LTC1504CS8

Final Electrical Specifications LTC1504 500mA Low Voltage Step-Down Synchronous Switching Regulator U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ 500mA Output Current at 3.3V Output Up to 92% Peak Efficiency Internal Reference Trimmed to 1% Output Can Source or Sink Current Requires as Few as Four External Components Input Voltage Range: 4V to 10V Adjustable Current Limit Small SO-8 Package 200kHz Switching Frequency Can be Synchronized Up to 500kHz U APPLICATIONS ■ ■ ■ ■ Small Portable Digital Systems Control Outputs Daisy-Chained Active Termination Auxiliary Output Voltage Supplies Minimum Part Count/Size Switchers The LTC®1504 is a self-contained, high efficiency synchronous buck switching regulator. It includes a pair of on-chip 1.5Ω power switches, enabling it to supply up to 500mA of load current. Efficiency peaks at 92%, minimizing heat and wasted power. The synchronous buck architecture allows the output to source or sink current as required to keep the output voltage in regulation. The LTC1504 is available in adjustable and fixed 3.3V output versions. An adjustable current limit circuit provides protection from overloads. The internal 1% reference combined with a sophisticated voltage feedback loop provides optimum output voltage accuracy and fast load transient response. The LTC1504 is specified to operate with input voltages between 4V and 10V. Contact the LTC factory for guaranteed specifications at 2.7V supply. The LTC1504 is available in a plastic SO-8 package. , LTC and LT are registered trademarks of Linear Technology Corporation. U TYPICAL APPLICATION Minimum Part Count 5V to 3.3V Regulator NC SHUTDOWN IMAX SHDN 5V to 3.3V Efficiency 100 90 VCC + CIN 3.3V AT 500mA LTC1504-3.3 GND SS CIN: AVX TPSC226M016R0375 COUT: AVX TAJC476M010 LEXT: COILTRONICS CTX50-1P 80 LEXT SW NC SENSE COMP + COUT 70 EFFICIENCY (%) 5V 60 50 40 30 1000pF 20 1504 • TA01 10 0 10 100 LOAD CURRENT (mA) 500 1504 • TA02 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 1 LTC1504 W U U W W U W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER I FOR ATIO (Note 1) Supply Voltage (VCC to GND) ................................... 10V Peak Output Current (SW) ....................................... ±1A Input Voltage (All Other Pins) ......... – 0.3V to VCC + 0.3V Operating Temperature Range ..................... 0°C to 70°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER TOP VIEW IMAX 1 8 COMP VCC 2 7 SS SW 3 6 SHDN GND 4 5 FB/SENSE* LTC1504CS8 LTC1504CS8-3.3 S8 PACKAGE 8-LEAD PLASTIC SO S8 PART MARKING *FB FOR LTC1504CS8, SENSE FOR LTC1504CS8-3.3 1504 15043 TJMAX = 115°C, θJA = 90°C/W Consult factory for Industrial and Military grade parts. ELECTRICAL CHARACTERISTICS VCC = 5V, TA = 25°C unless otherwise specified. (Note 2) SYMBOL PARAMETER CONDITIONS MIN VCC Minimum Supply Voltage (Note 7) ● 4 VFB Feedback Voltage LTC1504CS8 ● 1.25 ∆VFB Feedback Voltage PSRR Figure 1, 4V ≤ VCC ≤ 10V, LTC1504CS8 ● VSENSE Sense Pin Voltage LTC1504CS8-3.3 ● ∆VSENSE Sense Voltage PSRR Figure 1, 4V ≤ VCC ≤ 10V, LTC1504CS8-3.3 ICC Supply Current Figure 1, VSHDN = VCC, IOUT = 0 (Note 4) Figure 1, VSHDN = VCC, IOUT = 0, VFB/VSENSE = VCC (Note 4) VSHDN = 0V TYP MAX UNITS V 1.265 1.28 V 1.1 1.6 % 3.30 3.40 V ● 1.2 1.8 % ● ● 3 0.3 1.0 0.6 20 mA mA µA 200 250 kHz 1.3 2.0 Ω 3.20 fOSC Internal Oscillator Frequency ● RSW Internal Switch Resistance ● VIH SHDN Input High Voltage ● VIL SHDN Input Low Voltage ● IIN SHDN Input Current ● VOH Error Amplifier Positive Swing Figure 2 ● VOL Error Amplifier Negative Swing Figure 2 ● IOH, IOL Error Amplifier Output Current Figure 2 ● gmV Error Amplifier Transconductance (Note 5) AV Error Amplifier DC Gain (Note 5) gmI ILIM Amplifier Transconductance (Note 6) IMAX IMAX Sink Current VIMAX = VCC ● 8 12 16 µA ISS Soft Start Source Current VSS = 0V ● –8 – 12 – 16 µA tr, tf Output Switch Rise/Fall Time 5 50 ns DCMAX Maximum Duty Cycle 2 150 2.4 V ±1 µA 0.05 0.5 V ±50 ±100 ±200 µA ● 350 600 1100 µmho ● 40 48 1000 2000 ±0.1 4.5 ● VCOMP = VCC V 0.8 ● 84 4.95 90 V dB 3000 µmho % LTC1504 ELECTRICAL CHARACTERISTICS The ● 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: LTC1504 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 gmV 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. U W TYPICAL PERFORMANCE CHARACTERISTICS Supply Current vs Supply Voltage Supply Current vs Temperature 14 10 SUPPLY CURRENT (mA) SUPPLY CURRENT (mA) TA = 25°C IOUT = 0 12 10 8 VFB = VOUT 6 4 VCC = 5V IOUT = 0 VFB = VOUT 1 VFB = VCC 2 VFB = VCC 0 2.5 7.5 5 SUPPLY VOLTAGE (V) 0.1 –50 10 –25 0 25 50 75 TEMPERATURE (°C) Switch On-Resistance vs Temperature Current Limit Threshold vs RIMAX 3.5 700 CURRENT LIMIT THRESHOLD (mA) SWITCH ON-RESISTANCE (Ω) 3.0 VCC = 3.3V 2.0 VCC = 5V 1.5 VCC = 10V 1.0 0.5 0 –50 –25 125 1504 • TPC02 1504 • TPC01 2.5 100 50 25 75 0 TEMPERATURE (°C) 100 125 600 TA = 25°C VCC = 5V 500 400 300 200 100 0 10k 100k RIMAX (Ω) 1504 • TPC03 1504 • TPC04 3 LTC1504 U W TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Threshold vs Supply Voltage Current Limit Threshold vs Temperature 4.0 VCC = 5V 450 SHUTDOWN PIN THRESHOLD (V) CURRENT LIMIT THRESHOLD (mA) 500 400 RIMAX = 47k 350 300 250 200 RIMAX = 22k 150 100 50 0 –50 –25 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 25 50 75 TEMPERATURE (°C) 100 125 1504 • TPC05 3 5 7 SUPPLY VOLTAGE (V) 10 1504 • TPC07 U U U PIN FUNCTIONS IMAX (Pin 1): Current Limit Set. Connect a resistor from VCC to IMAX to set the current limit threshold. An internal 12µA current source from IMAX 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 IMAX floating. VCC (Pin 2): Power Supply Input. Connect to a power supply voltage between 4V and 10V. VCC requires a low impedance bypass capacitor to ground, located as close as possible to the LTC1504. See the Applications Information 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 COUT 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 LTC1504 S0-8 package and should be connected to 4 as large a copper area as possible to improve thermal dissipation. See the Thermal Considerations section for more information. FB (LTC1504CS8) (Pin 5): Feedback. Connect FB to a resistor divider from VOUT to GND to set the regulated output voltage. The LTC1504CS8 feedback loop will servo the FB pin to 1.265V. SENSE (LTC1504CS8-3.3) (Pin 5): Output Voltage Sense. Connect directly to the output voltage node. The LTC1504CS8-3.3 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 LTC1504CS8. SHDN (Pin 6): Shutdown, Active Low. When SHDN is at a logic High, the LTC1504 will operate normally. When SHDN is Low, the LTC1504 ceases all internal operation and supply current drops below 1µA. 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 LTC1504 is inactive. See the Applications Information section for more details. SS (Pin 7): Soft Start. Connect an external capacitor (usually 0.1µF) from SS to GND to limit the output rise time LTC1504 U U U PIN FUNCTIONS during power-up. CSS also compensates the current limit loop, allowing the LTC1504 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. W BLOCK DIAGRAM SHDN TO INTERNAL BLOCKS VCC SAW – Q1 PWM SW + COMP 12V Q2 SS ILIM – FB + MIN + – + MAX – + – IMAX 12V + – + – 40mV + – FB (ADJ ONLY) 40mV 20.4k VREF 1.265V SENSE (–3.3V ONLY) 12.6k 1504 • BD Figure 3. Block Diagram TEST CIRCUITS NC IMAX VCC SHDN VCC + CIN LTC1504 LEXT VOUT SW LTC1504 1µF GND SS FB/SENSE A FB/SENSE – B + COMP COUT COMP + VREF 0.1µF CIN: AVX TPSE107M016R0125 COUT: SANYO 16CV220GX LEXT: COILCRAFT D03316-473 7.5k 220pF A: TEST VOL, IOL B: TEST VOH, IOH 0.01µF 1504 • TC02 1504 • TC01 Figure 1 Figure 2 5 LTC1504 U W U U APPLICATIONS INFORMATION OVERVIEW The LTC1504 is a complete synchronous switching regulator controller (see Block Diagram). It includes two on-chip 1.5Ω power MOSFETs, eliminating the need for external power devices and minimizing external parts count. The internal switches are set up as a synchronous buck converter with a P-channel device (Q1) from the input supply to the switching node and an N-channel device (Q2) as the synchronous rectifier device from the switching node to ground. An external inductor, input and output bypass capacitors and the compensation network complete the control loop. The LTC1504 adjustable output parts require an additional pair of resistors to set the output voltage. The LTC1504-3.3 parts include an onboard resistor divider preset to a 3.3V output voltage. A functional 3.3V output regulator can be constructed with an LTC1504-3.3 and as few as four external components. The LTC1504 feedback loop includes a precision reference trimmed to 1% (VREF), a wide bandwidth transconductance feedback amplifier (FB) and an onboard PWM generator (SAW and PWM). Two additional feedback comparators (MIN and MAX) monitor the feedback voltage and override the primary feedback amplifier when the regulated out falls outside a ±3% window, improving 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 LTC1504 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 section for more information. Theory of Operation The LTC1504 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 6 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 comparator PWM. This raw pulse-width modulated signal is logically combined with the outputs of the transient comparators MIN and MAX before reaching the output stage. The output stage 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 1Ω and 3Ω, 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 MIN and MAX comparators in the feedback loop provide high speed fault correction in situations where the FB amplifier may not respond quickly enough. MIN compares the feedback signal to a voltage 40mV (3%) below the internal reference. At this point, MIN overrides the FB amplifier and forces the loop to full duty cycle. Similarly, MAX monitors the output voltage at 3% above the internal reference and forces the output to 0% duty cycle when tripped. These two comparators prevent extreme output perturbations with fast output transients, while allowing the main feedback loop to be optimally compensated for stability. The LTC1504 includes yet another feedback loop that controls operation in current limit. The ILIM 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 IMAX, indicating the current through Q1 has increased beyond the desired value, ILIM 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. LTC1504 U W U U APPLICATIONS INFORMATION EXTERNAL COMPONENT SELECTION External components required by the LTC1504 fall into three categories: input bypass, output filtering and compensation. Additional components to set up soft start and current limit are usually included as well. A minimum LTC1504 circuit can be constructed with as few as four external components; a circuit that utilizes all of the LTC1504s functionality usually includes eight or nine external components, with two additional feedback resistors required for adjustable parts. See the Typical Applications section for examples of external component hookup. Input Bypass The input bypass capacitor is critical to proper LTC1504 operation. The LTC1504 includes a precision reference and a pair of high power switches feeding from the same VCC pin. If VCC does not have adequate bypassing, the switch pulses introduce enough ripple at VCC to corrupt the reference voltage and the LTC1504 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, there isn’t adequate input bypass. Ideally, the LTC1504 requires a low impedance bypass right at the chip and a larger reservoir capacitor that can be located somewhat farther away. This requirement usually can be met with a ceramic capacitor right next to the LTC1504 and an electrolytic capacitor (usually 10µF to 100µF, depending on expected load current) located somewhere nearby. In certain cases, the bulk capacitance requirement can be met by the output bypass of the input supply. Applications running at very high load currents or at input supply voltages greater than 6V may require the local ceramic capacitor to be 1µF or greater. In some cases, both the low impedance and bulk capacitance requirements can be met by a single capacitor, mounted very close to the LTC1504. Low ESR organic semiconductor (OS-CON) electrolytic capacitors or surge tested surface mount tantalum capacitors can have low enough impedance to keep the LTC1504 happy in some circuits. Often the RMS current capacity of the input bypass capacitors is more important to capacitor selection than value. Buck converters like the LTC1504 are hard on input capacitors, since the current flow alternates between the full load current and near zero during every clock cycle. In the worst case (50% duty cycle or VOUT = 0.5VIN) the RMS current flow in the input capacitor is half of the total load current plus half the ripple current in the inductor— perhaps 300mA in a typical 500mA load current application. This current flows through the ESR of the input bypass capacitor, heating it up and shortening its life, sometimes dramatically. Many ordinary electrolytic capacitors that look OK at fist glance are not rated to withstand such currents—check the RMS current rating before you specify a device! If the RMS current rating isn’t specified, it should not be used as an input bypass capacitor. Again, low ESR electrolytic and surge tested tantalums usually do well in LTC1504 applications and have high RMS current ratings. The local ceramic bypass capacitor usually has negligible ESR allowing it to withstand large RMS currents without trouble. Table 1 shows typical surface mount capacitors that make acceptable input bypass capacitors in LTC1504 applications. Table 1. Representative Surface Mount Input Bypass Capacitors PART VALUE ESR MAX RMS TYPE HEIGHT AVX TPSC226M016R0375 22µF 0.38Ω TPSD476M016R0150 47µF 0.15Ω TPSE107M016R0125 100µF 0.13Ω 1206YC105M 1µF Low 1210YG106Z 10µF Low 0.54A 0.86A 1.15A >1A >1A Tantalum Tantalum Tantalum X7R Ceramic Y5V Ceramic 2.6mm 2.9mm 4.1mm 1.5mm 1.7mm Sanyo 16SN33M 16SN68M 16CV100GX 16CV220GX 33µF 0.15Ω 68µF 0.1Ω 100µF 0.44Ω 220µF 0.34Ω 1.24A 1.65A 0.23A* 0.28A* OS-CON 7mm OS-CON 7mm Electrolytic 6mm Electrolytic 7.7mm Sprague 593D476X0016D2W 593D107X0016E2W 47µF 100µ 0.93A 1.05A Tantalum Tantalum 0.17Ω 0.15Ω 2.8mm 4mm *Note: Use multiple devices in parallel or limit output current to prevent capacitor overload. Inductor The LTC1504 requires an external inductor to be connected from the switching node SW to the output node where the load is connected. Inductor requirements are fairly straightforward; it must be rated to handle continuous DC current equal to the maximum load current plus 7 LTC1504 U U W U APPLICATIONS INFORMATION half the ripple current and its value should be chosen based on the desired ripple current and/or the output current transient requirements. Large value inductors lower ripple current and decrease the required output capacitance, but limit the speed that the LTC1504 can change the output current, limiting output transient response. Small value inductors result in higher ripple currents and increase the demands on the output capacitor, but allow faster output current slew rates and are often smaller and cheaper for the same DC current rating. A typical inductor used in an LTC1504 application might have a maximum current rating between 500mA and 1A and an inductance between 33µH and 220µH. Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron rod core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC1504 requires to operate. Table 2 shows some typical surface mount inductors that work well in LTC1504 applications. Table 2. Representative Surface Mount Inductors PART VALUE MAX DC CORE TYPE CORE MATERIAL HEIGHT CoilCraft DT3316-473 DT3316-104 DO1608-473 DO3316-224 47µH 100µH 47µH 220µH 1A 0.8A 0.5A 0.8A Shielded Shielded Open Open Ferrite Ferrite Ferrite Ferrite 5.1mm 5.1mm 3.2mm 5.5mm Coiltronics CTX50-1 CTX100-2 CTX50-1P CTX100-2P 50µH 100µH 50µH 100µH 0.65A 0.63A 0.66A 0.55A Toroid Toroid Toroid Toroid KoolMµ ® KoolMµ Type 52 Type 52 4.2mm 6mm 4.2mm 6mm Sumida CDRH62-470 CDRH73-101 CD43-470 CD54-101 47µH 100µH 47µH 100µH 0.54A 0.50A 0.54A 0.52A Shielded Shielded Open Open Ferrite Ferrite Ferrite Ferrite 3mm 3.4mm 3.2mm 4.5mm Output Capacitor The output capacitor affects the performance of the LTC1504 in a couple of ways: it provides the first line of Kool Mµ is a registered trademark of Magnetics, Inc.. 8 defense during a transient load step and it has a large effect on the compensation required to keep the LTC1504 feedback loop stable. Transient load response of an LTC1504 circuit is controlled almost entirely by the output capacitor and the inductor. In steady load operation, the average current in the inductor will match the load current. When the load current changes suddenly, the inductor is suddenly carrying the wrong current and requires a finite amount of time to correct itself—at least several switch cycles with typical LTC1504 inductor values. Even if the LTC1504 had psychic abilities and could instantly assume the correct duty cycle, the rate of change of current in the inductor is still related to its value and will not change instantaneously. Until the inductor current adjusts to match the load current, the output capacitor has to make up the difference. Applications that require exceptional transient response (2% or better for instantaneous full-load steps) will require relatively large value, low ESR output capacitors. Applications with more moderate transient load requirements can often get away with traditional standard ESR electrolytic capacitors at the output and can use larger valued inductors to minimize the required output capacitor value. Note that the RMS current in the output capacitor is slightly more than half of the inductor ripple current— much smaller than the RMS current in the input bypass capacitor. Output capacitor lifetime is usually not a factor in typical LTC1504 applications. Large value ceramic capacitors used as output bypass capacitors provide excellent ESR characteristics but can cause loop compensation difficulties. See the Loop Compensation section. Loop Compensation Loop compensation is strongly affected by the output capacitor. From a loop stability point of view, the output inductor and capacitor form a series RLC resonant circuit, with the L set by the inductor value, the C by the value of the output capacitor and the R dominated by the output capacitor’s ESR. The amplitude response and phase shift due to these components is compensated by a network of Rs and Cs at the COMP pin to (hopefully) close the feedback loop in a stable manner. Qualitatively, the L and LTC1504 U U W U APPLICATIONS INFORMATION C of the output stage form a 2nd order roll-off with 180° of phase shift; the R due to ESR forms a single zero at a somewhat higher frequency that reduces the roll-off to first order and reduces the phase shift to 90°. show compensation values that work with several combinations of external components—use them as a starting point. For complex cases or stubborn oscillations, contact the LTC Applications Department. If the output capacitor has a relatively high ESR, the zero comes in well before the initial phase shift gets all the way to 180° and the loop only requires a single small capacitor from COMP to GND to remain stable (Figure 4a). If, on the other hand, the output capacitor is a low ESR type to maximize transient response, the ESR zero can increase in frequency by a decade or more and the output stage phase shift can get awfully close to 180° before it turns around and comes back to 90°. Large value ceramic, OS-CON electrolytic and low impedance tantalum capacitors fall into this category. These loops require an additional zero to be inserted at the COMP pin; a series RC in parallel with a smaller C to ground will usually ensure stability. Figure 4b shows a typical compensation network which will optimize transient response with most output capacitors. Adjustable output parts can add a feedforward capacitor across the feedback resistor divider to further improve phase margin. The typical applications in this data sheet External Schottky Diode VOUT RFB1* LTC1504 FB RFB2* COMP CC *ADJUSTABLE PARTS ONLY 1504 • F04a Figure 4a. Minimum Compensation Network VOUT RFB1* LTC1504 CFF* FB RFB2* COMP RC CF CC *ADJUSTABLE PARTS ONLY 1504 • F04b Figure 4b. Optimum Compensation Network An external Schottky diode can be included across the internal N-channel switch (Q2) to improve efficiency at heavy loads. The diode carries the inductor current during the nonoverlap time while the LTC1504 turns Q1 off and Q2 on and prevents current from flowing in the intrinsic body diode in parallel with Q2. This diode will improve efficiency by a percentage point or two as output current approaches 500mA and can help minimize erratic behavior at very high peak current levels caused by excessive parasitic current flow through Q2. A Motorola MBRS0530L is usually adequate, with the cathode connected to SW and the anode connected to GND. Note that this diode is not required for normal operation and has a negligible effect on efficiency at low (< 250mA) output currents. Soft Start and Current Limit Soft start and current limit are linked in the LTC1504. Soft start works in a straightforward manner. An internal 12µA current source connected to the SS pin will pull up an external capacitor connected from SS to GND at a rate determined by the capacitor value. COMP is clamped to a voltage one diode drop above SS; as SS rises, COMP will rise at the same rate. When COMP reaches roughly 2V below VCC, the duty cycle will slowly begin to increase until the output comes into regulation. As SS continues to rise, the feedback amplifier takes over at COMP, the clamp releases and SS rises to VCC. During a soft start cycle, the MIN feedback comparator is disabled to prevent it from overriding the COMP pin and forcing the output to maximum duty cycle. Current limit operates by pulling down on the soft start pin when it senses an overload condition at the output. The current limit amplifier (ILIM) compares the voltage drop across the internal P-channel switch (Q1) during its on time to the voltage at the IMAX pin. IMAX includes an internal 12µA pull-down, allowing the voltage to be set by a single resistor between VCC and IMAX . When the IR drop across 9 LTC1504 U W U U APPLICATIONS INFORMATION Q1 exceeds the drop across the IMAX resistor, ILIM pulls current out of the external soft start capacitor, reducing the voltage at SS. A soft start capacitor should always be used if current limit is enabled. SS, in turn, pulls down on COMP, limiting the output duty cycle and controlling the output current. When the current overload is removed, the ILIM amplifier lets go of SS and allows it to rise again as if it were completing a soft start cycle. The size of the external soft start capacitor controls both how fast the current limit responds once an overload is detected and how fast the output recovers once the overload is removed. The soft start capacitor also compensates the feedback loop created by the ILIM amplifier. Because the ILIM loop is a current feedback loop, the additional phase shift due to the output inductor and capacitor do not come into play and the loop can be adequately compensated with a single capacitor. Usually a 0.1µF ceramic capacitor from SS to GND provides adequate soft start behavior and acceptable current limit response. This type of current limit circuit works well with mild current overloads and eliminates the need for an external current sensing resistor, making it attractive for LTC1504 applications. These same features also handicap the current limit circuit under severe short circuits when the output voltage is very close to ground. Under this condition, the LTC1504 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 LTC1504 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 LTC1504 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 RON of Q2 inside the LTC1504. This current is usually nondestructive and dissipates a limited amount of power since the output voltage is very low. A typical LTC1504 circuit can withstand such a short for many seconds without damage. The test circuit in Figure 1 will 10 typically withstand a direct output short for more than 30 seconds without damage to the LTC1504. Eventually, however, a continuous short may cause the die temperature to rise to destructive levels. Note that the current limit is primarily designed to protect the LTC1504 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 RON 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 IMAX pin; the internal current source will pull IMAX to GND and the ILIM amplifier will be disabled. Shutdown The LTC1504 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 1µA 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 LTC1504 is shut down, but prevents other devices from powering the output when the LTC1504 is disabled. External Clock Synchronization The LTC1504 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 immediately when it goes low, but waits 50µs before shutting down the rest of the internal circuitry. A clock signal applied directly to the SHDN pin will force the LTC1504 internal oscillator to lock to its frequency as long as the external clock runs faster than the internal oscillator LTC1504 U W U U APPLICATIONS INFORMATION frequency. Attempting to synchronize to a frequency lower than the 250kHz maximum internal frequency may result in inconsistent pulse widths and is not recommended. 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 frequencies will reduce the sawtooth amplitude to the point that the LTC1504 may run erratically. THERMAL CONSIDERATIONS Each of the LTC1504 internal power switches has approximately 1.5Ω 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 supplying 500mA of load current will cause a continuous dissipation of approximately 375mW. The SO-8 package has a thermal resistance of approximately 90°C/W, meaning that the die will begin to rise toward 34°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 temperatures, this cycle may continue until the chip melts, since the LTC1504 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 115°C die temperature is not exceeded. The LTC1504 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. Applications which will operate the LTC1504 near maximum 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 LTC1504 requires special care in layout to ensure optimum performance. 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 LTC1504 circuit may look nothing at all like a regulator. Wire-wrap or plug-in prototyping boards are not useful for breadboarding LTC1504 circuits! Perhaps most critical to proper LTC1504 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 LTC1504 ground pin. The compensation network and soft start capacitor can be connected together on 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 LTC1504 VCC and GND pins as physically possible. Ideally, the capacitor should be located right next to the package, straddling the SW pin. High peak current applications or applications with VCC greater than 6V may require a 1µF 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- 11 LTC1504 U U W U APPLICATIONS INFORMATION 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 compromised to make the layout work, this is the one. Note that long traces at the SW node may aggravate EMI considerations—don’t get carried away. If a Schottky diode is used at the SW node, it should be located at the LTC1504 end of the trace, close to the device pins. The LTC Applications Department has constructed literally hundreds of layouts for the LTC1504 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 component 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. U TYPICAL APPLICATIONS SCSI-2 Active Terminator High Efficiency 5V to 2.5V Converter with Current Limit RIMAX* VCC 5V IMAX SHDN CIN LEXT LTC1504 1µF GND SS MBRS0530L FB 11.8k IMAX SHDN COUT 7.5k 0.1µF 220pF 110Ω NC 110Ω + COMP • • • VOUT 2.5V SW VCC + 110Ω SHDN TERMPWR 15k LTC1504 GND 12.1k 110Ω SW VCC 4.7µF CERAMIC LEXT SS FB NC 1504 • TA03 110Ω + COUT COMP 12k 0.01µF CIN: AVX TPSE107M016R0125 COUT: SANYO 16CV220GX LEXT: COILCRAFT DO3316-473 *SELECT RIMAX VALUE USING CURRENT LIMIT THRESHOLD GRAPH ON PAGE 3 18 TO 27 LINES 7.5k 220pF 0.01µF COUT: AVX TPSC107M006R0150 LEXT: SUMIDA CD54-470 1504 • TA04 RELATED PARTS PART NUMBER LTC1174 DESCRIPTION 600mA, High Efficiency Step-Down Converter COMMENTS Nonsynchronous, Better Low Load Efficiency LT ® 1307 Single Cell 600kHz DC/DC Converter Boost Mode, Micropower LT1372 1.5A, 500kHz Step-Up Switching Regulator Boost Mode, High Power LT1376 1.5A, 500kHz Step-Down Switching Regulator Nonsynchronous, 1.5A Max Current LTC1433/LTC1434 450mA, Low Noise Current Mode Step-Down Converters Nonsynchronous, Better Low Load Efficiency LT1507 1.5A, 500kHz Monolithic Buck Regulator Nonsynchronous, 1.5A Max Current 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● (408) 432-1900 FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com 1504i LT/TP 0897 4K • PRINTED IN USA  LINEAR TECHNOLOGY CORPORATION 1997