the following schematic by an ideal switch. v bat1 r3 0.2 2 l2 22 h 1 d1 mbr030 r31 5 0 r5 1 + + ? 0 0 0 0 s vq + ic = 0 s1 v on = 1 v v off = 0.0 v figure 2. typical boost topology converter NCP5008/9 constantly monitors the current flowing through the inductor. in order to achieve maximum efficiency, NCP5008/9 particularly controls the inductor peak current, and constantly switches the inductor charge and discharge between two boundaries ipeak and ivalley. figure 3. typical current/voltage diagrams (spice simulation) 3.75 v 2.5 v 1.25 v 0.09 v 487 ma 438 ma 375 ma 313 ma sel >> 275 ma 3.9750 v 3.9625 v 3.9500 v 2.6328 ms 2.6332 ms 3.6336 ms 2.63490 ms 2.6348 ms 2.6348 ms 2.6351 ms time phase 1 phase 2 i inductor vmiddle v(vmiddle) i(l2) v out
and8126/d http://onsemi.com 4 the basic operation of the boost converter can be divided in two phases: 1. phase 1 switch s1 is on (see figure 2). therefore, the input voltage vin is applied across the inductor l1. inductor current linearly ramps up from its original value imin (say 0 a) to the peak value ipk (when the switch turns off). during this period, energy is stored in the core material as per following equation: energy 0.5 * l * (ipk?imin) 2 (eq. 1) di dt vbat l (eq. 2) and 2. phase 2 switch s1 is off (see figure 2). since the inductor current must not be discontinuous, the inductor voltage flies back . rectifier diode d1 now turns on, and energy from the inductor is transferred to the output reservoir capacitor c1. as the inductor voltage flies back above the input voltage, the output voltage is then higher than the input voltage such as: di dt (vout?vbat) l (eq. 3) from equations 1, 2, and 3, it is obvious that overall output power (and thus average output current) is directly linked to the battery voltage vbat. consequently, any battery voltage variation will be noticeable on the output current. how to compensate battery variation NCP5008/9 derives the peak current (which determines the on time of the inductor) from the external resistor rref (between pin 1 and ground). following diagram depicts internal blocks. figure 4. v bat v bat m2 m1 i peak = k * i ref to inductor controller m3 0 r ref v ref i ref + - out 1 4 v? 8 v+ u20a 3 2 bandgap = 1.24 v i ref i peak k*i ref (eq. 4) since i ref v ref r r e f 1.24 r r e f (eq. 5) and it appears that when the battery voltage varies, the peak current threshold remains the same. hence, the output current will vary along with the battery voltage: the more vbat is high, the more current we will have on the output. compensating the battery variation means that when vbat is increasing, we need to decrease the iref (thus decreasing ipeak) and conversely, when vbat is decreasing, we need to increase iref.
and8126/d http://onsemi.com 5 figure 5. v bat v bat m2 m1 i peak = k * i ref to inductor controller m3 0 r ref v ref i ref + - out 1 4 v? 8 v+ u20a 3 2 bandgap = 1.24 v i ref i comp v bat r comp with such compensation circuitry, we now have: v ref r ref i ref v bat ?v ref r comp (eq. 6) a little of calculation and we find: i ref v bat r comp (eq. 7) negative sign means that when vbat is decreasing, compensated current is positive: that's exactly what we want. this compensation method has been applied to the ncp5009 for several output current values. experimental results open loop operation the following curves are based on experimental results at ambient temperature. a resistive divider has been used to set the reference current. it has been connected between vbat and the cs input to prevent a dc current to flow when the circuit is disabled. the open?loop operation is necessary to allow the photo?transistor dimming (ncp5009 only) and the digital control. v bat r1 r2 cs ncp5009 l1 l2 cs gnd i ref figure 6. compensation schematic
and8126/d http://onsemi.com 6 figure 7. output current with four white leds as a function of battery voltage 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2.7 3.0 4.0 4.5 5.0 3.5 battery voltage current figure 8. output current as a function of battery voltage with three leds 25 20 15 10 5 0 2.7 3.0 4.0 4.5 5.0 3.5 4.1 r1 82 k, r2 22 k r1 100 k, r2 27 k r1 162 k, r2 44 k battery voltage led current r1 100 k, r2 24 k r1 130 k, r2 30 k r1 160 k, r2 39 k r1 187 k, r2 53.6 k the ratio between r1 and r2 can be modified to reduce the variation on a given battery voltage range. the chosen values are corresponding to 2.7 v to 4.1 v range.
and8126/d http://onsemi.com 7 closed loop operation when high accuracy is requested, the compensation method previously described, is not sufficient to reduce the current variations. the iref pin is a true current input. this means it is equivalent to a voltage generator having zero output resistor. an additional transistor has been used to increase the open?loop gain, and reduce the error voltage. v bat ncp5009 l1 l2 i ref figure 9. closed loop compensation 22 h 1 f i1 r sense 1 f 1.24 v bc557c 22 k 47 k figure 10. 10.8 10.6 10.4 10.2 10 9.8 9.6 2.7 3 3.5 4 4.5 5 5.5 battery voltage led current led current adjust is quite straightforward. it is only imposed by the sense resistor value. sixty?eight ohm resistor has been used to get the above curve. the 47 k resistor prevents high current to flow from iref pin to ground at circuit start?up. this would results in very high peak current and over?heating. the bc557c pnp transistor is recommended because it exhibits high beta for collector current values.
and8126/d http://onsemi.com 8 on semiconductor and are registered trademarks of semiconductor components industries, llc (scillc). scillc reserves the right to mak e changes without further notice to any products herein. scillc makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does scillc assume any liability arising out of the application or use of any product or circuit, and s pecifically disclaims any and all liability, including without limitation special, consequential or incidental damages. atypicalo parameters which may be provided in scillc data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. all operating parameters, including atypicalso must be validated for each customer application by customer's technical experts. scillc does not convey any license under its patent rights nor the rights of others. scillc products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body , or other applications intended to support or sustain life, or for any other application in which the failure of the scillc product could create a sit uation where personal injury or death may occur. should buyer purchase or use scillc products for any such unintended or unauthorized application, buyer shall indem nify and hold scillc and its of ficers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and re asonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized u se, even if such claim alleges that scillc was negligent regarding the design or manufacture of the part. scillc is an equal opportunity/affirmative action employ er. publication ordering information japan : on semiconductor, japan customer focus center 2?9?1 kamimeguro, meguro?ku, tokyo, japan 153?0051 phone : 81?3?5773?3850 on semiconductor website : http://onsemi.com for additional information, please contact your local sales representative. and8126/d literature fulfillment : literature distribution center for on semiconductor p.o. box 5163, denver, colorado 80217 usa phone : 303?675?2175 or 800?344?3860 toll free usa/canada fax : 303?675?2176 or 800?344?3867 toll free usa/canada email : orderlit@onsemi.com n. american technical support : 800?282?9855 toll free usa/canada
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