3a step-down voltage regulator general description the ams2576 series are step-down switching regulators with all require d active functions. it is capable of driving 3a load with excellent line and load regulations. these dev ices are available in fixed output voltages of 3.3v, 5v, 12v and an adjustable output version. the ams2576 series offers a high-efficiency replacement for popular three-terminal linear regulators. also it requires a minimum number of external components. it substantially not only reduces the area of board size but also the size of heat sink, and in some cases no heat sink is required. the 4% tolerance on output voltage within specified input voltages and output load conditions is guar anteed. also, the oscillator frequency accuracy is within 10%. external shutdown is included. feat uring 80a (typical) standby current. the output switch includes cycle-by-cycle current limiting, as well as thermal shutdown for full protecti on under fault conditions. this series are offered in 5-pin to-263, to-220 package. features ? 3.3v, 5v, 12vand adjustable versions ? wide input voltage range, up to 40v ? internal oscillator of 52khz fixed frequency ? guaranteed 3a output current ? wide adjust version outp ut voltage range from 1.23v to 37v 4% max. at over line and load conditions ? low standby current, typ. 80a, at shutdown mode ? requir es only 4 external components ? thermal shutdown and current limit protection applications ? lcd monitors. ? add-on cards switching regulators ? high efficiency step-down regulators ? efficient pre-regulator for linear regulators ? positive to negative converter (buck-boost) block diagram + - 1.23v band-gap reference 52khz oscillator + - reset current limited thermal shutdown 2 3 fixed gain error amp. comparator driver 3amp. switch internal regulator enable 4 r2 r1 1k 1 5 feed back + vin enable output cout d1 + load dc input l1
ordering information part no. operating temp. (ambient) package ams2576t-xx -20 ~ +85 to-220-5l ams2576t-adj ams2576s- adj to-263-5l AMS2576S-XX note: where xx denotes voltage option, available are 12v, 5v and 3.3v. leave blank for adjustable version. contact fact ory for additional voltage options. pin assignment to-220 to-263 absolute maximum ratings input voltage vin +45 v enable pin input volt age vin (operate) -0.3v v vin v power dissipation p d internal limited w operating junction temperature range t j -40 ~ +150 storage temper ature range t stg -65 ~ +150 lead soldering temperature (260 ) to-220-5l / to-263-5l package 10 s 1. input 2. output 3. ground 4. feedback 5. enable
electrical characteristics vin = 12v, i l = 500ma, ta = 25 o c unless otherwise specified. parameter conditions min typ max unit ams2576-3.3v output voltage (note 1) vin=12v, i l =0.5a (figure 1) 0.98|vo| 3.3 1.02|vo| v output voltage (note 1) 0.5a i l 3a, 6v vin 45v (figure 1) 0.96|vo| 1.04|vo| efficiency vin=12v, i l =3a -- 75 -- % ams2576-5v output voltage (note 1) vin=12v, i l =0.5a(figure 1) 0.98|vo| 5.0 1.02|vo| v output voltage (note 1) 0.5a i l 3a, 8v vin 45v (figure 1) 0.96|vo| 1.04|vo| efficiency vin=12v, i l =3a -- 77 -- % ams2576-12v output voltage (note 1) vin=25v, i l =0.5a (figure 1) 0.98|vo| 12 1.02|vo| v output voltage(note 1) 0.5a i l 3a, 15v vin 45v (figure 1) 0.96|vo| 1.04|vo| efficiency vin=15v, i l =3a -- 88 -- % ams2576 feedback voltage (note 1) vin=12v, vout=5v,i l =0.5a (figure 2) 0.98|vo| 1.23 1.02|vo| v feedback voltage (note 1) 0.5a i l 3a, 8v vin 45v, vout=5v (figure 2) 0.96|vo| 1.04|vo| efficiency vin=12v, vout=5v, i l =3a -- 77 -- %
all output voltage v ersion (vin=12v for 5v, 3.3v and adjustable version, vin=25v for 12v version, i l = 500ma) oscillator frequency (note 2) 47 52 58 khz quiescent current (note 3) -- 5 10 ma standby current enable =5v -- 80 250 ua saturation voltage (note 4) i load =3a -- 1.2 1.5 v feedback bias current v out =5v (adj. version only) -- 10 100 na duty cycle (note 5) operating (on) 93 98 -- % current limit (note 2)(note 4) 4.2 7.0 8.8 a output leakage current (note 3) v out =0v -- 0.3 2 ma v out =-1v -- 7.5 30 enable threshold voltage v ih (v out =0v) 2.2 1.4 -- v v il (v out =normal output voltage) -- 1.2 1.0 enable input current i ih (enable =5v) -- 12 30 ua i ih (enable =0v) -- 0 10 note 1: external components such as the catch diode, i nductor, input and output ca pacitors can affect switching regulator system performance. refe r to application information for details. note 2: the oscillator frequency reduces to approximately 11khz in the event of fault conditions, such as output short or overload. a nd the regulated output voltage w ill drop approximately 40% from the nominal output voltage. this self-protection feature lowers the average power dissipation by lowering the minimum duty cycle fr om 5% down to approximately 2%. note 3: for these parameters, fb is removed from vo ut and connected to +12v to force the output transistor off. note 4: vout pin sourcing current. no diode, i nductor or capacitor connected to vout. note 5: fb is removed from vout and connected to 0v.
application information it is required that vin must be bypassed with at least a 100f electrolytic capacitor for stability. also, it is strongly recommended the capacitor?s leads must be dept short, and located near the regulator as possible. for low operating temperature range, for example, below -25 , the input capacitor value may need to be larger. this is due to the reason that the capacitance value of el ectrolytic capacitors decreases and the esr increases with lower temperatures and ago. paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures. output capacitor an output capacitor is also required to filter the output voltage and is needed for loop stability. the capacitor should be located near the ams2576 using short pc board traces. low esr types capacitors are recommended for low output ripple voltage and good stability. generally, low value or low voltage (less than12v) electrolytic capacitors usually have higher esr numbers, for example, the lower capacitor values (220f - 1000f) will yield typically 50mv to 150mv of output ripple voltage, while larger-value capacitors will reduce the ripple to approximately 20mv to 50mv. the amount of output rippl e voltage is primarily a function of the esr (equivalent series resistance) of the output capacitor and the amplitude of the inductor ripple current ( iind) output ripple voltage = ( iind) x (esr of cout) some capacitors called ?high-frequency?, ?low-inductance?, or ?low-esr? are recommended to use to further reduce the output ripple voltage to 10mv or 20mv. however, very low esr capacitors, such as tantalum capacitors, should be carefully evaluated. catch diode this diode is required to return path for the inductor current when the switch is off. it should be located close to the ams2576 using short leads and short printed circuit traces as possible. to satisfy the need to fast switching speed and low forward voltage drop, schottky diodes are widely used to provide the best efficiency, especially in low output voltage switching regulato rs (less than 5v). beside, fast-recovery, high-efficiency, or ultra fast recovery diodes are also suitable. but some types with an abrupt turn-off characteristic may cause instability and emi problems. a fast recovery diode with soft recovery characteristics is better choice. inductor selection the ams2576 can be used for either continuous or discontinuous modes of operation. each mode has distinctively different operating characteristics, which can affect the regulator performance and requirements. with relatively heavy load currents, the circuit operates in the continuou s mode (inductor current always flowing). but under light load conditions, the circuit will be force to the discontinuous mode (inductor current falls to zero for a period of time). for light loads (less than approximately 300ma) it may be desirable to operate the regulator in the discontinuous mode, prim arily because of the lower inductor values required for the discontinuous mode. indictors are available in different styles such as pot core, toroid, e-frame, bobbin core, et., as well as different core materials such as ferrites and powdered iron. the least expensive, the bobbin core type, consists of wire wrapped on a ferrite rod core. this type of construction makes for an inexpensive inductor, but since the magnetic flux is not completely contai ned within the core, it generates more electromagnetic interference (emi). this emi can caus e problems in sensitive circuits, or can give incorrect scope readings because of induced volta ge in the scope probe. an inductor should not be operated beyond its maximum rated current because it may saturate. when an inductor begins to saturate, the
inductance decreases r apidly and the inductor begins to look mainly resistive (the dc resistance of the winding). this will cause the switch current to rise very rapidly. different inductor types have different saturation characteristics, and this should be well considered when selecting as inductor. feedback connection for fixed output voltage ve rsion, the fb (feedback) pin must be connected to vout. for the adjustable version, it is important to place the output voltage ratio resistors near ams2576 as possible in order to minimize the noise introduction. enable input it is required that the enable must not be left open. for normal operati on, connect this pin to a ?low? voltage (typically, below 1.6v). on the other hand, for standby mode, connect this pin with a ?high? voltage. this pin can be safely pulled up to +vin without a resistor in series with it. grounding to maintain output voltage stability, the power ground connections must be low-impedance. for the 5-lead to-220 and to-263 style package, both the tab and pin 3 are ground and rather connection may be used. thermal characteristics the output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the esr of the output capacitor. the output ripple voltage of a switching pow er supply will contain a sawtooth ripple voltage at the switcher frequency, typically about 1% of the output voltages, and may also contain short voltage spokes of the sawtooth waveform. due to the fast switching action, and the parasitic inductance of the output filt er capacitor, there is voltage spikes presenting at the peaks of the sawtooth waveform. cautions must be taken for stray capacitance. wiri ng inductance, and even the scope probes used for transients evaluation. to minimize these voltage spikes, shortening the lead length and pcb traces is always the first thought. further more, an additional small lc filter (30h & 100f) (as shown in figure 3) will possibly provide a 10x r eduction in output ripple voltage and transients. heatsink and thermal consideration although the ams2576 requires only a small heatsink for most cases, the following thermal consideration is important for all operation. with the package thermal resistances ? ja and ? jc , total power dissipation can be estimated as follows: pd= (vin x iq) + (vout / vin) (iout x vsat); when no heatsink is used, the junction temperature rise can be determined by the following: ? tj = pd x ? ja with the ambient temperatur e, the actual junction temperature will be: tj = ? tj + ta if the actual operating junction temperature is out of the safe operating junction temperature (typically 125 ), then a heatsink is required. when using a heatsink, t he junction temperature rise will be reduced by the following: ? tj= pd x ( ? jc + ? interface + ? heatsink ); also one can see from the above, it is important to choose an heatsink with adequate size and thermal resistance, such that to maintain the regulator?s junction temperature below the maximum operating temperature.
application circuit ams2576 fixed voltage load + feedback l1 vout cout 1000f 100h unregulated dc input cin 100f gnd enbale vin + output mbr340 1 35 2 4 figure 1 fixed voltage version ams2576 adjustable voltage load + feedback l1 vout cout 1000f 100h unregulated dc input cin 100f gnd enbale vin + output mbr340 1 35 2 4 r2 r1 vout vout=1.23(1+r2/r1) figure 2: adjustable voltage version ams2576 adjustable voltage + feedback l1 vout cout 1000f 100h unregulated dc input cin 100f gnd enbale vin + output mbr340 1 35 2 4 r2 r1 vout output l2 20h c1 100f figure 3: lc filter for low output ripple
package description to-220-5l mechanical drawing to-263-5l mechanical drawing to-220 dimension dim millimeters inches min max min max a 10.00 10.50 0.394 0.413 b 3.240 4.440 0.128 0.175 c 2.440 2.940 0.096 0.116 d 0.260 1.020 0.010 0.040 e 1.570 1.830 0.062 0.072 f 13.31 14.13 0.524 0.556 g 4.475 5.225 0.176 0.206 h 1.170 1.370 0.046 0.054 i 27.60 29.44 1.087 1.159 j 2.175 2.925 0.086 0.115 k 0.297 0.477 0.012 0.019 l 8.280 8.800 0.326 0.346 m 6.010 6.510 0.237 0.256 n 14.29 15.31 0.563 0.603 to-263 dimension dim millimeters inches min max min max a 10.220 10.260 0.402 0.404 b 14.600 15.870 0.575 0.625 c 0.750 0.770 0.030 0.030 d 1.573 1.827 0.062 0.072 e 4.560 4.570 0.179 0.180 f 1.240 1.270 0.049 0.050 g 2.280 2.790 0.090 0.110 h 0.280 0.320 0.011 0.013 i 8.240 8.280 0.324 0.326 j 1.540 1.800 0.060 0.071
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