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a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 1 - de s cri p t i o n f eat u r es the a7108 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. the device is available in an adjustable version . supply current with no load is 300ua and drops to <1ua in sh utdown. the 2.5v to 6.5v input voltage range makes the a7108 ideally suited for single li - ion, two to four aa battery - powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. pwm pulse skipping mode operation provides very low output ripple voltage for noise sensitive applications. switching frequency is internally set at 1.5mhz, allowing the use of small surface mount inductors and capacitors. the internal synchronous switch increases efficiency and eliminates the need for an external schottky diode. low output voltages are easily supported with the 0.6v feedback reference voltage. the a7108 is available in sot -25 package. o rde ri ng i nf o rm a t i o n p ac k ag e t y pe p ar t n um ber sot -25 e5 a7 10 8e5r- adj a7 10 8e5 vr - adj note adj: adjustable output voltage v: halogen free package r: tape & reel ait provides all rohs products suffix v means halogen free package ? high efficiency: up to 96% ? 1.5mhz constant switching frequency ? 800ma output current at v in =3v ? integrated main switch and synchronous rectifier. no schottky diode required ? 2.5v to 6.5v input voltage range ? output voltage as low as 0.6v ? 100% duty cycle in dropout ? quiescent current: 300 a(input < 4.2v) ? slope compensated current mode control for excellent line and load transient response ? short circuit protection ? <1ua shutdown current ? soft start ? available in sot -25 package appl i c at i o n ? cellular and smart phones ? microprocessors and dsp core supplies ? wireless and dsl modems ? pdas ? mp3 / mp4 /pmp player ? digital still and video cameras ? portable instruments t ypi c al ap pl i c at i o n figure 1. basic application circuit with a7108 adjustable version
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 2 - p i n de s cri p t i o n top view p i n # s y mb ol f u nc t i on 1 run regulator enable control input. drive run above 1.5v to turn on the part. drive run below 0.6v to turn it off. in shutdown, all functions are disabled draw ing <1a supply current. do not leave run floating. 2 gnd ground 3 sw power switch output. it is the switch note connection to inductor. this pin connects to the drains of the internal p - ch and n - ch mosfet switches. 4 v in supply input pin. must be closely decoupled to gnd, pin 2, with a 2.2f or greater ceramic capacitor. 5 v fb v fb ( a7108 ): feedbac k input pin. connect fb to the center point of the external resistor divider. the feedback threshold voltage is 0.6v.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 3 - abso l u t e m ax i m u m r at i n g s input supply voltage - 0.3v ~ +7v run, v fb voltages - 0.3v ~v in +0.3v sw voltages - 0.3v ~ v in +0.3v p- channel switch source current (dc) 1000ma n- channel switch sink current (dc) 1000ma peak sw sink and source current 1.4a package thermal resistance note1 ja 220c/w jc 110c/w operating temperature range - 40c ~ +85c junction temperature note2 +125c storage temperature range - 65c ~ +150c lead temperature (soldering, 10s) +260c stress beyond above listed absolute maximum ratings may lead permanent damage to the device. these are stress ratings only and operations of the device at these or any other conditions beyond those indicated in the operational sections of the specifications are not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. note1: thermal resistance is specified with approximately 1 square of 1 oz copper. note2: t j is calculated from the ambient temperature t a and power dissipation pd according to the following formula: a7108 : t j = t a + (p d )x(220c/w)
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 4 - e l e ct ri ca l cha ra ct e ri s t i cs no t e 3 v in =v run = 3.6v, t a = 25c, unless otherwise noted p ar a met er c ond i t i ons mi n. t y p. m ax . u n it input voltage range 2.5 6.5 v input dc supply current active mode shutdown mode v fb =0.5v or v out =90% v fb =0v, v in =4.2v 300 0.1 400 0.1 a a regulated feedback voltage t a = +25c 0.5880 0.6000 0.6120 v t a = 0c t a 85c 0.5865 0.6000 0.6135 t a = - 40c t a 85c 0.5820 0.6000 0.6180 v fb input bias current v fb = 0.65v 30 na reference voltage line regulation v in = 2.5v to 5.5v, i out =10ma 0.5 0.60 %/v output voltage line regulation v in = 2.5v to 5.5v i out =10ma 0.5 0.60 %/v output voltage load regulation i out =10 to 800ma 0.5 % peak inductor current v in =3v, v fb =0.5v or v out =90% duty cycle <35% 1.0 a oscillator frequency v fb =0.6v or v out =100% 1.2 1.5 1.8 mhz r ds(on) of p - ch mosfet i sw= 300ma 0. 3 0.35 r ds(on) of n - ch mosfet i sw =- 300ma 0.3 0.35 sw leakage v run = 0v, v sw = 0v or 5v, v in =5v 0.01 1 a soft start 0.3 ms run threshold low - 40c t a 85c 0.6 v run threshold high - 40c t a 85c 1.5 v run leakage current 0.01 1 a thermal sh utdown 165 c note3 : 100% production test at +25c. specifications over the temperature range are guaranteed by design and characterization.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 5 - t y p i ca l p e rf o rm a n ce cha ra ct e ri s t i cs test figure 1 a bove unless otherwise specified 1. vs . i o ( v o =1 .8 v) 2. vs . i o ( v o = 3.3 v) 3. v o vs . i o (v o =1.8v) 4. v o vs . i o (v o =3.3v) 5. normal operation ch1:v o ch2:v sw ch3:i sw 6. load transient ch1:v o ch3:i o
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 6 - b l o ck di a g ra m
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 7 - d e t a il e d in f or m a t ion o p er at i on a7108 is a monolithic switching mode step - down dc - dc converter. it utilizes internal mosfets to achieve high efficiency and can generate very low output voltage by using internal reference at 0.6v. it operates at a fix ed switching frequency, and uses the slope compensated current mode architecture. c ur r ent mo de p w m c ont r ol slope compensated current mode pwm control provides stable switching and cycle - by - cycle current limit for excellent load and line responses and pro tection of the internal main switch (p - ch mosfet) and synchronous rectifier (n - ch mosfet). during normal operation, the internal p - ch mosfet is turned on for a certain time to ramp the inductor current at each rising edge of the internal oscillator, and sw itched off when the peak inductor current is above the error voltage. the current comparator, i comp , limits the peak inductor current. when the main switch is off, the synchronous rectifier will be turned on immediatel y and stay on until either the inductor current starts to reverse, as indicated by the current reversal compar ator, i zero , or the beginning of the next clock cycle. the ovdet comparator controls output transient overshoot s by turning the main switch off and keeping it off until the fault is no longer present. i dl e m ode o per a t i on at very light loads, the a7108 automatically enters pulse skipping mode. in the pulse skipping mode, the inductor current may reach zero or reverse on each pulse. the pwm control loop will automaticall y skip pulses to maintain output regulation. the bottom mosfet is turned off by the cur rent reversal comparator, i zero , and the switch voltage will ring. this is discontinuous mode operation, and is normal behavior for the switching regulator. d r o po ut o p er at i on when the input voltage decreases toward the value of the output voltage, the a7108 allows the main s witch to remain on for more than one switching cycle and increases the duty cycle n o t e 4 until it reaches 100%. the output voltage then is the input voltage minus the voltage drop across the main switch and the ind uctor. at low input supply voltage, the r ds(on) of the p - channel mosfet increases, and the efficiency of the converter decreases. caution must be exercised to ensure the heat dissipated not to exceed the maxim um junction temperature of the ic.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 8 - n o t e 4 : the duty cycle d of a step - down converter is defined as : where t on is the main switch on time and f osc is the oscillator frequency (1.5mhz). m ax i mum lo ad c ur r en t the a7108 will operate with input supply voltage as low as 2.5v, however, the maximum load current decreases at lower input due to large ir drop on the main switch and synchronous rectifier. the slop e compensation signal reduces the peak inductor current as a function of the duty cycle to prevent sub - harmonic oscillations at duty cycles greater than 50%. conversely the current limit increases as the duty cycle decreases. l ay out g ui da nc e when laying out the pcb board, the following suggestions should be taken to ensure proper opera tion of the a7108 . th ese items are also illustrated graphically in figure 2 . 1. the power traces, including the gnd trace, the sw trace and the v in trace should be kept short, direct and wide. 2. the v fb pin should be connected directly to the feedback resistor. the resistive divider r1/r2 must be connected between the (+) plate of c out and ground. 3. connect the (+) plate of c1 to the v in pin as closely as possible. this capacitor provides the ac current to internal power mosfet. 4. keep the switching node, sw, away from the sensitive v fb node. 5. keep the ( - ) plates of c1 and c3 as close as possible. figure 2. a7108 -a dj output suggested layout
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 9 - s et t i n g t he o u t pu t v ol t ag e figure 1 above shows the basic application circuit with a710 8 adjustable output version. the exter nal resistor sets the output voltage according to the following equation: r1= 200 k for all outputs; r2= 200 k for v out =1.2v, r2=300 k for v out =1.5v, r2=400 k for v out =1.8v, and r2=633.3 k for v out =2.5v. i nduc t or s el ec t i o n for most designs, the a710 8 o perates with inductors of 1h to 4.7h. low inductance values are physically smaller but require faster switching, which results in some efficiency loss. the inductor value c an be derived from the following equation: where i l is inductor ripple current. large value inductors lower ripple current and small value inductors result in high ripple currents. choose inductor ripple current approximately 35% of the maximum load current 800ma, or i l =280ma. for output voltages above 2.0v, when light - load efficie ncy is important, the minimum recommended inductor is 2.2h. for optimum voltage - positioning load transients, choose an inductor with dc series resistance in the 50m to 150m range. for higher efficiency at heavy loads (above 200ma), or minimal load regul ation (but some transient overshoot), the resistance should be kept below 100m. 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 (800ma+140ma). table 1 lists some typical surface mount inductors that meet target applications for the a710 8. p ar t # l ( h ) m ax dcr ( m ) r at e d d .c . c ur r e nt ( a ) s iz e w xl xh ( m m ) sumida cr43 1.4 2.2 3.3 4.7 56.2 71.2 86.2 108.7 2.52 1.75 1.44 1.15 4.5x4.0x3.5 sumida cdrh4d18 1.5 2.2 3.3 4.7 75 110 162 1.32 1.04 0.84 4.7x4.7x2.0 toko d312c 1.5 2.2 3.3 4.7 120 140 180 240 1.29 1.14 0.98 0.79 3.6x3.6x1.2 table 1typical surface mount inductors
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 10 - i npu t c a pac i t or s e l ec t i on the input capacitor reduces the surge current drawn from the input and switching noise from the device. the input capacitor impedance at the switching frequency shall be less than input sour ce impedance to prevent high frequency switching current passing to the input. a low esr input capacitor sized for maximum rms curr ent must be used. ceramic capacitors with x5r or x7r dielectrics are highly recommended because of their low esr and small temperature coefficients. a 4.7f ceramic capac itor for most applications is sufficient. o ut put c a pac i t or s el ec t i on the output capacitor is required to keep the output voltage ripple small and to ensure regul ation loop stability. the output capacitor must have low impedance at the switching frequency. cerami c capacitors with x5r or x7r dielectrics are recommended due to their low esr and high ripple current. the output rippl e v out is determined by:
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 11 - p a c k a ge in f or m a t ion dimension in sot - 25 (unit: mm) s y mb ol m in m ax a 1.050 1.250 a1 0.000 0.100 a2 1.050 1.150 b 0.300 0.500 c 0.100 0.200 d 2.820 3.020 e 1.500 1.700 e1 2.650 2.950 e 0.950(bsc) e1 1.800 2.000 l 0.300 0.600 0 8
a i t s em i c ond u ctor i n c . www.ait - ic.com a 71 08 dc - dc converter/ buck ( step - down ) 1.5mhz, 800ma synchronous rev 2.1 - no v 200 8 released , may 201 6 updated - - 12 - i m p o rt a nt no t i ce ait semiconductor inc. (ait) reserves the right to make changes to any its product, specificat ions, to discontinue any integrated circuit product or service without notice, and advises its customers to obtain the latest version of relevant information to verify, before placing orders, that the information being relied on is current. ait semiconductor inc.'s integrated circuit products are not designed, i ntended, authorized, or warranted to be suitable for use in life support applications, devices or systems or other critical applications. use of ait products in such applications is understood to be fully at the risk of the customer. as used herei n may involve potential risks of death, personal injury, or servere property, or environmental damage. in order to minimize risks associated with the customer's applications, the customer should provide adeq uate design and operating safeguards. ait semiconductor inc . assumes to no liability to customer product design or applica tion support. ait warrants the performance of its products of the specifications applicable at the time of sa le.

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