DEMO MANUAL DC187/DC188 LT1374 Monolithic 4A Switcher 5.5V to 25V Input 3.3V or 5V Output DESCRIPTIO U

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1 EMO MNUL 187/188 EMO MNUL NO ESIGN 187/188 SWITHER LT137 Monolithic Switcher.V to V Input 3.3V or V Output ESRIPTIO U emonstration circuits 187/188 are complete / step-down regulators using the LT 137, constant frequency, high efficiency converter in 7-pin (187) and SO-8 (188) packages. These circuits are primarily used in personal computers, disk drives, portable hand- held devices and in larger systems, as local onboard regulators. High frequency switching allows the use of small inductors, making these all surface mount solutions ideal for space-conscious systems., LT and LT are registered trademarks of Linear Technology orporation. PERFORmNE SU U W WW RY T =, V IN = 1V, I LO =, V OUT = 3.3V (Jumper J1 inserted), SHN pin open unless otherwise noted. PRMETER ONITIONS MIN TYP MX UNITS Output Voltage Jumper J1 Removed (Note 1).91.. V Jumper J1 Inserted V Maximum I LO (Note ) Input Voltage Range (Note 3). V Switching Frequency 6 khz Output Ripple Voltage 6 mv P-P Line Regulation.V to V 8 mv Load Regulation I LO = 1m to mv SHN Lockout Threshold Shutdown Option Boards V SHN Shutdown Threshold Shutdown Option Boards V Synchronization Range SYN Option Boards 8 1 khz Supply urrent SHN = V µ Note 1: Output voltage variations include the ± tolerance of the feedback-divider network. For tighter voltage range, use higher tolerance resistors or a fixed V output device, the LT137-. Note : For 188 additional thermal restrictions apply. Note 3: For operating voltages down to V consult LT Marketing for details on the LT16. BOR PHOTOS

2 EMO MNUL 187/188 TYPIL PERFOR E HRTERISTIS UW EFFIIENY (%) V IN, V OUT Efficiency IE TEMPERTURE RISE ( ) Temperature Rise vs Load urrent V IN = 1V V OUT = V IE TEMPERTURE RISE ( ) Temperature Rise vs Time, 188 V IN = 1V V OUT = V I OUT = I OUT = 3 I OUT = I OUT = LO URRENT () 1 3 LO URRENT () 1 3 TIME (SEONS) 187/88 T1 187/88 T 187/88 T3 PKGE SHE TI IGR SM W U MMB91LT1 W 187 E1 V IN.V TO V E S/ E µF V *SYN FUNTION REPLES SHN FOR LT137R-SYN PRTS 3 OPTIONL * 6.33µF 16V BOOST V IN SW U1 LT137R SHN FB V R1 OPTIONL OPTIONL L1 6.8µH 1 MBR83L 1 1pF V + 1µF 1V + 7 OPTIONL TB IS R.36k J1 3.3V/V LINK R3.99k R.7k FRONT VIEW R PKGE 7-LE PLSTI PK + 6.7µF V LT137R (LT137R-SYN) E V OUT 3.3V/ OR V/ E 187 S FB BOOST V IN V SW SHN (SYN) V

3 PKGE SHE TI IGR SM W U MMB91LT1 EMO MNUL 187/188 W 188 E1 V IN.V TO V E S/ E µF V 3 OPTIONL 1 7* *SYN FUNTION REPLES SHN FOR LT137S8-SYN PRTS.33µF 16V BOOST V IN U1 SW LT137S8 BIS SHN FB V R1 OPTIONL OPTIONL L1 6.8µH 1 MBR83L 1 1pF V + 1µF 1V + 7 OPTIONL R.36k J1 3.3V/V LINK R3.99k R.7k V IN 1 BOOST FB 3 F + 6.7µF V E V OUT 3.3V/ OR V/ E 188 S TOP VIEW S8 PKGE 8-LE PLSTI SO 8 V SW 7 SHN (SYN) 6 V BIS LT137S8 (LT137S8-SYN) PRTS LIST REFERENE ESIGNTOR QUNTITY PRT NUMBER ESRIPTION VENOR TELEPHONE MTS 1pF V X7R hip apacitor VX (83) 96-36, 7 Optional apacitor 3 1 THR6E1H16ZT 1µF V YU hip apacitor Marcon (87) Y33MTS.33µF 16V X7R hip apacitor VX (83) TPS17M1R1 1µF 1V TPS Tantalum apacitor VX (7) G7MT3S.7µF V YV hip apacitor VX (83) MBR83L SMT iode Motorola (6) MMB91LT1 iode Motorola (6) Optional iode E1 to E 1- Turret Mill-Max (16) 9-6 J1 1 8S--G.79" enter -Pin Header omm on (818) 31- P1 1 IJMM-138-G.79" enter -Pin Shunt omm on (818) 31- L1 1 O3316P µH % Inductor oilcraft (87) R1 1 Optional Resistor R 1 R1-361F-T.36k 1/8W hip Resistor T (71) -913 R3 1 R1-991F-T.99k 1/8W hip Resistor T (71) -913 R 1 R1-71F-T.7k 1/8W hip Resistor T (71) -913 U1 (187) 1 LT137R (-SYN) I LT (8) 3-19 U1 (188) 1 LT137S8 (-SYN) I LT (8)

4 EMO MNUL 187/188 OPERTIO U 187 vs 188 (Temperature vs Package Size) The 187 and 188 demonstration boards are intended for evaluation of the LT137 switching regulator in the 7-pin and SO-8 packages, respectively. The boards are electrically identical; they differ only in package layout (the BIS pin is not available in the package). The primary reason for choosing the SO-8 over the package is board space. The 187 ( package) occupies an active board area of approximately.7 square inches. Optimizing the 188 board, by using a Sumida coil and removing the layout options and voltage-selector jumper, a total active area of. square inches is possible. The package is more suitable for higher power or higher ambient-temperature applications. lthough both boards will supply of output current, the 188 must be thermally derated to 3 continuous at ambient to prevent excessive die temperatures. The 187 can run at 7 ambient at output current. The SO-8 package can, however, be used for dynamic loads up to the full rated switch current. The Temperature Rise vs Time, 188 graph shows the dynamic thermal response of the 188 board for loads up to. LT137 Operation The LT137 data sheet gives a complete description of the part, operation and applications information. The data sheet must be read in conjunction with this demo manual. Hook-Up Solid turret terminals are provided for easy connection to supplies and test equipment. The jumper should be inserted if a 3.3V output is required. It should be removed for V operation. onnect a V to V,. power supply across the V IN and terminals and the load across the V OUT and terminals. When measuring load/line regulation, remember to Kelvin connect to the turrets. lso, when measuring output ripple voltage with an oscilloscope probe, the wire from the probe to the ground clip will act as an antenna, picking up noise. For improved results, the ground clip should be removed from the probe. The tip should be touched against the output turret, with the bare ground shield pressed against the ground turret. This reduces the noise seen on the waveform. Shutdown Option (evice Markings LT137R or 137) For normal operation, the S/ pin can be left floating. S/ has two output-disable modes, lockout and shutdown. When the pin is taken below the lockout threshold, switching is disabled. This is typically used for input undervoltage lockout. Grounding the S/ pin places the LT137 in shutdown mode. This reduces total board supply current to µ. Synchronization Option (evice Markings LT137R-SYN or 137SN) For normal operation the S/ pin can be left floating. To synchronize switching to an external clock, apply a logiclevel signal to the S/ pin. mplitude must be from a logical low to greater than.v with a duty cycle from 1% to 9%. Synchronization frequency must be greater than that of the free-running oscillator and less than 1MHz. dditional circuitry may be required to prevent subharmonic oscillation refer to the data sheet for more details. OMPONENTS Inductor L1 The inductor is a oilcraft O3316P-68, a 6.8µH unshielded ferrite unit. It is selected for its low cost, small size and.6 I ST rating. The equivalent oiltronics UP- 6R8 unit can be substituted. If board space is at a premium and higher ripple current is acceptable, 188 has the solder pads available for the Sumida 3-1R8 inductor. This 1.8µH unit has a.9 I ST rating. With this coil, ripple at 1V IN, V OUT is ±1.. This gives a maximum output current of (. 1.) = 3. t input voltages above 1V, 7 should be inserted in parallel with to increase output capacitor ripple rating. Input/Output apacitors 3,, 6 and 7 The input capacitor, 3, is a Tokin ceramic capacitor. It was selected for its small size, high voltage rating and low ESR (effective series resistance). The input ripple current for a buck converter is high, typically I OUT /. Tantalum capacitors become resistive at higher frequencies, requiring careful ripple-rating selection to prevent excessive

5 EMO MNUL 187/188 OPERTIO U heating. eramic capacitors ESL (effective series inductance) tends to dominate their ESR, making them less susceptible to ripple-induced heating. The output capacitor,, is an VX tantalum capacitor. ceramic is not recommended as the main output capacitor since loop stability relies on a resistive characteristic at higher frequencies to form a zero. The VX TPS series, specifically designed for use in switch mode power supplies, has very low ESR. t switching frequencies, ripple voltage is more a function of ESR than of absolute capacitance value. If lower output ripple voltage is required, use the optional capacitor, 7, to reduce ESR, rather than increasing the capacitance of. For very low ripple, an additional L filter on the output may be a cheaper solution. The output contains very narrow voltage spikes because of the parasitic inductance of. small ceramic capacitor, 6, removes these spikes on the demo board. In application circuits, trace inductance and local bypass capacitors may perform this function, negating the need for 6. atch iode 1 Use diodes designed for switching applications, such as Schottky or ultrafast diodes, with adequate current rating and fast turn-on times. In selecting a diode, the basic parameters of interest are forward voltage, maximum reverse voltage, average operating current and peak current. Lower forward voltage yields higher circuit efficiency and lowers power dissipation in the diode. The MBR83L has a maximum forward drop of.v at 3. The reverse voltage rating must be greater than the input voltage. verage diode current is always less than output current, but under a shorted output condition, diode current can equal the switch current limit. If the application must withstand this condition, the diode must be rated for maximum switch current. ompensation: 1, and R1 detailed discussion of frequency compensation can be found in the LT137 data sheet. 1, a 1pF capacitor from V to ground, gives a stable loop response over a wide range of input and output conditions. Options R1 and are included to optimize the dynamic response for specific applications. Boost:, 3 and boost voltage of at least.8v is required throughout the on time of the switch to guarantee that it remains saturated. For output voltages of 3.3V or more, diode provides sufficient boost voltage to. Below 3.3V, can be moved to position 3, powering boost from V IN. PB LYOUT In many cases, the layout of the demonstration board may be dropped directly into the application with minimal changes. If this is not practical, there are several precautions that must be taken when laying out high frequency converter circuits. The high frequency switching path runs from ground, through 3 to the V IN pin of the LT137, out of the SW pin, through 1 and back to ground. This loop acts as an antenna and will radiate noise if not kept as short as possible. lso, at higher switching currents the associated trace inductance can cause excessive voltage spikes across the switch. The use of a ground plane will reduce many noise problems. The ground pin of the LT137 contains some high frequency signal currents, but more importantly, it is the V reference for the output voltage. onnect the ground pin directly to the ground plane. The FB and V components should be kept away from the power components as much as possible. The ground for these components should be separated from power grounds. Run a Kelvin sense to V OUT as required, but keep the divider network close to the LT137 to prevent noise pick-up on the FB node. Noise pickup on the V pin appears as various problems, including poor load regulation, subharmonic oscillation and instability. Thermal management must also be considered. The SO-8 package has a fused ground pin. Soldering this pin to a large copper area will significantly reduce its thermal resistance. Solder filled feedthroughs close to the ground pin provide a good thermal path to the ground plane. For the package, the grounded tab should be treated in the same way. For more information or advice, contact the LT pplications department.

6 EMO MNUL 187/188 U W PB LYOUT FIL 187 omponent Side Silkscreen omponent Side omponent Side Solder Mask Solder Side Solder Side Solder Mask Pastemask 6

7 EMO MNUL 187/188 U W PB LYOUT FIL 188 omponent Side Silkscreen omponent Side omponent Side Solder Mask Solder Side Solder Side Solder Mask Pastemask Information furnished by Linear Technology orporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology orporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 7

8 EMO MNUL 187/188 P FB RWI GS U 187. B. NOTES: UNLESS OTHERWISE SPEIFIE 1. MTERIL: FR OR EQUIVLENT EPOXY, OZ OPPER L, THIKNESS.6 ±.6 TOTL OF LYERS. FINISH: LL PLTE HOLES.1 MIN/.1 MX OPPER PLTE, ELETROEPOSITE TIN-LE OMPOSITION BEFORE REFLOW, SOLER MSK OVER BRE OPPER (SMOB) 3. SOLER MSK: BOTH SIES USING GREEN SR1 OR EQUIVLENT. SILKSREEN: USING WHITE NONONUTIVE EPOXY INK. LL IMENSIONS IN INHES SYMBOL B IMETER TOTL HOLES NUMBER OF HOLES F 188. E E. NOTES: UNLESS OTHERWISE SPEIFIE 1. MTERIL: FR OR EQUIVLENT EPOXY, OZ OPPER L, THIKNESS.6 ±.6 TOTL OF LYERS. FINISH: LL PLTE HOLES.1 MIN/.1 MX OPPER PLTE, ELETROEPOSITE TIN-LE OMPOSITION BEFORE REFLOW, SOLER MSK OVER BRE OPPER (SMOB) 3. SOLER MSK: BOTH SIES USING GREEN SR1 OR EQUIVLENT. SILKSREEN: USING WHITE NONONUTIVE EPOXY INK. LL IMENSIONS IN INHES B SYMBOL B E IMETER TOTL HOLES NUMBER OF HOLES F 8 Linear Technology orporation 163 Mcarthy Blvd., Milpitas, (8) 3-19 FX: (8) 3-7 TELEX: dc1878 LT/TP 98 PRINTE IN US LINER TEHNOLOGY ORPORTION 1998