1 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A high efficiency fast response, 2a, 18v input synchronous step - down regulator ? general description AX3811A develops high efficiency synchronous stepdown dc - dc converter capable of delivering 2a load current. AX3811A operates over a wide input voltage range from 4. 3 5v to 18v and integrates main switch and synchronous switch with very low r ds(on) to minimize the conduction loss. AX3811A adopts the instant pwm architecture to achieve fast transient responses for high step down applications and high efficiency a t light loads. in addition, it operates at pseudo - constant frequency of 500 khz under heavy load conditions to minimize the size of inductor and capacitor. ? features - low r ds(on) for internal switches (top/bottom):140/130 m - 4. 3 5 - 18v input voltage range - 2a load current capability - instant pwm architecture to achieve fast transient responses internal softstart limits the inrush current - 2% 0.6v reference - rohs compliant and halogen free - compact package: sot23 - 6 applications - set top box - portable tv - access point router - dsl modem - lcd tv
2 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? pin assignment the package of AX3811A is sot - 23 - 6l ; the pin assignment is given by : name description bs boot - strap p in. supply high side gate driver. decouple this pin to lx pin with 0.1uf c eramic cap. gnd ground pin fb output f eedback p in. connect this pin to the center point of the output resistor divider (as shown in figure 1) to program the output voltage: vout=0.6*(1+r1/r2) en enable control. pull high to turn on. do not float. in in put pin. decouple this pin to gnd pin with at least 1 0 uf ceramic cap lx inductor pin. connect this pin to the switching node of inductor ? o rder/marking information order information top marking ? a bsolute maximum ratings characteristics symbol rating unit supply input voltage 19 v lx, en voltage v in +0.3v v fb, bs - lx voltage 4 v power dissipation ( @ t a = 25c ) p d 0.6 w package thermal resistance (note 2) jc 75 c /w ja 170 c /w junct ion temperature range t j 1 50 c /w lead temperature (soldering, 10 sec.) 260 c storage temperature range C 65 to 150 c recommended operating conditions (note 3 ) supply input voltage 4. 35 to 18 v junction temperature range - 40 to 125 c ambient te mperature range - 40 to 85 c ( t o p v i e w ) s o t - 2 3 - 6 l 3 6 4 1 g n d e n b s l x 5 f b i n 2 p a c k i n g a : t a p i n g x x c : s o t - 2 3 - 6 l b l a n k : t u b e p a c k a g e t y p e a x 3 8 1 1 a n w e e k c o d e l o t n u m b e r c o d e b x z y y e a r c o d e a x 3 8 1 1 a
3 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? e lectrical c haracteristics (v in = 12v, v out = 1.2v, l = 4.7 uh, c out = 22 uf *2 , t a = 25c, i out = 1a unless otherwise specified) characteristics symbol conditions min typ max units input voltage range v in 4. 3 5 - 18 v qui escent current i q i out =0, v fb =v ref 105% - 400 a shutdown current i shdn en=0 - 5 10 a feedback reference voltage v ref 0.588 0.6 0.612 v fb input current i fb v fb =v in - 50 50 na top fet ron r ds(on)1 - 0.1 3 0.1 5 ? bottom fet ron r ds(on)2 - 0.1 2 0 .1 4 ? bottom fet valley current limit i lim 2 - 3 a en rising threshold v enh 1.5 - - v en falling threshold v enl - - 0.4 v input uvlo threshold v uvlo - - 4. 3 5 v min on time - 40 50 100 ns min off time - 60 100 150 ns soft - start time t ss - 4 00 - us thermal shutdown temperature t sd - 150 - c thermal shutdown hysteresis t hys - 15 - c note 1: stresses listed as the above absolute maximum ratings may cause permanent damage to the device. these are for stress ratings. functional operatio n of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliabilit y. note 2: ja is measured in the natural convection at t a = 25c on a low effective single layer thermal conductivity test board of jedec 51 - 3 thermal measurement standard. pin 2 of sot - 23 - 6 packages is the case position for jc measurement. note 3: the d evice is not guaranteed to function outside its operating conditions.
4 / 1 0 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? application circuit figure 1. schematic diagram figure 2. efficiency figure ? f unction d escriptio ns operation AX3811A is a synchronous buck regulator ic that integrates the pwm control, top and bottom switches on the same die to minimize the switching transition loss and conduction loss. with ultra low rds(on) power switches and proprietary pwm contr ol, this regulator ic can achieve the highest efficiency and the highest switch frequency simultaneously to minimize the external inductor and capacitor size, and thus achieving the minimum solution footprint. AX3811A provides protection functions such as cycle by cycle current limiting and thermal shutdown protection. AX3811A will sense the output voltage conditions for the fault protection. v o u t 3 . 3 v l x i n g n d f b e n v i n 4 . 3 5 ~ 1 8 v c i n 1 0 u f l 1 4 . 7 u h r 1 1 0 0 k c o u t 2 2 u f * 2 b s r 2 2 2 . 1 k c 1 1 0 0 n f c 4 ( o p t . ) o n / o f f 5 4 6 3 2 1 e f f i c i e n c y v s . l o a d c u r r e n t l o a d c u r r e n t ( a ) e f f i c i e n c y ( % ) 5 0 6 0 7 0 8 0 9 0 1 0 0 0 0 . 5 1 1 . 5 2 5 v i n , 3 . 3 v o u t 1 2 v i n , 3 . 3 v o u t 1 6 v i n , 3 . 3 v o u t
5 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? typical characteristics e f f i c i e n c y v s . l o a d c u r r e n t l o a d c u r r e n t ( m a ) e f f i c i e n c y ( % ) 6 0 6 5 7 0 7 5 8 0 8 5 9 0 9 5 1 0 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 7 v i n , 5 v o u t 1 2 v i n , 5 v o u t 1 6 v i n , 5 v o u t e f f i c i e n c y v s . l o a d c u r r e n t l o a d c u r r e n t ( m a ) e f f i c i e n c y ( % ) 6 0 6 5 7 0 7 5 8 0 8 5 9 0 9 5 1 0 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 7 v i n , 3 . 3 v o u t 1 2 v i n , 3 . 3 v o u t 1 6 v i n , 3 . 3 v o u t e f f i c i e n c y v s . l o a d c u r r e n t l o a d c u r r e n t ( m a ) e f f i c i e n c y ( % ) 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 5 v i n , 1 . 2 v o u t 1 2 v i n , 1 . 2 v o u t 1 6 v i n , 1 . 2 v o u t o u t p u t r i p p l e t i m e ( 1 u s / d i v ) ( v i n = 1 2 v , v o u t = 3 . 3 v , i l o a d = 2 a ) i l 2 a / d i v v o u t ( a c ) 1 0 m v / d i v v l x 1 0 v / d i v l o a d t r a n s i e n t ( v i n = 1 2 v , v o u t = 3 . 3 v , i l o a d = 0 . 2 - 1 a ) t i m e ( 4 0 u s / d i v ) i l 5 0 0 m a / d i v v o u t ( a c ) 1 0 0 m v / d i v l o a d t r a n s i e n t ( v i n = 1 2 v , v o u t = 3 . 3 v , i l o a d = 1 - 2 a ) t i m e ( 4 0 u s / d i v ) i l 1 a / d i v v o u t ( a c ) 1 0 0 m v / d i v
6 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? typical characteristics (countinous) s t a r t u p t i m e ( 1 0 0 u s / d i v ) ( v i n = 1 2 v , v o u t = 3 . 3 v , i l o a d = 2 a ) i l 2 a / d i v v o u t 2 v / d i v e n 2 v / d i v v l x 5 v / d i v s h u t d o w n t i m e ( 4 u s / d i v ) ( v i n = 1 2 v , v o u t = 3 . 3 v , i l o a d = 2 a ) i l 2 a / d i v v o u t 2 v / d i v e n 2 v / d i v v l x 5 v / d i v s h o r t c i r c u i t p r o t e c t i o n t i m e ( 2 m s / d i v ) ( v i n = 1 2 v , v o u t = 3 . 3 v , o p e n t o s h o r t ) i l 1 a / d i v v o u t 2 v / d i v s h o r t c i r c u i t p r o t e c t i o n t i m e ( 2 m s / d i v ) ( v i n = 1 2 v , v o u t = 3 . 3 v , 2 a t o s h o r t ) i l 1 a / d i v v o u t 2 v / d i v
7 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? application information because of the high integration in the ax381 1a ic, the application circuit based on this regulator ic is rather simple. only input capacitor c in , output capacitor c out , output i nductor l and feedback resistors (r1 and r2) need to be selected for the targeted applications specifications. feedback resistor dividers r1 and r2: choose r1and r2 to program the proper output voltage. to minimize the power consumption under light loads , it is desirable to choose large resistance values for both r1 and r2. a value of between 10k and 1 00k is highly recommended for both resistors. if vout is 3.3v, r1=100k is chosen, then using following equation, r2 can be calculated to be 22.1k: input capacitor c in : the ripple current thro ugh input capacitor is calculated as: to minimize the potential noise problem, place a typical x5r or better grade ceramic capacitor really close to the in and gnd pins. care should be taken to minimize the loop area formed by c in , and in/gnd pins. in this case, a 10 uf low esr ceramic capacitor is recommended. output capacitor c out : the output capacitor is selected to handle the output ripple noise requirements. both steady state ripple and transient requirements must be taken in to consideration when selecting this capacitor. for the best performance, it is recommended to use x5r or better grade ceramic capacitor greater than 22uf capacitance. 1 2 6 . 0 6 . 0 r v v v r out ? ? ) d 1 ( d i i out rms _ cin ? ? ? 0 . 6 v fb gnd r 1 r 2 v out
8 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A output inductor l: there are several considerations in choosing this induct or. (1) choose the inductance to provide the desired ripple current. it is suggested to choose the ripple current to be about 40% of the maximum output current. the inductance is calculated as: where f sw is the switching frequency and i out , max is the maximum load current. the AX3811A regulator ic is quite tolerant of different ripple current amplitude. consequently, the final choice of inductance can be slightly off the calculation value without significantly impacting the performance. (2) the saturation current rating of the inductor must be selected to be greater than the peak inductor current under full load conditions. (3) the dcr of the inductor and the core loss at the switching frequency must be low enough to achieve the desired efficiency requirement. it is desirable to choose an inductor with dcr<50m to achieve a good overall efficiency. external boo t strap cap this capacitor provides the gate driver voltage for internal high side mosefet. a 100nf low esr ceramic capacitor connected between bs pin and lx pin is recommended. 40% i f ) /v v (1 v l max out, sw max in, out out = l f 2 ) /v v - sw max in, out ? ? 1 ( v + i > i out max , out min , sat
9 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A load transient considerations: the AX3811A regulator ic integrates the compensation components to achieve good stability and fast transient responses. in some applicati ons, adding a 22pf ceramic cap in parallel with r1 may further speed up the load transient responses and is thus recommended for applications with large load transient step requirements. layout design: the layout design of ax381 1a regulator is relatively simple. for the best efficiency and minimum noise problem, we should place the following components close to the ic: c in , l, r1 and r2. (1) it is desirable to maximize the pcb copper area connecting to gnd pin to achieve the b est thermal and noise performance. if the board space allowed, a ground plane is highly desirable. (2) c in must be close to p ins in and gnd. the loop area formed by c in and gnd must be minimized. (3) the pcb copper area associated with lx pin must be minimized to avoid the potential noise problem. (4) the components r1 and r2, and the trace connecting to the fb pin must not be adjacent to the lx net on the pcb layout to avoid the noise problem. (5) if the system chip interfacing with the en pin has a high impedance s tate at shutdown mode and the in pin is connected directly to a power source such as a li - ion battery, it is desirable to add a pull down 1mohm resistor between the en and gnd pins to prevent the noise from falsely turning on the regulator at shutdown mode . l 1 v o u t c 1 ( o p t . ) c o u t r 1 r 2
10 / 10 axelite confidential materials, do not cop y or distribute without written consent . rev. 1. 2 nov . 0 4 , 20 1 3 AX3811A ? package outlines recommended pad layout top view notes: all dimension in mm all dimension dont not include mold flash and metal burr 0.95 2.40 1.90 0.60 1.00 0.95 2.40 1.90 0.60 1.00 1.50 - 1.70 0.30 - 0.50 2.80 - 3.10 2.70 - 3.00 1.50 - 1.70 0.30 - 0.50 2.80 - 3.10 2.70 - 3.00 0.1 - 0.15 0.25 ref 0.3 - 0.6 0.1 - 0.15 0.25 ref 0.3 - 0.6 1.0 - 1.3 0.01 - 0.1 1.90 typ 0.95 typ 1.0 - 1.3 0.01 - 0.1 1.90 typ 0.95 typ
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