1 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 1. 25 m hz, 2 a synchronous step - down converter ? general description AX3531 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 wi th no load is 2 0 0 ua and drops to <1ua in shutdown. the 2. 8 v to 5.5 v input voltage range makes AX3531 ideally suited for single li - ion, two to three aa battery - powered applications. 100% duty cycle provides low dropout operation, extending battery life in port able systems. switching frequency is internally set at 1. 25 m hz, allowing the use of small surface mount inductors and capacitors. the internal synchronous switch increases efficiency and decreases need of an external s chottky diode. low output voltages are easily supported with the 0.6v feedback reference voltage. AX3531 is available in small tsot - 23 - 6 l package. ? features - 2. 8 v to 5.5 v input voltage range - output voltage from 0.6v to vin - high efficiency: up to 9 6 % - 1. 25 m hz constant frequency operation - up to 2 a output current - quiescent current: 2 0 0 ua ( typical ) - no schottky diode required - current mode operation for excellent line and load transient response - current limit, enable function - short circuit protect (scp) - build in soft start function - 1 a shutdown cur rent - tsot - 23 - 6 l pb - free package
2 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? b lock d iagram ? pin assignment the package of AX3531 is tsot - 23 - 6 l ; the pin assignment is given by : name description en enable pin h vin power supply input p in sw switch output pin. connect external inductor here. minimize trace area at this pin to reduce emi. gnd ground pin fb output feedback pin ? order /marking information order information top marking (tsot - 23 - 6 l) f b e n v i n s w g n d o v e r - t e m p e r a t u r e a n d s h o r t - c i r c u i t p r o t e c t i o n r e f s o f t - s t a r 0 . 6 v + - - + i c o m p - + i s e n s e a m p o s c s l o p e c o m p p w m l o g i c n o n - o v e r l a p c o n t r o l r e s e t s e t e n a b l e l o g i c ( t o p v i e w ) e n g n d s w v i n f b 1 2 3 4 5 t s o t - 2 3 - 6 l 6 g n d p a c k a g e t y p e a x 3 5 3 1 p a c k i n g b l a n k : b a g x x x c t : t s o t - 2 3 - 6 l a : t a p i n g g g w w : 0 1 ~ 2 6 ( a ~ z ) 2 7 ~ 5 2 ( a ~ z ) y e a r : a = 2 0 1 0 1 = 2 0 1 1 i d c o d e : i n t e r n a l a x 3 5 3 1 y w x
3 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? a bsolute m aximum r atings (at t a =25 c ) characteristics symbol rating unit v in pin voltage v in v ss - 0.3 to v ss + 6.5 v feedback pin voltage v fb v ss - 0.3 to v in + 0.3 v en pin voltage v en v s s - 0.3 to v in + 0.3 v switch pin voltage v sw v ss - 0.3 to v in + 0.3 v power dissipation pd ( t j - t a ) / ja mw storage temperature range t st - 40 to +150 c operating temperature range t op - 4 0 to + 8 5 c junction temperature t j +125 c thermal resistance from junction to case tsot - 23 - 6l jc 50 c /w thermal resistance from junction to ambient tsot - 23 - 6l j a 100 c /w note: ja is measured with the pcb copper area of approximately 1 in 2 (multi - layer). ? electrical characteristics ( v in = v en =3.6v, t a =25 c , unless otherwise specified ) characteristics symbol conditions min typ max units input voltage range v in 2. 8 - 5.5 v v in uvlo threshold v in - 2. 2 - v v in uvlo hysteresis v in (hys) - 0. 35 - v feedback voltage v fb t a = +25c 0.588 0.600 0.612 v feedback bias current i fb v fb =0.65v - - 30 na quiescent current i cc q v fb = 1 v - 20 0 3 50 ua shutdown supply curr ent i sd v en =0v - 0.1 1 ua switching current limit i limit 2. 2 3 - a line regulation out /v out v in = 2.5v to 5.5 v - 0. 2 - % /v load regulation out /v out i out = 0.01 to 2 a - 1 - % oscillation frequency f osc i out =300ma 1. 0 1. 25 1. 5 mhz r ds(on) of p - ch mosfet r dson v in = 5 v - 120 - m
4 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? electrical characteristics (c ountinous ) ( v in = v en =3.6v , t a =25 c , unless otherwise specified ) characteristics symbol conditions min typ max units r ds(on) of n - ch mosfet r dson v in = 5v (note 1) - 100 - m en pin logic input threshold voltage v enl - - 0.4 v v enh 1.2 - - en pin input current i en - 0.1 1 ua efficiency effi v in =5v, v out =3.3v, i out =0.5a - 9 4 - % thermal shutdown t ds - 150 - c thermal shutdown hysteresis t sh - 30 - c note 1 : guaranteed by design . ? a pplication c ircuit ? f unction d escriptions operation AX3531 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 fixed switching frequency, and uses th e slope compens ated current mode architecture. this step - down dc - dc converter su pplies minimum 2 000m a output current at input voltage range from 2. 8 v to 5.5 v. v o u t = 0 . 6 * ( 1 + r 1 / r 2 ) r 2 1 0 0 k c 1 1 0 u f c 2 1 0 u f 2 v o u t = 1 . 8 v l 1 2 . 2 u h r 1 2 0 0 k a x 3 5 3 1 v i n e n g n d s w f b c 3 2 7 p f v i n = 2 . 8 v ~ 5 . 5 v r 2 = 1 0 k ~ 1 5 0 k
5 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 current mode pwm control slope compensated current mode pwm control provides stable switchi ng and cycle - by - cycle current limit for excellent load and line transient responses and protection 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 certa in time to ramp the inductor current at each rising edge of the internal oscillator, and switched 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 immediately and stay on until either the inductor current starts to reverse, as indicated by the current reversal comparator, i zero , or the beginnin g of the next clock cycle. ? a pplication i nformation setting the output voltage application circuit item shows the basic application circuit with AX3531 adjustable output version. the external resistor sets the output voltage according to the following equation: table 1 resistor select for output voltage setting v out r2 r1 1.2v 1 0 0 k 1 0 0 k 1.5v 1 00k 1 50k 1.8v 1 00k 2 00k 2.5v 15 0k 47 0k 3.3v 120k 540k inductor selection for most designs, the AX3531 operates with inductors of 2.2 h to 3.3 h. low inductance values are physically smaller bu t require faster switching, which results in some efficiency loss. the inductor value can be derived from the following equation: where is inductor ripple current. large value inductors lower ripple current and small value induct ors result in high ripple currents. choose inductor ripple current approximately 20 % of the maximum load current 2 0 00ma, i l = 4 0 0ma. ? ? ? ? ? ? ? ? ? 2 1 1 6 . 0 r r v v out ? ? f i v v v v l osc l in out in out ? ? ? ? ? ?
6 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 table 2 inductor select for output voltage setting ( v in =3.6v) v out 1.2v 1.5v 1.8v 2.5v inductor 2. 2 uh 2.2 uh 2. 2 uh 2. 2 uh part number we - tpc 74404300 2 2 74404300 2 2 74404300 2 2 744043002 2 note: part type mh or m (www.we - online.com) for opt imum 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 regulation (but some transient overshoot), the resistance shou ld 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 ( 2 0 00ma+ 2 0 0 ma). input capacitor selection the input capacitor reduces the surge c urrent drawn from the input and switching noise from the device. the input capacitor impedance at the switching frequency shall be less than input source impedance to prevent high frequency switching current passing to the input. a low esr input capacitor sized for maximum rms current must be used. ceramic capacitors with x5r or x7r dielectrics are highly recommended because of their low esr and small tempe rature coefficients. a 10 f ceramic capacitor for most applications is sufficient. output capacitor selection the output capacitor is required to be 10uf 2 to keep the output voltage ripple small and to ensure regulation loop stability. the output capacitor must have low impedance at the switching frequency. ceramic capacitors with x5r or x7r dielectri cs are recommended due to their low esr and high ripple current. compensation capacitor selection the compensation capacitors for increasing phase margin provide additi onal stability. it is required 27 p f , please r efer to demo board schematic to design .
7 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? t ypical characteristics v out = 1.2v v out = 3.3v i out =500ma
8 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? t ypical characteristics (c ountinous ) i out =500ma i out =500ma
9 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? t ypical characteristics (c ountinous ) v in = 5v, v out = 1.2v, no load v in = 5v, v out = 1.2v, r load = 0.5 v in = 5v, v out = 1.2v, no load v in = 5v, v out = 1.2v, r load = 0.5 v in = 3.3v, v out = 1.2v, i out = 0 a ? 2 a v in = 5v, v out = 1.2v, out = 0 a ? 2 a
10 / 10 axelite confidential materials, do not copy or distribute without written consent . rev. 1.0 jul . 22 , 20 1 1 ax3 5 31 ? p ackage o utlines symbol dimensions in millimeters dimensions in inches min. nom. max. min. nom. max. a - - 1.10 - - 0.043 a1 0.00 - 0.10 0 - 0.004 a2 0.70 0.90 1.00 0.028 0.035 0.039 b 0.30 0.40 0.50 0.012 0.016 0.020 c 0.08 0.14 0.20 0.003 0.006 0.008 d 2 .80 2.90 3.00 0.110 0.114 0.118 e 2.60 2.80 3.00 0.102 0.110 0.118 e1 1.50 1.60 1.70 0.059 0.063 0.067 e 0.95 bsc. 0.037 bsc. e1 1.90 bsc. 0.075 bsc. l 0.30 0.45 0.60 0.012 0.018 0.024 l1 0.60 ref. 0.024 ref. l2 0.25 bsc. 0.010 bsc. y - - 0.10 - - 0.004 r 0.10 - - 0.004 - - 0 o - 8 o 0 o - 8 o jedec outline: mo - 193 aa e 1 e e e 1 b d a a 1 a 2 y l 1 r l c l 2
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