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1 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. 21v, 4 a , 500khz synchronous pwm - buck dc/dc converter pin assignment s sp package (sop - 8 expose pad ) fig ure 1. pin assignment of FR9808 ordering information de s cription FR9808 is a high - efficiency synchronous step - down dc/dc converter that employs a special process technique to obtain very low r ds (on) for the internal metal C oxide C semiconductor field - effect transistor (mosfet) . the input operation voltage is in a wide 4.75v to 21v , and continuous load current capability is 4a . control circuit is design ed by a particular current mode which provides fast transient response and eases loop stabilization. this product has a very low standby current less than 1a in shutdown mode. when the en/sync pin voltage is less than 0.4v, FR9808 will turn off. fault protection includes over current protection (ocp), under voltage lockout protection (uvlo) and o ver t e mperature p rotection (otp) function . this high - efficiency current mode step - down green power converter offers the standard so p - 8 package with an exposed pad. feature s high e fficiency up to 90% internal mosfet r ds (on): 120m/20m i nternal c ompen sation input operation v oltage range : 4.75v to 21v 4 a continuous o utput c urrent output v oltage down to 0.80 5 v 500 khz oscillation f requenc y sync to e xternal c lock from 300 khz to 800 khz cycle - by - cycle c urrent l imit under voltage lockout over - temperature protection w ith auto recovery <1a s hutdown c urrent thermal enhanced sop - 8 (exposed pad) package rohs compliant application s networking equipment olpc, netbook distributed power system lcd monitor , tv , stb external hdd se curity system fr9 808 g : green tr: tape / re e l package type s p : sop - 8 (exposed pad) s w g n d s w v i n e n / s y n c f b v c c b s 2 3 4 6 5 7 8 9 g n d 1
2 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. typical application circuit figure 2. output 1.2 v applicati on circuit figure 3 . high input voltage application circuit f r 9 8 0 8 5 e n / s y n c g n d s w 2 , 3 v o u t v i n 4 . 7 5 v t o 1 8 v f b 6 l 1 1 . 8 h 8 , 9 7 v c c c 1 2 2 f / 2 5 v c e r a m i c r 1 4 . 9 9 k ? / 1 % r 2 1 0 k ? / 1 % c 4 0 . 1 f 4 b s c 3 0 . 1 f c 2 4 7 f / 6 . 3 v c e r a m i c r 3 1 0 0 k ? 1 . 2 v 1 v i n c 6 ( o p t i o n a l ) f r 9 8 0 8 5 e n / s y n c g n d s w 2 , 3 v o u t v i n 1 8 v t o 2 1 v f b 6 l 1 1 . 8 h 8 , 9 7 v c c c 1 2 2 f / 2 5 v c e r a m i c r 1 4 . 9 9 k ? / 1 % r 2 1 0 k ? / 1 % c 4 0 . 1 f 4 b s c 3 0 . 1 f c 2 4 7 f / 6 . 3 v c e r a m i c r 3 1 0 0 k ? 1 . 2 v 1 v i n 3 3 0 f / 2 5 v e c x 1 c 5 c 6 ( o p t i o n a l )
3 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. functional pin description i/o pin name pin no. pin function i fb 6 voltage feedback input pin. fb and vout are connected by a resistive voltage divider. thi s ic senses feedback voltage via fb and regulates it a t 805 mv. i vin 1 power supply input pin. drive 4.75v to 21v voltage to this pin to power on this chip. a 22f ceramic bypass capacitor is connected between vin and gnd to eliminate noise. o vcc 7 b ias supply output pin. a 0.1f capacitor must be connected from this pin to gnd. i en/sync 5 this pin provides a digital control to turn the converter on or off. for automatic start - up, connect the en pin to vin pin with a 100k resistor. an external clock from 300khz to 800 khz can be applied to the en pin to change o scillation frequency. o sw 2,3 power switching output pin. this is the output pin that internal high - side nmos switches to supply power. o bs 4 high - side gate - drive boost input. a 0.1f capacitor is connected from this pin to sw . it can boost the gate drive to fully turn on the internal high - side nmos. i gnd 8 ground pin. this pin is connected to the exposed pad with copper. i exposed pad 9 ground pin. the exposed pad must be soldered to a large pcb area and connected to gnd for maximum power dissipation. block diagram figure 4 . block diagram of FR9808 + + - o s c i l l a t o r 1 5 0 k h z / 5 0 0 k h z v i n e n / s y n c b s s w 5 v c u r r e n t s e n s e a m p l i f i e r h i g h - s i d e m o s f e t l o w - s i d e m o s f e t 1 m c u r r e n t l i m i t c o m p a r a t o r + - c l k v c c r e g u l a t o r v c c f b c o n t r o l l o g i c p w m c o m p a r a t o r c u r r e n t l i m i t l o w - s i d e r c o m p c c o m p v r e f 2 + - - e r r o r a m p l i f i e r + v r e f 1 e n / s y n c g n d + -
4 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. absolute maximum ratings input supply voltage v in ------ - --------------------- -- --------------- --------------------------- -- ----- - - 0.3v to + 2 3 v sw voltage v sw -------------- - ------- -- ---------------------------------------------------------------- - -- - 0.3v to v in +1v b oos t voltage v bs ------------ - ----------------- -- ------------------------------------------------- ------ - v sw - 0.3v to v sw + 6v all other pins voltage ---------------- - ---------------------------------------------------------------- - - - 0.3v to + 6v maximu m junction temperature (t j ) ------------ - --------------------------------------------------- +150 c storage temperature (t s ) --------------------- - -------------------------------------------------------- - 65 c to +150 c lead temperature (soldering, 1 0sec.) -------- - ----------------------------------------------------- +260c power dissipation @t a =25 c , (p d ) sop - 8 (exposed pad ) ------- - ------------------------------------------------------------ - - 2.08w package thermal resistance, ( ja ): s op - 8 exposed pad ----------- - ------------------------------------------------------------ - 6 0 c /w package thermal resistance, ( jc ): sop - 8 exposed pad --------- - -------------------------------------------------------------- - 1 5 c /w note 1 stresses beyond those listed under absolute maximum ratings" may cause permanent damage to the device. recommended operating conditions input supply voltage (v in ) - --- ----- - ------------------------------------------------ --------------- - -- -- + 4. 7 5 v to + 21 v op eration temperature range ------ - ----------------------------------- -------------------------- - - - - - 4 0c to +85c
5 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. electrical characteristics (v in = 12 v, t a =25 c , unless otherwise specified.) parameter symbol conditions min typ max unit input supply voltag e v in 4.75 21 v vin shutdown supply current i sd v en = 0v 1 a ddq v en = 2 v, v fb = 1 v 1.1 ma feedback voltage v fb 4.75v Q in Q ds (on) (note 2 ) hsr ds (on) 120 m ds (on) (note 2 ) lsr ds (on) 20 m sw (l eak) v en = 0v, v sw = 0v 0 10 a limit 6 .5 a maximum duty cycle d max v fb = 0.7 v 90 % oscillation frequency f sw 350 500 650 khz short - circuit oscillation frequency f sw (short) v fb = 0.3 v 15 0 khz sync fr equency range f sync 300 800 k hz input uvlo threshold v uvlo (vth) v in rising 4 v under voltage lockout threshold hysteresis v uvlo (hys) 2 00 mv en/sync input low voltage v en (l) 0.4 v en/sync input high voltage v en (h) 2.0 v en input current i e n v en = 2 v 2 a cc 4.5 v soft - start time t ss 600 s sd 1 6 0 c note 2 guarantee by design.
6 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. typical performance curves v in = 12v, v out = 1.2v, c1 =10 f x 2, c2 = 22 f x 2, l1 =1.8 h , ta = +25 c , unless other wise noted. figure 5 . efficiency vs. loading figure 6 . efficiency vs. loading figure 7 . efficiency vs. loading figure 8 . feedback voltage vs. temperature figure 9 . frequency vs. temperature v out = 1 . 2v v in = 5 v v in = 12 v v in = 21 v v out = 3.3 v v in = 5 v v in = 12 v v in = 21 v v out = 5 v v in = 12 v v in = 21 v 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 3.5 4 load current (a) efficiency (%) 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 3.5 4 load current (a) efficiency (%) 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 3.5 4 load current (a) efficiency (%) 0.785 0.79 0.795 0.8 0.805 0.81 0.815 0.82 0.825 0.83 0.835 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 case temperature ( degrees c ) vfb (v) 350 375 400 425 450 475 500 525 550 575 600 625 650 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 case temperature ( degrees c ) frequency (khz)
7 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. typical performance curves (continued) v in = 12v, v out = 1.2 v, c1 =10 f x 2 , c2 = 22 f x 2, l1 = 1.8 h , ta = +25 c , unless otherwise noted. i out =0a i out = 4 a figure 1 0 . dc ripple waveform figure 1 1 . dc ripple waveform i out =0a i out = 4 a figure 1 2 . startup through enable waveform figure 1 3 . startup th rough enable waveform i out = 0 a i out = 4 a figure 1 4 . s hutdown through enable waveform figure 1 5 . s hutdown through enable waveform v in 2 0mv/div. v out 20 mv/div. i l 2 a/div. v sw 5v/div. 2 s/div. v in 2 0 0 mv/div. v out 20 mv/div. i l 2 a/div. v sw 5v/div. 2 s/div. v en 5 v/div. v out 0.5 v/div. v sw 5v/div. 4 00s /div. v en 5 v/div. v out 0.5 v/div. v sw 5v/div. 4 00s /div. v en 5 v/div. v out 0.5 v/div. i l 2 a/div. v sw 5v/div. 2m s/div. v en 5 v/div. v out 0.5 v/div. i l 2 a/div. v sw 5v/div. 40s /div.
8 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. typical performance curves (continued) v in = 12v, v out = 1.2 v, c1 =10 f x 2 , c2 = 22 f x 2, l1 = 1.8 h , ta = +25 c , unless otherwise noted. i out =0a i out =4a figure 1 6 . startup through vin waveform figure 1 7 . startup through vin waveform i out = 0a to 4a i out = 2a to 4a figure 1 8 . load transient waveform figure 19 . load transient w aveform 10m s/d iv. 10m s/div. v out 100m v/div. i l 2 a/div. 8 0s/div. v in 5 v/div. v out 0.5 v/div. i l 2 a/div. v sw 5v/div. v in 5 v/div. v out 0.5 v/div. i l 2 a/div. v sw 5v/div. 8 0s/div. v out 10 0m v/div. i l 2 a/div.
9 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. function description introduction FR9808 is a constant - frequency current - mode step - down synchronous dc/dc converter. it regulates input voltage from 4.75v to 21v and provide s 4a of continuous load current. to achieve bias power supply, FR9808 co ntains an internal voltage regulator to support the internal circuits. for application s in which vin is less than 4.5 v, output decreases , and a 0.1f ceramic capacitor is required for decoupling. if vin is greater than 4.5v, the output of the regulator i s in full regulation. the error amplifier compares the fb voltage with the internal 0.80 5 v reference . a nd t he voltage of error amplifier output is compared to the switch current to control the rs flip - flop . at the beginning of each clock cycle, the high - side nmos turn s on when the oscillator sets the rs flip - flop , an d turns off when current comparator resets the rs flip - flop . then the low - side nmos turns on until the clock period ends. internal soft - start the internal soft - start function is used to elimi nate the output voltage overshooting during start - up. when the chip initiates , the internal reference voltage rises slowly to 0.80 5 v and the internal comp signal rises slowly to achieve output voltage. the soft - start time is approximate 600 s . en/sync the FR9808 en/sync pin provides digital control to turn on/turn off the regulator. for automatic start - up, tie en/sync and vin with a resist er , as shown in the figure. the recommended value of r3 is 100k? . the f r9808 can be synchronized with an external clock from 300khz to 800khz by using the en/sync pin. device protection: 1. input under voltage lockout when the power of FR9808 is on, the internal circuits are held inactive until vin exceeds the input uvlo thre shold voltage. the regulator is disabled when vin falls below the input uvlo threshold voltage. the hysteretic of the uvlo comparator is 2 00mv. 2. over current protection the FR9808 o c p protection function was designed to sense the cycle - by - cycle over c urrent limit signal to prevent device damage due to short . when the inductor current peak value reaches the current limit threshold, the output voltage starts to drop until the fb voltage is less than 30% of the reference. FR9808 subsequently enters hicc up mode to periodically restart the part and exits the hiccup mode once the over current condition is lift ed. 3. over temperature protection the FR9808 incorporates an over temperature protection circuit to protect itself from overheating. when the junctio n temperature exceeds the thermal shutdown threshold temperature, the regulat or will shutdown. when the junction temperature is less than the recovery threshold temperature , the chip will re - enable . v i n r 3 e n / s y n c f r 9 8 0 8
10 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. application information output voltage setting the ou tput voltage v out is set using a resistive divider from the output to fb. the fb pin regulated voltage is 0.805 v. thus the output voltage is: table 1 lists recommended values of r1 and r2 for most used output voltage. t able 1 recommended resistance values v out r1 (1%) r2 (1%) 5v 30.9 k? 5.76 k? 3.3 v 30.9 k? 9.76 k? 2.5v 4.99 k? 2.32 k? 1.8 v 4.99 k? 3.92 k? 1.2 v 4.99 k? 10 k? resistors r1 and r2 should be placed close to the fb pin to prevent stray pickup. input capacitor selection the use of the input capacitor is filtering the input voltage ripple and the mosfets switching spike voltage. because the input current to the step - down converter is discontinuous, the input capacitor is required to supply the current to the converter to keep the dc input voltage. the capacitor voltage rating should be 1.25 to 1.5 times greater than the maximum input voltage. the input capacitor ripple current rms value is calculated as: where d is the duty cycle of the power mosfet. this function reaches the maximum value at d=0.5 and the equivalent rms current is equal to i out /2. the following diagram is the graphical representation of above equation. a low esr capacitor is required to keep the noise minimum. ceramic capacitors are better, but tantalum or low esr electrolytic capacitors may also suffice. when using tantalum or electrolytic capacitors, a 0.1 f ceramic capacitor should be placed as close to the ic as possible. it is recommended that the input ec capacitor should be added for applications if the FR9808 will suffer high spike input voltage (ex. hot plug test). it can eliminate the spike voltage and induced the ic damage from high input voltage stress (see note1). output capacitor selection the output capacitor is used to keep the dc output voltage and sup pli es the load transient current. when operating in constant c urrent mode, the output ripple is determined by four components: the following figures show the form of the ripple contributions. v ripple(esr) (t) + v ripple(esl) (t) + v ripple(c) (t) 4 a 3a 2 a (t) (t) (t) out r1 v 0.805 1 v r2 ?? ? ? ? ?? ?? ? ? ? ? in rms out out in i i d 1-d v d v ? ? ? ? f r 9 8 0 8 c 5 3 3 0 f / 2 5 v e c x 1 v i n v i n 1 8 v t o 2 1 v c 1 2 2 f / 2 5 v m l c c x 1 ? ? ? ? ? ? ? ? ? ? ripple ripple(c) ripple(esr) ripple(esl) noise v t v t v t v t v t ?? ??
11 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. application information (continue d) + v noise (t) = v ripple (t) where f osc is the switching frequency, l is the inductance value, v in is the input voltage, es r is the equivalent series resistance value of the output capacitor, esl is the equivalent series inductance value of the output capacitor and the c out is the output capacitor. low esr capacitors are preferred to use . ceramic, tantalum or low esr electrolytic capacitors can be used depend ing on the o utput ripple requirement . when using the ceramic capacitors , the esl component is usually negligible. it is important to use the proper method to eliminate high frequency noise when measuring the output ripple. the figure shows how to locate the probe a cross the capacitor when measuring output ripple. removing the scope probe plastic jacket in order to expose the ground at the tip of the probe. it gives a very short connection from the probe ground to the capacitor and eliminating noise. inductor selection the output inductor is used for storing energy and filtering output ripple current. but the trade - off condition often happens between maximum energy storage and the physical size of the inductor. the first consideration for selecting the output inductor is to make sure that the inductance is large enough to keep the converter in the continuous current mode. that will lower ripple current and result in lower output ripple voltage. the i l is inductor peak - to - peak ripple current: the following diagram is an example to graphical represent i l equation. a good compromise value between size and efficiency is to set the peak - to - peak inductor ripple current i l equal to 30% of the maximum load current. but setting the peak - to - peak inductor ripple current i l between 20%~50% of the maximum load current is also acceptable. then the inductance can be calculated with the following equation: to guarantee sufficient output current, peak inductor current must be lower than the FR9808 high - side mosfet current limit. the peak inductor current is as below: (t) probe ground ceramic capacitor vout gnd v in = 12 v, f osc = 500 khz l= 1.8 h l= 2.2 h l= 4.7 h out out ripple(esr, p-p) osc in vv v 1 esr f l v ?? ? ? ? ? ?? ? ?? ripple(esl, p-p) in esl vv l esl ?? ? out out ripple(c, p-p) 2 in osc out vv v1 v 8 f l c ?? ? ? ? ?? ? ? ? ?? out out l osc in vv i1 f l v ?? ? ? ? ? ?? ? ?? l out(max) i 0.3 i ? ? ? ? ? in out out in osc l v - v v l v f i ? ? ? ? ? l peak out(max) i i =i + 2
12 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. application info rmation (continued) feedforward capacitor selection internal compensation function allows users saving time in design and saving cost by reducing the number of external components. the use of a feedforward capacitor c6 in the feedback network is recommend ed to improve the transient response or higher phase margin. for optimizing the feedforward capacitor, knowing the cross frequency is the first thing . the cross frequency (or the converter bandwidth) can be determined by usin g a network analyzer. when getting the cross frequency with no feedforward capacitor identified, the value of feedforward capacitor c6 can be calculated with the following equation : where f cross is the cross frequency. to reduce transient ripple, the feedforward capacitor value can be increased to push the cross frequency to higher region. although this can improve transient response, it also decrease phase margin and cause more ringing. in the other hand, if more phase margin is desired, the feedforward capacitor value can be decreased to push the cross frequency to lower region. in general, the feedforward capacitor range is between 10pf to 1nf. external diode selection for 5v input applications, it is recommended to add an e xternal bootstrap diode . this helps improving efficiency. the bootstrap diode can be a low cost one such as 1n4148. pcb layout recommendation the devices performance and stability are dramatically affected by pcb layout. i t is recommended to follow these general guidelines sho w n a s below: 1. place the input capacitors and output capacitors as close to the device as possible. trace to these capacitors should be as short and wide as possible to minimize parasitic inductance and resistance. 2. place feedback resistors close to the fb pin. 3. keep the sensitive signal (fb) away from the switching signal ( sw ). 4 . the exposed pad of the package should be soldered to an equivalent area of metal on the pcb. this area should connect to the gnd plane and have multiple via connections to the back of the pcb as well as connections to intermediate pcb layers. the gnd plane area connecting to the exposed pad should be maximized to improve thermal performance. 5 . multi - layer pcb design is recomme nded. figure 2 0 . FR9808 sop - 8 (exposed pad) package c in / cout with ec capacitors recommended pcb layout diagram f r 9 8 0 8 v o u t f b r 1 r 2 c 6 cross 1 1 1 1 c6 2 f r1 r1 r2 ? ?? ? ? ? ? ?? ? ?? f r 9 8 0 8 v i n b s v i n s w d 1 1 n 4 1 4 8 5 v c 4 1 2 3 4 5 6 7 8 l 1 l x g n d v o u t v i n e x p o s e d p a d g n d c 4 c 5 r 1 r 2 r 3 c 3 c 6 c 2 + C C
13 FR9808 - 1.1 - sep - 2011 FR9808 85t fitipower integrated technology lnc. outline information sop - 8 (exposed pad) package (unit: mm) symbols unit dimension in millimeter min max a 1.25 1.70 a1 0.00 0.15 a2 1.25 1.55 b 0.31 0.51 d 4.80 5.00 d1 3.04 3. 50 e 3.80 4.00 e1 2.15 2.41 e 1.20 1.34 h 5.80 6.20 l 0.40 1.27 note followed from jedec mo - 012 - e. carrier dimensions life support policy fitipowers products are not authorized for use as critical components in life support devices or other medical systems .

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