CD54-101

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Daisy-Chained Control Outputs

FEATURES

1504A is a self-contained, high efficiency syn- chronous buck switching regulator. It includes a pair of on-chip 1.3W power switches, enabling it to supply up to 500mA of load current. Efficiency peaks at 92%, minimiz- ing heat and wasted power. The synchronous buck archi- tecture allows the output to source or sink current as required to keep the output voltage in regulation. 100% duty cycle operation minimizes dropout voltage. The LTC1504A is available in adjustable and fixed 3.3V output versions. An adjustable current limit circuit pro- vides protection from overloads. The internal 1% refer- ence combined with a sophisticated voltage feedback loop provides optimum output voltage accuracy and fast load transient response. The LTC1504A is specified to operate with input voltages between 4V and 10V. Contact the LTC factory for guaranteed specifications at 2.7V supply. The LTC1504A is a pin-compatible, functional upgrade to the LTC1504. The LTC1504A is available in a plastic SO-8 package. DESCRIPTIONU n 500mA Output Current at 3.3V Output n Up to 92% Peak Efficiency n 100% Maximum Duty Cycle n Internal Reference Trimmed to 1% n Output Can Source or Sink Current n Requires as Few as Four External Components n Input Voltage Range: 4V to 10V n Adjustable Current Limit n Small SO-8 Package n 200kHz Switching Frequency Can Typically be Synchronized Up to 500kHz Minimum Part Count 5V to 3.3V Regulator 5V to 3.3V Efficiency LOAD CURRENT (mA) EFFICIENCY (%) 100 100 500 1504 • TA02 IMAX SHDN SS COMP VCC GND SW SENSE SHUTDOWNNC NC 1000pF 3.3V AT 500mA LEXT 50µH CIN: AVX TPSC226M016R0375 COUT: AVX TAJC476M010 LEXT: COILTRONICS CTX50-1P 1504A • TA01 LTC1504A-3.3CIN 22µF COUT 47µF APPLICATIONSU n Small Portable Digital Systems n Active Termination n Auxiliary Output Voltage Supplies n Minimum Part Count/Size Switchers TYPICAL APPLICATIONU , LTC and LT are registered trademarks of Linear Technology Corporation.

ELECTRICAL CHARACTERISTICS

The l denotes specifications which apply over the full operating temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. Note 3: This parameter is guaranteed by correlation and is not tested directly. Note 4: LTC1504A quiescent current is dominated by the gate drive current drawn by the onboard power switches. With FB or SENSE pulled to VCC the output stage will stop switching and the static quiescent current can be observed. With FB or SENSE hooked up normally, the output stage will be switching and total dynamic supply current can be measured. Note 5: Fixed output parts will appear to have g mV and AV values 2.6 times lower than the specified values, due to the internal divider resistors. Note 6: The ILIM amplifier can sink but not source current. Under normal (not current limited) operation, the ILIM output current will be zero. Note 7: Contact factory for guaranteed specifications at 2.7V supply. TYPICAL PERFORMANCE CHARACTERISTICSUW TEMPERATURE (°C) –50 3.5 3.0 2.5 2.0 1.5 1.0 0.5 25 75 1504A • TPC03 –25 0 50 100 125 SWITCH ON-RESISTANCE (Ω ) VCC = 10V VCC = 5V VCC = 3.3V Switch On-Resistance vs TemperatureSupply Current vs Supply Voltage SUPPLY VOLTAGE (V) 2.5 1504A • TPC01 5 7.5 10 SUPPLY CURRENT (mA) VFB = VOUT VFB = VCC TA = 25°C IOUT = 0 Current Limit Threshold vs RIMAX RIMAX (Ω ) 10k CURRENT LIMIT THRESHOLD (mA) 700 600 500 400 300 200 100 100k 1504A • TPC04 TA = 25°C VCC = 5V TEMPERATURE (°C) –50 –25 0 25 50 75 100 125 SUPPLY CURRENT (mA) 0.1 1504A • TPC02 VFB = VOUT VFB = VCC VCC = 5V IOUT = 0 Supply Current vs Temperature Shutdown Threshold vs Supply Voltage SUPPLY VOLTAGE (V) 35 7 SHUTDOWN PIN THRESHOLD (V) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 1504A • TPC07 Current Limit Threshold vs Temperature TEMPERATURE (°C) –50 CURRENT LIMIT THRESHOLD (mA) –25 0 25 50 75 1504A • TPC05 125 500 450 400 350 300 250 200 150 100 100 VCC = 5V RIMAX = 47k RIMAX = 22k SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS ISS Soft Start Source Current V SS = 0V, Commercial l –8 –12 –16 mA VSS = 0V, Industrial l –8 –12 –17 mA tr, tf Output Switch Rise/Fall Time 10% to 90% Ouput Swing l 55 0 n s DCMAX Maximum Duty Cycle V COMP = VCC l 100 % VCC = 5V, TA = 25°C unless otherwise specified. (Note 2)

voltage. The LTC1504A feedback loop will servo the FB pin to 1.265V. SENSE (LTC1504A-3.3) (Pin 5): Output Voltage Sense. Connect directly to the output voltage node. The LTC1504A-3.3 feedback loop will servo SENSE to 3.3V. SENSE is connected to an internal resistor divider which will load any external dividers. For output voltages other than 3.3V, use the LTC1504A. SHDN (Pin 6): Shutdown, Active Low. When SHDN is at a logic High, the LTC1504A will operate normally. When SHDN is Low, the LTC1504A ceases all internal operation and supply current drops below 1mA. In shutdown, the SW pin is pulled low. This ensures that the output is actively shut off when SHDN is asserted, but it prevents other supplies from providing power to the output when the LTC1504A is inactive. See the Applications Information section for more details. SS (Pin 7): Soft Start. Connect an external capacitor (usually 0.1mF) from SS to GND to limit the output rise time during power-up. C SS also compensates the current limit loop, allowing the LTC1504A to enter and exit current limit cleanly. See the Applications Information section for more details. COMP (Pin 8): External Compensation. An external RC network should be connected to COMP to compensate the feedback loop. COMP is connected to the output of the internal error amplifier. I MAX (Pin 1): Current Limit Set. Connect a resistor from VCC to IMAX to set the current limit threshold. An internal 12mA current source from I MAX to GND sets the voltage drop across this resistor. This voltage is compared to the voltage drop across the internal high-side switch (Q1) while it is turned on. See the Applications Information section for more information. To disable current limit, leave I MAX floating. VCC (Pin 2): Power Supply Input. Connect to a power supply voltage between 4V and 10V. V CC requires a low impedance bypass capacitor to ground, located as close as possible to the LTC1504A. See the Applications Infor- mation section for details on capacitor selection and placement. SW (Pin 3): Power Switch Output. This is the switched node of the buck circuit. Connect SW to one end of the external inductor. The other end of the inductor should be connected to C OUT and becomes the regulated output voltage. Avoid shorting SW to GND or VCC. GND (Pin 4): Ground. Connect to a low impedance ground. The input and output bypass capacitors and the feedback resistor divider (adjustable parts only) should be grounded as close to this pin as possible. Pin 4 acts as a heat sink in the LTC1504A S0-8 package and should be connected to as large a copper area as possible to improve thermal dissipation. See the Thermal Considerations section for more information. FB (LTC1504A) (Pin 5): Feedback. Connect FB to a resistor divider from V OUT to GND to set the regulated output

APPLICATIONS INFORMATIONWU UU 3W , depending on supply voltage. This high power pulse train is filtered by the external inductor and capacitor, providing a steady DC value at the output node. This node returns to FB or SENSE, closing the loop. The LTC1504A includes a second feedback loop that controls operation in current limit. The I LIM amplifier monitors the voltage at the SW pin while Q1 is on. It compares this voltage to the voltage at the IMAX pin. As the peak current through Q1 rises, the voltage drop across it due to its RON increases proportionally. When SW drops below I MAX, indicating the current through Q1 has in- creased beyond the desired value, I LIM starts pulling a controlled amount of current out of SS, the external soft start pin. As SS falls, it pulls COMP down with it, limiting the duty cycle and reducing the output voltage to control the current. The speed at which the current limit circuit reacts is set by the value of the external soft start capacitor. EXTERNAL COMPONENT SELECTION External components required by the LTC1504A fall into three categories: input bypass, output filtering and com- pensation. Additional components to set up soft start and current limit are usually included as well. A minimum LTC1504A circuit can be constructed with as few as four external components; a circuit that utilizes all of the LTC1504A’s functionality usually includes eight or nine external components, with two additional feedback resis- tors required for adjustable parts. See the Typical Applica- tions section for examples of external component hookup. Input Bypass The input bypass capacitor is critical to proper LTC1504A operation. The LTC1504A includes a precision reference and a pair of high power switches feeding from the same V CC pin. If V CC does not have adequate bypassing, the switch pulses introduce enough ripple at V CC to corrupt the reference voltage and the LTC1504A will not regulate accurately. Symptoms of inadequate bypassing include poor load regulation and/or erratic waveforms at the SW pin. If an oscilloscope won’t trigger cleanly when looking at the SW pin, the LTC1504A doesn’t have adequate input bypass. output bypass capacitors and a compensation network complete the control loop. The LTC1504A adjustable output parts require an additional pair of resistors to set the output voltage. The LTC1504A-3.3 parts include an onboard resistor divider preset to a 3.3V output voltage. A functional 3.3V output regulator can be constructed with an LTC1504A-3.3 and as few as four external components. The LTC1504A feedback loop includes a precision refer- ence trimmed to 1% (V REF ), a wide bandwidth transconductance feedback amplifier (FB) and an onboard PWM generator (SAW and PWM). The PWM generator is capable of generating pulse widths from 0% to 100%, minimizing dropout and maximizing transient response. The internal sawtooth oscillator typically runs at 200kHz. Q1 and Q2 are capable of carrying peak currents in excess of 500mA, with the continuous output power level limited primarily by the thermal dissipation of the SO-8 package. With a 5V input and a 3.3V output, the LTC1504A can supply 500mA of continuous output current with an appropriate layout. An on-chip current limit circuit, set with a single external resistor, can be used to help limit power dissipation. See the Thermal Considerations sec- tion for more information. Theory of Operation The LTC1504A primary feedback loop consists of the main error amplifier FB, the PWM generator, the output drive logic and the power switches. The loop is closed with the external inductor and the output bypass capacitor. The feedback amplifier senses the output voltage directly at the SENSE pin for fixed output versions or through an external resistor divider in the adjustable output version. This feedback voltage is compared to the 1.265V internal reference voltage by FB and an error signal is generated at the COMP pin. COMP is a high impedance node that is brought out to an external pin for optimizing the loop compensation. COMP is compared to a 200kHz sawtooth wave by com- parator PWM. The output stage takes the PWM signal and generates nonoverlapping drive for the onboard P- and N- channel power MOSFETs, which drive the SW pin with a low impedance image of the PWM waveform. Typical open-loop output impedance at SW is between 1 W and

impedance to keep the LTC1504A happy in some circuits. tors is more important to capacitor selection than value. bypass capacitors in LTC1504A applications. Table 1. Representative Surface Mount Input Bypass Capacitors *Note: Use multiple devices in parallel or limit output current to prevent capacitor overload. and an inductance between 33mH and 220mH. current and price/current relationship of an inductor. well in LTC1504A applications.

Table 2. Representative Surface Mount Inductors and cannot change instantaneously. rent, the output capacitor has to make up the difference. quire relatively large value, low ESR output capacitors. in typical LTC1504A applications. first order and reduces the phase shift to 90°. mize transient response with most output capacitors. Kool Mm is a registered trademark of Magnetics, Inc..

APPLICATIONS INFORMATIONWU UU output voltage is very close to ground. Under this condi- tion, the LTC1504A must run at extremely narrow duty cycles (< 5%) to keep the current under control. When the on-time falls below the time required to sense the current in Q1, the LTC1504A responds by reducing the oscillator frequency, increasing the off-time to decrease the duty cycle and allow it to maintain some control of the output current. The oscillator frequency may drop by as much as a factor of 10 under severe current overloads. Under extreme short circuits (e.g., screwdriver to ground) the on-time will reduce to the point where the LTC1504A will lose control of the output current. At this point, output current will rise until the inductor saturates, and the current will be limited by the parasitic ESL of the inductor and the R ON of Q2 inside the LTC1504A. This current is usually nondestructive and dissipates a limited amount of power since the output voltage is very low. A typical LTC1504A circuit can withstand such a short for many seconds without damage. The test circuit in Figure 1 will typically withstand a direct output short for more than 30 seconds without damage to the LTC1504A. Eventually, however, a continuous short may cause the die tempera- ture to rise to destructive levels. Note that the current limit is primarily designed to protect the LTC1504A from damage and is not intended to be used to generate an accurate constant-current output. As the die temperature varies in a current limited condition, the R ON of the internal switches will change and the current limit threshold will move around. RON will also vary from part to part due to manufacturing tolerance. The external IMAX resistor should be chosen to allow enough room to account for these variations without allowing the current limit to engage at the maximum expected load current. A current limit setting roughly double the expected load is often a good compromise, eliminating unintended current limit operation while preventing circuit destruction under actual fault conditions. If desired, current limit can be disabled by floating the I MAX pin; the internal current source will pull IMAX to GND and the ILIM amplifier will be disabled. Shutdown The LTC1504A includes a micropower shutdown mode controlled by the logic level at SHDN. A logic High at SHDN allows the part to operate normally. A logic Low at SHDN stops all internal switching, pulls COMP, SS and SW to GND and drops quiescent current below 1 mA typically. Note that the internal N-channel power MOSFET from SW to GND turns on when SHDN is asserted. This ensures that the output voltage drops to zero when the LTC1504A is shut down, but prevents other devices from powering the output when the LTC1504A is disabled. External Clock Synchronization The LTC1504A SHDN pin can double as an external clock input for applications that require a synchronized clock or a faster switching speed. The SHDN pin terminates the internal sawtooth wave and resets the oscillator immedi- ately when it goes low, but waits 50 ms before shutting down the rest of the internal circuitry. A clock signal applied directly to the SHDN pin will force the LTC1504A internal oscillator to lock to its frequency as long as the external clock runs faster than the internal oscillator frequency. Attempting to synchronize to a frequency lower than the 250kHz maximum internal frequency may result in inconsistent pulse widths and is not recom- mended. Because the sawtooth waveform rises at a fixed rate internally, terminating it early by synchronizing to a fast external clock will reduce the amplitude of the sawtooth wave that the PWM comparator sees, effectively raising the gain from COMP to SW. 500kHz is the maximum recommended synchronization frequency; higher frequen- cies will reduce the sawtooth amplitude to the point that the LTC1504A may run erratically. THERMAL CONSIDERATIONS Each of the LTC1504A internal power switches has ap- proximately 1.3W of resistance at room temperature and will happily carry more than the rated maximum current if the current limit is set very high or is not connected. Since the inductor current is always flowing through one or the other of the internal switches, a typical application supply- ing 500mA of load current will cause a continuous dissi- pation of approximately 325mW. The SO-8 package has a thermal resistance of approximately 90 °C/W, meaning that the die will begin to rise toward 30°C above ambient

at this power level. The RON of the internal power switches increases as the die temperature rises, increasing the power dissipation as the feedback loop continues to keep the output current at 500mA. At high ambient tempera- tures, this cycle may continue until the chip melts, since the LTC1504A does not include any form of thermal shutdown. Applications can safely draw peak currents above the 500mA level, but the average power dissipation should be carefully calculated so that the maximum 125°C die temperature is not exceeded. The LTC1504A dissipates the majority of its heat through its pins, especially GND (Pin 4). Thermal resistance to ambient can be optimized by connecting GND to a large copper region on the PCB, which will serve as a heat sink. mum power levels or which must withstand short circuits of extended duration should maximize the copper area at all pins and ensure that there is some airflow over the part to carry away excess heat. For layout assistance in situa- tions where power dissipation may be a concern, contact the LTC Applications Department. The current limit circuit can be used to limit the power under mild overloads to a safe level, but severe overloads where the output is shorted to ground may still cause the die temperature to rise dangerously. For more information on current limit behavior, see the Current Limit section. LAYOUT CONSIDERATIONS Like all precision switching regulators, the LTC1504A requires special care in layout to ensure optimum perfor- mance. The large peak currents coupled with significant DC current flow will conspire to keep the output from regulating properly if the layout is not carefully planned. A poorly laid out op amp or data converter circuit will fail to give the desired performance, but will usually still act like an op amp or data converter. A poorly laid out LTC1504A circuit may look nothing at all like a regulator. Wire-wrap or plug-in prototyping boards are not useful for bread- boarding LTC1504A circuits! Open-core inductors lo- cated close to the LTC1504A can cause erratic regulation due to stray flux coupled into PC board traces or the LTC1504A itself. Changing the orientation of the inductor or switching to a shielded type will solve the problem. Perhaps most critical to proper LTC1504A performance is the layout of the ground node and the location of the input and output capacitors. The negative terminals of both the input and output bypass capacitors should come together at the same point, as close as possible to the LTC1504A ground pin. The compensation network and soft start capacitor can be connected together with their own trace, which should come directly back to this same common ground point. The input supply ground and the load return should also connect to this common point. Each ground line should come to a star connection with Pin 4 at the center of the star. This node should be a fairly large copper region to act as a heat sink if required. Second in importance is the proximity of the low ESR (usually ceramic) input bypass capacitor. It should be located as close to the LTC1504A V CC and GND pins as physically possible. Ideally, the capacitor should be located right next to the package, straddling the SW pin. High peak current applica- tions or applications with V CC greater than 6V may require a 1mF or larger ceramic capacitor in this position. One node that isn’t quite so critical is SW. Extra lead length or narrow traces at this pin will only add parasitic induc- tance in series with the external inductor, slightly raising its value. The SW trace need only be wide enough to support the maximum peak current under short circuit conditions—perhaps 1A. If a trace needs to be compro- mised to make the layout work, this is the one. Note that long traces at the SW node may aggravate EMI consider- ations—don’t get carried away. If a Schottky diode is used at the SW node, it should be located at the LTC1504A end of the trace, close to the device pins. The LTC Applications Department has constructed liter- ally hundreds of layouts for the LTC1504A and related parts, many of which worked and some of which are now archived in the Bad Layout Hall of Fame. If you need layout assistance or you think you have a candidate layout for the Hall of Fame, give Applications a call at (408) 954-8400. Demo boards with properly designed layouts are available and specialized layouts can be designed if required. The applications team is also experienced in external compo- nent selection for a wide variety of applications, and they have a never-ending selection of tall tales to tell as well. When in doubt, give them a call. APPLICATIONS INFORMATIONWU UU Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

ª LINEAR TECHNOLOGY CORPORATION 1 997 1504afs, sn1504a LT/TP 1098 4K • PRINTED IN USA TYPICAL APPLICATIONSU High Efficiency 5V to 2.5V Converter with Current Limit SCSI-2 Active Terminator IMAX SHDN SS COMP VCC GND SW FB 1µF 0.1µF RIMAX* CIN VCC SHDN 7.5k 0.01µF 220pF 11.8k 12.1k MBRS0530L COUT VOUT 2.5V LEXT CIN: AVX TPSE107M016R0125 COUT: SANYO 16CV220GX LEXT: COILCRAFT DO3316-473 *SELECT RIMAX VALUE USING CURRENT LIMIT THRESHOLD GRAPH ON PAGE 3 1504A • TA03 LTC1504A IMAX SHDN SS COMP VCC GND SW FB 4.7µF CERAMIC TERMPWR 7.5k 0.01µF 220pF 15k 12k COUT LEXT COUT: AVX TPSC107M006R0150 LEXT: SUMIDA CD54-470 1504A • TA04 NC NC LTC1504A 110Ω 110Ω 110Ω 110Ω 110Ω TO LINES PART NUMBER DESCRIPTION COMMENTS LTC1174 600mA, High Efficiency Step-Down Converter Nonsynchronous, Better Low Load Efficiency LTC1430 High Power Step-Down DC/DC Controller 5V to 1.xV – 3.xV Voltage Conversion for High End Processors LTC1433/LTC1434 450mA, Low Noise Current Mode Step-Down Converters Nonsynchronous, Better Low Load Efficiency LTC1474 Low Quiescent Current, High Efficiency Step-Down Converter 10 mA Standy Current, 92% Efficiency, MSOP Package LT1507 1.5A, 500kHz Monolithic Buck Regulator Nonsynchronous, 1.5A Max Current LTC1627 Monolithic Synchronous Step-Down Switching Regulator 2.65V to 8.5V Input Range, 95% Efficiency, SO-8 Package RELATED PARTS PACKAGE DESCRIPTIONU Dimensions is inches (millimeters) unless otherwise noted. 1 2 3 4 0.150 – 0.157** (3.810 – 3.988) 8 7 6 5 0.189 – 0.197* (4.801 – 5.004) 0.228 – 0.244 (5.791 – 6.197) 0.016 – 0.050 0.406 – 1.270 0.010 – 0.020 0°– 8° TYP 0.008 – 0.010 (0.203 – 0.254) SO8 0996 0.053 – 0.069 (1.346 – 1.752) 0.014 – 0.019 (0.355 – 0.483) 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) TYPDIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 l FAX: (408) 434-0507 l www.linear-tech.com