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? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 1 tel:805-498-2111 fax:805-498-3804 web:http:// www.semtech.com ordering information part number (1) package temp. range (t a ) sc1408is.tr so-8 -40 to +85c sc1408ims.tr msop-8 note: (1) only available in tape and reel packaging. a reel contains 2500 devices. description the sc1408 is a low voltage boost controller that oper- ates from a 1.8v to 16.5v input range. a shutdown pin allows the user to turn the controller off reducing sup- ply current to less than 2a typical. output voltage can be preset to 5v or is adjustable from 3v to 16.5v with a resistor divider. the controller changes frequency in low load conditions to improve efficiency. the sc1408 was designed for two cell alkaline or sin- gle cell lithium ion battery applications. with the proper external components it can be used as a boost converter or a buck/boost converter. a current sense is implemented with an external resis- tor that is not in the load current path. the sc1408 operates in ?bootstrapped? mode. when the input voltage to the device is less than 2.5v during startup, a low voltage 50% fixed duty cycle oscillator is switched in to initiate boost action. features ? = 1.8v to 16.5v input range ? = up to 10w output power ? = preset 5v or adjustable output ? = up to 300khz switching frequency ? = 10a max shutdown current ? = industrial temperature range ? = so-8 or msop-8 package applications ? = pda power supplies ? = battery powered applications ? = positive lcd bias generator ? = portable communications (cellular phones) ? = peripheral card supplies ? = industrial power supplies typical application - boost configuration vin (1.8v to 16.5v) vout (3v to 16.5v) u1 sc1408is 8 1 2 3 4 5 6 7 isense gate bst fb shdn ref agnd gnd l1 r2 q1 d1 + c2 r4 r3 + c3 c5
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 2 absolute maximum ratings parameter symbol maximum units input voltage bst to gnd -0.3 to 18 v small signal ground to power ground gnd to agnd 0.1 v gate to gnd -0.3 to vbst+0.3 v fb, shdn, ref, isense to gnd -0.3 to min. of vbst+0.3 or 5 v operating temperature t a -40 to +85 c junction temperature range t j -40 to +150 c storage temperature t stg -65 to +160 c lead temperature (soldering) 10 seconds t l +300 c thermal resistance, junction to ambient so-8 msop-8 = ja 165 206 c/w thermal resistance, junction to case so-8/msop-8 = jc 40 c/w electrical characteristics (1) unless specified: v out = 5v; i load = 0ma; t a = +25c parameter sym conditions min typ max units input voltage t a = 25c 1.8 16.5 v t a = -40c to +85c 1.8 16.5 v supply current v out = 16.5v, shdn = 0.4v t a = -40c to +85c 110 140 ua v out = 10v, 1.6v = shdn = 5v t a = -40c to +85c 2 10 a output voltage v in = 2.0v to 5.0v, t a = -40c to +85c 4.800 5.0 5.200 v load regulation v in = 2.0v, v out = 5v, i load = 0ma to 500ma 60 mv/a line regulation v in = 2.7v to 4.0v, v out = 5v, i load = 500ma 7 mv/v minimum start up voltage no load 1.8 v maximum switch on time t on 9.6 16 22.4 s minimum switch off time t off 1.4 2.3 3.2 s efficiency v in = 4v, v out = 5v, i load = 500ma 87 % reference voltage v ref i ref = 0a t a = -40c to +85c 1.176 1.200 1.224 v reference load regulation 0a < i ref < 100a -4 10 mv reference line regulation 5v < v out < 16.5v 40 100 v/v fb trip point voltage v fb t a = 25c t a = -40c to +85c 1.176 1.200 1.224 v fb input current i fm t a = 25c t a = -40c to +85c -4 + 40 na
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 3 electrical characteristics (1) unless specified: v out = 5v; i load = 0ma; t a = +25c parameter sym conditions min typ max units shdn input high voltage v ih v out = 2.7v to 16.5v 1.6 v shdn input low voltage v il v out = 2.7v to 16.5v 0.4 v shdn input current i in v out = 16.5v, shdn = 0v or 5v + 1a current limit trip level v cs v out = 3v to 16.5v t a = 25c t a = -40c to +85c 85 80 100 115 120 mv isense input current i sense 0.01 + 1a gate rise time v out = 5v, 1nf from gate to gnd 50 ns gate fall time v out = 5v, 1nf from gate to gnd 50 gate on resistance gate = high or low 15 30 ? note: (1) this device is esd sensitive. use of standard esd handling precautions is required.
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 4 block diagram internal bias trig q trig q r s q + - + - + - fb 50mv + - isense 0.1v bst 2.5v gate shdn agnd ref gnd 1.20v reference low voltage oscillator min off time one shot 2.3us max on time one shot 16us bias current sense amp error comp mode detect start up comp pin # pin name pin function 1 gate gate drive output. 2 bst supply voltage. 3 fb voltage feedback. 4 shdn logic high shuts down the converter. 5 ref reference output pin. 6 agnd small signal analog and digital ground. 7 gnd power ground. 8 isense current sense pin. pin description pin configuration top view (so-8/msop-8)
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 5 fig. 1: typical application - boost configuration + c3 100uf c5 0.1uf d1 b130t r3 31.6k vin=3.3v c9 (1) u1 sc1408is 8 1 2 3 4 5 6 7 isense gate bst fb shdn ref agnd gnd + c8 100uf r4 10.0k coilcraft do3316p-223 + c2 100uf q1 irlr024n l1 22uh (1) see component selection r2 0.05ohm vout=5v fig. 2: typical application - buck/boost (sepic) configuration c9 (1) r2 0.05ohm c4 0.1uf d1 b130t vin=2.7v to 6v d2 1n4148 + c1 100uf + c2 100uf l1a 22uh 1 2 l1a/l1b is coupled inductor pulse pe-53718 or equivalent r4 10.0k q1 irll3303 + c3 100uf u1 sc1408is 8 1 2 3 4 5 6 7 isense gate bst fb shdn ref agnd gnd vout=5v (1) see component selection r3 31.6k l1b 22uh 4 3 + c8 100uf c5 0.1uf
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 6 fig. 3: achieving output voltages greater than 16.5v + c3 100uf 25v out q1 irl3103s l1 22uh d3 12v 3.3v in d1 b130t + c8 100uf r4 10.0k u1 sc1408is 8 1 2 3 4 5 6 7 isense gate bst fb shdn ref agnd gnd r2 0.05ohm r6 470 c4 0.1uf r3 200k + c2 100uf d4 1n4148 c5 0.1uf fig. 4: implementing shutdown with input/output isolation r3 90k 3.3v in u1 sc1408is 8 1 2 3 4 5 6 7 isense gate bst fb shdn ref agnd gnd l1 22uh + c2 100uf + c3 100uf 12v out c5 0.1uf d1 b130t c4 0.1uf r2 0.05ohm j1 1 2 r5 5.1k q1 irll3303 short = run open = shutdown + c8 100uf r4 10.0k q2 si2301ds
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 7 theory of operation the sc1408 is a modified hysteretic boost converter controller. the power switch is turned on when the out- put voltage falls slightly below it?s setpoint. it remains on for approximately 16 s, or until the inductor current reaches limit, whichever occurs first. the power switch is then turned off for 2.3 s, or until the output voltage once again falls below setpoint, whichever occurs last. the sc1408 is normally powered from the output volt- age. internal circuitry, such as the bandgap, compara- tors and one shots will not function properly until the bst pin voltage reaches 2.5v. to ensure startup at low input voltages, the normal control circuitry is disabled and a special, low voltage start up oscillator generates an ap- proximate square wave at the gate pin, initiating boost action. when the output voltage reaches 2.5v, the nor- mal control circuitry is enabled and the start up oscillator shuts down. to conserve power, a shdn pin is provided which, when pulled high, shuts down most internal circuitry. the output voltage will then be one diode drop below the in- put. component selection boost converter r sense the value of the sense resistor is the primary determin- ing factor for maximum output current. the sc1408 has a fixed current limit voltage threshold, which is devel- oped by the peak inductor current flowing through r sense . r sense may be determined either from the maxi- mum output current curves or from the equation below: in the equation above, the use of 2.3s for t off may lead to slightly optimistic current values for low v o /v in ratios. the theoretical curves use the actual value of t off , v f =0.5v, v fet =0.3v and v cs =0.08v and are generated for l=22 h. output voltage output voltage can be set to 5v by connecting the fb pin to gnd, or to any voltage in the 3.0v to 16.5v range using external divider resistors. the bottom resistor in the divider chain (r4) should be 300k ? or less and the top resistor (r3 in the application circuits) can be calculated from: inductor the sc1408 will work with a wide range of inductor val- ues. a good choice for most applications is 22 h. smaller inductor values result in higher peak currents and increased output ripple, while larger values will re- sult in slower loop response. transistor selection normally the power switch will be an n-channel mos- fet, although in certain circumstances an npn bipolar may be substituted. the choice of fet can be critical, especially in battery powered applications where the converter must be able to use all of the available energy in the battery. this re- quires that the converter be capable of starting up from very low input voltages. for example a two cell alkaline system?s terminal voltage will drop to 1.8v as it ap- proaches full discharge. for these demanding applica- tions, a fet with low v gs(th) is required. a good rule of thumb is that v gs(th) should be at least 0.5v less than the minimum input voltage. diode for most applications, a schottky diode should be used as the output rectifier. it will be subjected to reverse volt- ages of at least v o , and average current will be equal to the output current. industry standard 1n5817 series or an equivalent surface mount part would be suitable. output capacitors output capacitors should be low esr to minimize ripple voltage and maximize efficiency. low esr tantalums, oscons or the newer polymer capacitors should be used. ripple voltage will be approximately: input capacitors input capacitors on a boost converter are less critical than the output capacitors, since there are no fast cur- rent pulses drawn from the input supply. a 100 f tanta- lum will be adequate for most applications. ()( ) dcr inductor and r fet, across voltage v drop voltage forward diode output v : where v v v v v v v v l 2 t v v v v v v 1 r v i sense fet f fet f o in f o fet in off fet f o in f o sense cs ) max ( o = = ? + ? + ? ? ? + ? + ? = ? = 1 v v 4 r 3 r ref o sense esr cs ripple r r * v v =
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 8 feed forward capacitor although converters based on the sc1408 are stable, with no possibility of oscillation, they are susceptable to ?mode switching? at intermediate current levels. this leads to increased ripple voltage and slightly reduced efficiency. mode switching can be eliminated by a suit- able choice of c9. it is possible to empirically select c9. start with a value of about 100pf and adjust until all low frequency output ripple is eliminated at the desired out- put current. see an99-15 ?sc1408 operating modes? for a full dis- cussion of the origins of ?mode switching? and a more analytical approach to the selection of c9. component selection sepic converter r sense again, with the sepic topology, the value of the sense resistor is the primary determining factor for maximum output current. the simplest approach to select r sense is to add vin to vo and use this value as the output volt- age in the output current curves or in the equation for boost converter. output voltage output voltage setting works exactly the same in sepic topology as in boost, including the ability to set to 5v by connecting the fb pin to gnd. care must be taken to ensure that the ic supply (pin2; bst) does not exceed its 16.5v rating. in the circuit of fig.2: this requires max- imum output voltage to be limited to 16.5v-vin. higher output voltages are possible with different ic supply strategies. inductor the sepic topology requires a coupled inductor . again a good choice for most applications is 22uh. smaller in- ductor values result in higher peak currents and increase output ripple, while larger values will result in slower loop response. transistor selection the choice of fet can be critical, especially in battery powered applications where the converter must be able to use all of the available energy in the battery. this re- quires that the converter be capable of starting up from very low input voltages. for example a two cell alkaline system?s terminal voltage will drop to 1.8v as it ap- proaches full discharge. for these demanding applica- tions, a fet with low v gs(th) is required. a good rule of thumb is that v gs(th) should be at least 0.5v less than the minimum input voltage. diode for most applications, a schottky diode should be used as the output rectifier. it will be subjected to reverse volt- ages of at least v o +v in and average current equal to the output current. industry standard 1n5817 series or an equivalent surface mount part would be suitable. output capacitors output capacitors should be low esr to minimize ripple voltage and maximize efficiency. low esr tantalums, oscons or the newer polymer capacitors should be used. input capacitors input capacitors on a sepic converter are less critical than the output capacitors, since there are no fast cur- rent pulses drawn from the input supply. a 100 f tanta- lum will be adequate for most applications. series capacitors the series capacitor(s) must be capable of handling an rms current given by:- feed forward capacitor the feed forward capacitor should be selected in the same way as for a boost application. layout guidelines the sc1408 is fairly insensitive to layout, however fol- lowing some simple guidelines will help ensure success- ful implementation. whenever possible, lay the circuitry out over a ground plane. connect agnd and gnd together at the ic and return to the ground plane with one or two vias. place the current sense resistor (r2), the ref decoupling ca- pacitor (c5) and the divider resistors (r3 and r4) close to the ic, return the ground side of these components to the ground plane close to the agnd/gnd pin vias. connect the top side of the sense resistor to isense with a short trace and the source of q1 directly to the sense resistor. make the output loop q1, d1 and the output capacitors (c3, c8) as small as possible to mini- mize emi. in o o rms v 5 . 0 v i i + =
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 9 vo=3.3v 100mohm 50mohm 35mohm 25mohm 20mohm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2.0 2.2 2.4 2.6 2.8 3.0 3.2 vin (v) io (a) l=22uh vo=5v 100mohm 50mohm 35mohm 25mohm 20mohm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 vin (v) io (a) l=22uh vo=12v 100mohm 50mohm 35mohm 25mohm 20mohm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2.0 4.0 6.0 8.0 10.0 12.0 vin (v) io (a) l=22uh vo=15v 100mohm 50mohm 35mohm 25mohm 20mohm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 vin (v) io (a) l=22uh figs. 5 - 8: maximum output current vs. input voltage and sense resistor value. (boost mode) vo=15v 60% 70% 80% 90% 100% 1 10 100 1000 io (ma) efficiency (%) vin=12v vin=9.0v vin=5.0v vin=3.0v vin=1.8v vo=12v 60% 70% 80% 90% 100% 1 10 100 1000 io (ma) efficiency (%) vin=5.0v vin=3.0v vin=1.8v vo=5v 60% 70% 80% 90% 100% 1 10 100 1000 io (ma) efficiency (%) vin=5.0v vin=4.0v vin=3.3v vin=3.0v figs. 9-11:efficiency in the boost application circuit of fig.1
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 10 output ripple voltage; vin=3v, vo=5v, io=470ma ch1 = output ripple ch2 = voltage at gate pin output ripple voltage; vin=3v, vo=5v, io=810ma ch1 = output ripple ch2 = voltage at gate pin load transient; vin=3v, vo=5v, io=0 to 500ma ch1 = output voltage ch2 = load current (0.5a/div) load transient; vin=2v, vo=5v, io=0 to 500ma ch1 = output voltage ch2 = load current (0.5a/div)
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 11 outline drawing - so-8 jedec ref: ms-012aa land pattern - so-8
? 2000 semtech corp. 652 mitchell road newbury park ca 91320 low voltage, boost dc-dc controller sc1408 august 22, 2000 12 outline drawing - msop-8 land pattern - msop-8 ecn00-1275

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