BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 1 / 9 synchronous buck converter in a sot23-6 plastic package. high efficiency: up to 93%,600khz frequency oper ation,2a output curre nt, no schottky diode required,3v to 18v input volt age range,0.6v reference, slope compensated current mode control for excellent line and load transient response, integrated internal compensation, stable with low esr ceramic output capacitors, over current protection with hiccup-m ode, thermal shutdown, inrush current limit and soft start, available in sot23-6 package,-40c to +85c temperature range. distributed power systems,digital set top boxes, flat panel television and monitors,wireless and dsl modems,notebook computer. ? / marking see marking instructions. / descriptions / features ? / applications ? / equivalent circuit / pinning pin name pin number pin function bs 1 bootstrap. a capacitor connected between lx and bst pins is required to form a floating supply across the high-side switch driver. gnd 2 ground fb 3 adjustable version feedback input. connect fb to the c enter point of the external resistor divider. en 4 drive this pin to a logic-high to enable the ic. drive to a logic-low to disable the ic and enter micro-power shutdown mode. vin 5 power supply pin lx 6 switching pin 1 3 2 4 5 6
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 2 / 9 parameter symbol ? rating unit vin voltage v dd -0.3 to 20 v lx voltage v m -0.3 to (vin+0.5v) v en voltage v co -0.3 to 20 v fb voltage v do -0.3 to 6 v bs voltage t opr -0.3 to (vlx+5v) v junction temperature t stg 150 c operating temperature -40c to +85 c parameter symbol test conditions ? min ? typ ? max unit input voltage range v dd 4.5 16 v uvlo threshold c o 3.0 v supply current in operation v st ven=2.0v, vfb=1.1v 0.4 0.6 ma supply current in shutdown r short ven =0 or en = gnd 1 ua regulated feedback voltage t a = 25c, 4.5v vin 18v 0.588 0.6 0.612 v high-side switch on-resistance 90 m ? low-side switch on-resistance 70 m ? high-side switch leakage current ven=0v, vlx=0v 0 10 ua upper switch current limit i dd minimum duty cycle 3 a oscillation frequency i st1 0.6 mhz maximum duty cycle vfb=0.6v 92 % minimum on-time 60 ns thermal shutdown 160 ? / absolute maximum ratings(ta=25 ) ? / electrical characteristics(ta=25 )
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 3 / 9 internal regulator the BRCD3470 is a current mode st ep down dc/dc converter that pr ovides excellent transient response with no extra external compensation co mponents. this device contains an internal, low resistance, high voltage power mosfet, and operates at a high 600k operating frequency to ensure a compact, high efficiency design with excellent ac and dc performance. error amplifier the error amplifier compares the fb pin voltage wi th the internal fb reference (vfb) and outputs a current proportional to the difference between the tw o. this output current is then used to charge or discharge the internal compensation network to form the comp voltage, which is used to control the power mosfet current. the optimized internal compensation network minimizes the external component counts and simplifies the control loop design. internal soft-start the soft-start is implemented to prevent the converter output voltage from overshooting during startup. when the chip starts, the internal circuitry generates a soft-start voltage (ss) ramping up from 0v to 0.6v. when it is lower than the internal reference (r ef), ss overrides ref so the error amplifier uses ss as the reference. when ss is hi gher than ref, ref regains control. the ss time is internally fixed to 1 ms. over-current-protection and hiccup the BRCD3470 has cycle-by-cycle over current li mit when the inductor current peak value exceeds the set current limit threshold. meanwhile, outpu t voltage starts to drop until fb is below the under-voltage (uv) threshold, typically 30% below the reference. once a uv is triggered, the BRCD3470 enters hiccup mode to periodically restar t the part. this protec tion mode is especially useful when the output is dead-short to ground. the average short ci rcuit current is greatly reduced to alleviate the thermal issue and to protect the regulator. the BRCD3470 ex its the hiccup mode once the over current condition is removed. startup and shutdown if both vin and en are higher than their appropriate thresholds, the chip star ts. the reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. the regulator provides stable supply for the remain ing circuitries. three events can shut down the chip: en low, vin low and thermal shutdown. in t he shutdown procedure, the signaling path is first blocked to avoid any fault triggering. the comp voltage and the internal supply rail are then pulled down. the floating driver is not subject to this shutdown command.
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 4 / 9 setting the output voltage the external resistor divider is used to set the output voltage (see typical a pplication on page 1). the feedback resistor r1 also sets the feedback loop bandwid th with the internal compensation capacitor. choose r1 to be around 100k ? for optimal transient respons e. r2 is then given by: 1 / 1 2 ? = fb out v v r r selecting the inductor a 4.7 h to 22 h inductor with a dc current rating of at least 25% percent higher than the maximum load current is recommended for most applications. for highest effici ency, the inductor dc resistance should be less than 15m ? . for most designs, the inductance val ue can be derived from the following equation. osc l in out in out f i v v v v l ? = ) ( where il is the inductor ripple current. choose inductor ripple current to be appr oximately 30% if the maximum load current, 2a. the maximum inductor peak current is: 2 ) ( l load max l i i i + = under light load conditions below 100ma, larger inductance is recommended for improved efficiency. selecting the output capacitor the output capacitor (c2) is required to maintain the dc output voltage. ceramic, tantalum, or low esr electrolytic capacitors are recommended. low esr capacitors are preferred to keep the output voltage ripple low. the output vo ltage ripple can be brcdmated by: ? ? ? ? ? ? + ? ? ? ? ? ? ? = 2 8 1 1 c f r v v l f v v s esr in out s out out
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 5 / 9 where l is the inductor value and resr is the equiva lent series resistance (esr) value of the output capacitor. in the case of ceramic capacitors, the impedance at the switch ing frequency is dominated by the capacitance. the output volt age ripple is mainly caused by the capacitance. for simplification, the output voltage ripple can be brcdmated by: ? ? ? ? ? ? ? = in out s out out v v c l f v v 1 8 2 2 in the case of tantalum or electrolytic capacito rs, the esr dominates the impedance at the switching frequency. for simplification, the output ripple can be approximated to: es r in out s out out r v v l f v v ? ? ? ? ? ? ? = 1 the characteribrcdcs of the output capacitor also af fect the stability of t he regulation system. the BRCD3470 can be optimized for a wide range of capacitance and esr values. pcb layout guide pcb layout is very important to achieve stable operation. it is highly reco mmended to duplicate evb layout for optimum performance. if change is ne cessary,please follow these guidelines and take figure 4 for reference. 1) keep the path of switching current short and mi nimize the loop area formed by input capacitor, high-side mosfet and low-side mosfet. 2) bypass ceramic capacitors are suggest ed to be put close to the vin pin. 3) ensure all feedback connections are shor t and direct. place the feedback resistors and compensation components as close to the chip as possible. 4) vout, lx away from sensitive analog areas such as fb. 5) connect in, lx, and especially gnd respectively to a large copper area to cool the chip to improve thermal performance and long-term reliability.
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 6 / 9 ?? / block diagram
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 7 / 9 ?? / package dimensions
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 8 / 9 ?? / marking instructions ? 3470 ??? **** ???? note: 3470: product type code. ****: lot no. code, code change with lot no. 3470 ****
BRCD3470 rev.d oct.-2015 data sheet http://www.fsbrec.com 9 / 9 ?? ( ? ) / temperature?profile?for?ir?reflow?soldering(pb-free) ? note: 1 ?? 25 150 ? 60 90sec; 1.preheating:25~150 , time:60~90sec. 2 ?? 245 5 ?? 5 0.5sec; 2.peak temp.:245 5 , duration:5 0.5sec. 3 ???? 2 10 /sec. 3. cooling speed: 2~10 /sec. ? / resistance to soldering heat test conditions ?? 260 5 ?? 10 1 sec. temp.:260 5 time:10 1 sec ? / packaging spec. ?? / reel package type ?? units ? dimension ?? (unit mm 3 ) units/reel ? / reels/inner box / units/inner box ? / inner boxes/outer box / units/outer box ? / reel inner box outer box sot23-5/6 3,000 10 30,000 4 120,000 7 8 210205205 445230435 ?? / notices
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