LTC1574 LTC1574-3.3/LTC1574-5 High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: Up to 94% Usable in Noise-Sensitive Products Peak Inductor Current Independent of Inductor Value Short-Circuit Protection Internal Low Forward Drop Schottky Diode Only Three External Components Required Wide VIN Range: 4V to 18.5V (Absolute Maximum) Low Dropout Operation Low-Battery Detector Pin Selectable Current Limit Internal 0.9Ω Power Switch: VIN < 11V Standby Current: 130µA Active Low Micropower Shutdown U APPLICATIO S ■ ■ ■ ■ ■ ■ The LTC®1574 is a family of easy-to-use current mode DC/DC converters ideally suited for 9V to 5V, 5V to 3.3V and inverting operation. With an internal 0.9Ω switch (at a supply voltage of 12V) and a low forward drop Schottky diode (0.450V typ at 200mA, TA = 25°C), the LTC1574 requires only three external components to construct a complete high efficiency DC/DC converter. Under no load condition, the LTC1574 draws only 130µA. In shutdown, it draws a mere 2µA making this converter ideal for battery-powered applications. In dropout, the internal P-channel MOSFET switch is turned on continuously allowing the user to maximize the life of the battery source. The maximum inductor current of the LTC1574 family is pin selectable to either 340mA or 600mA, optimizing efficiency for a wide range of applications. Operation up to 200kHz permits the use of small surface mount inductors and capacitors. Inverting Converters Step-Down Converters Memory Backup Supply Portable Instruments Battery-Powered Equipment Distributed Power Systems and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation. For applications requiring higher output current or ultrahigh efficiency, see the LTC1148 or LTC1265 data sheets. For detailed applications information, see the LTC1174 data sheet. U TYPICAL APPLICATIO LTC1574-5 Efficiency High Efficiency Step-Down Converter VIN 5.5V to 16V 100 L = 100µH VOUT = 5V IPGM = 0V + 5 VIN 12 11 6 SHDN LBIN LTC1574-5 VOUT LBOUT IPGM SW 7 10 3, 14 100µH† + GND 22µF* 35V 2, 4, 13, 15 5V 175mA 100µF* 10V 1574 TA01 * AVX TPSD226K035 ** AVX TPSD107K010 † COILTRONICS CTX100-4 EFFICIENCY (%) 95 VIN = 6V 90 VIN = 9V 85 80 75 70 1 10 LOAD CURRENT (mA) 100 200 1574 TA02 1 LTC1574 LTC1574-3.3/LTC1574-5 W W W AXI U U U W PACKAGE/ORDER I FOR ATIO U ABSOLUTE RATI GS (Note 1) (Voltage Referred to GND Pin) Input Supply Voltage (Pin 5) ................. – 0.3V to 18.5V Switch Current (Pin 3, 14) ........................................ 1A Switch Voltage (Pin 3, 14) .......................... VIN – 18.5V Operating Temperature Range .................... 0°C to 70°C Junction Temperature (Note 2) ............................ 125°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C TOP VIEW NC 1 GND 2 15 GND SW 3 14 SW GND 4 13 GND VIN 5 12 LBIN IPGM 6 SHDN 7 NC 8 ORDER PART NUMBER 16 NC LTC1574CS LTC1574CS-3.3 LTC1574CS-5 11 LBOUT 10 VOUT (VFB*) 9 NC S PACKAGE 16-LEAD PLASTIC SO *ADJUSTABLE OUTPUT VERSION TJMAX = 125°C, θJA = 110°C/W Consult factory for Industrial and Military grade parts. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 9V, SHDN = VIN, IPGM = 0V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS IFB Feedback Current into Pin 10 LTC1574 VFB Feedback Voltage LTC1574 ● 1.20 VOUT Regulated Output Voltage LTC1574-3.3 LTC1574-5 ● ● 3.14 4.75 ∆VOUT Output Voltage Line Regulation Output Voltage Load Regulation IQ Input DC Supply Current (Note 4) Active Mode Sleep Mode Shutdown (Note 5) MIN MAX UNITS 1 µA 1.25 1.30 V 3.30 5.00 3.46 5.25 V V VIN = 6V to 12V, ILOAD = 100mA, IPGM = VIN (Note 3) 10 70 mV LTC1574-3.3 (Note 3) 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN –5 – 45 – 70 – 70 mV mV LTC1574-5 (Note 3) 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN –5 – 50 – 70 – 70 mV mV 450 130 2 600 180 25 µA µA µA 1.25 1.4 V 0.5 µA 4V < VIN < 16V, IPGM = 0V 4V < VIN < 16V SHDN = 0V, 4V < VIN < 16V VLBTRIP Low-Battery Trip Point ILBIN Current into Pin 12 ILBOUT Current Sunk by Pin 11 VHYST Comparator Hysteresis IPEAK Current Limit RON ON Resistance of Switch tOFF Switch Off Time VOUT at Regulated Value VIH SHDN Pin High Minimum Voltage at Pin 7 for Device to Be Active VIL SHDN Pin Low Maximum Voltage at Pin 7 for Device to Be in Shutdown 2 TYP VLBOUT = 0.4V, VLBIN = 0V VLBOUT = 5V, VLBIN = 10V IPGM = VIN, VOUT = 0V IPGM = 0V, VOUT = 0V ● ● 0.5 1.0 1.5 1.0 mA µA 7.5 15 30 mV 0.54 0.27 0.60 0.34 0.83 0.53 A A 0.9 1.55 Ω 4 5 µs ● 3 1.2 V 0.75 V LTC1574 LTC1574-3.3/LTC1574-5 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 9V, SHDN = VIN, IPGM = 0V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN IIH SHDN Pin Input Current SHDN = 16V IIL SHDN Pin Input Current 0 ≤ SHDN ≤ 0.8V VF Schottky Diode Forward Voltage Forward Current = 200mA IR Schottky Reverse Current Reverse Voltage = 5V Reverse Voltage = 18.5V TYP MAX UNITS 2 µA 0.5 µA 0.450 0.570 V 10 100 25 250 µA µA Note 3: Guaranteed by design. Note 4: Does not include Schottky reverse current. Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: Current into Pin 5 only, measured without electrolytic input capacitor. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: TJ = TA + (PD • 110°C/W) U W TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Load Current Efficiency vs Load Current EFFICIENCY (%) EFFICIENCY (%) VIN = 5V 90 VIN = 9V 80 70 L = 50µH VOUT = 3.3V IPGM = VIN COIL = CTX50-4 60 Efficiency vs Input Voltage 100 95 95 94 VIN = 9V 85 80 L = 50µH VOUT = 5V IPGM = VIN COIL = CTX50-4 75 1 10 100 LOAD CURRENT (mA) 1 500 10 100 LOAD CURRENT (mA) Efficiency Using Different Types of Inductor Core Material 180 CTX50-4P 80 70 VIN = 5V VOUT = 3.3V IPGM = VIN 5 500 6 8 9 10 11 12 INPUT VOLTAGE (V) 7 13 14 1574 • TPC03 1.7 VIN = 13.5V 160 1.6 140 1.5 120 1.4 100 80 60 TA = 25°C 1.3 1.2 1.1 1.0 40 0.9 0.8 0 10 100 LOAD CURRENT (mA) 91 Switch Resistance vs Input Voltage 20 50 ILOAD = 100mA IPGM = 0V 89 400 RDS(ON) (Ω) EFFICIENCY (%) LEAKAGE CURRENT (nA) CTX50-4 1 ILOAD = 300mA IPGM = VIN Switch Leakage Current vs Temperature 100 60 92 1574 • TPC02 1574 • TPC01 90 93 90 70 50 VOUT = 5V L = 100µH COIL = CTX100-4 VIN = 6V 90 EFFICIENCY (%) 100 0 20 40 60 TEMPERATURE (°C) 80 100 1574 • TPC04 1574 • TPC05 0.7 4 6 8 10 12 14 16 INPUT VOLTAGE (V) 18 20 1574 • TPC06 3 LTC1574 LTC1574-3.3/LTC1574-5 U U U PI FU CTIO S NC (Pins 1, 8, 9, 16): No Connection. GND (Pins 2, 4, 13, 15): Ground. SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and Cathode of Schottky Diode. VOUT or VFB (Pin 10): For the LTC1574, this pin connects to the main voltage comparator input. On the LTC1574-5 and LTC1574-3.3, this pin goes to an internal resistive divider which sets the output voltage. VIN (Pin 5): Input Supply Voltage. It must be decoupled close to ground (Pin 4). LBOUT (Pin 11): Open drain of an N-Channel Pull-Down. This pin will sink current when (Pin 12) LBIN goes below 1.25V. IPGM (Pin 6): This pin selects the current limit of the P-channel switch. With IPGM = VIN, the current trip point is 600mA and with IPGM = 0V, the current trip point is reduced to 340mA. LBIN (Pin 12): The (–) Input of the Low-Battery Voltage Comparator. The (+) input is connected to a reference voltage of 1.25V. SHDN (Pin 7): Pulling this pin to ground keeps the internal switch off and puts the LTC1574 in micropower shutdown. U W U U APPLICATIO S I FOR ATIO Operating Frequency and Inductor Since the LTC1574 utilizes a constant off-time architecture, its operating frequency is dependent on the value of VIN. The frequency of operation can be expressed as: 1 VIN − VOUT t OFF VIN + VD (Hz) where tOFF = 4µs and VD is the voltage drop across the internal Schottky diode. Note that the operating frequency is a function of the input and output voltage. Although the size of the inductor does not affect the frequency or inductor peak current, it does affect the ripple current. The peak-to-peak ripple current is given by: IPGM = VIN 100mA/DIV f= IPGM pin, the limit is either set to 340mA or 600mA. In addition, the off-time of the switch is increased to allow the inductor current to decay far enough to prevent any current build-up (see Figure 1). IPGM = 0 GND V + VD IRIPPLE = 4 • 10 −6 OUT L (AP-P ) When choosing a small inductor, core loss will increase due to higher ripple current. Therefore, a low ESR output capacitor has to be used. Short-Circuit Protection The LTC1574 is protected from output short circuits by its internal current limit. Depending on the condition of the 4 L = 100µH VIN = 13.5V 20µs/DIV 1574 • F01 Figure 1. Inductor Current with Output Shorted Low-Battery Detector The low-battery indicator senses the input voltage through an external resistive divider. This divided voltage connects to the “–” input of a voltage comparator (Pin 12) which is compared with a 1.25V reference voltage. With the current LTC1574 LTC1574-3.3/LTC1574-5 U W U U APPLICATIO S I FOR ATIO going into Pin 12 being negligible, the following expression is used for setting the trip limit: R4 VLBTRIP = 1.251 + R3 VIN LTC1574 R4 12 difference between the absolute maximum voltage rating and the output voltage. A maximum of 12V is specified in Figure 4, giving the circuit 1.5V of headroom for VIN. Note that the circuit can operate from a minimum of 4V, making it ideal for a four NiCd cell application. For a higher output current circuit, please refer to the Typical Applications section. INPUT VOLTAGE 4V TO 12V – + R3 5 1.25V REFERENCE VIN 1574 • F02 12 Figure 2. Low-Battery Comparator 11 LTC1574 Adjustable Applications 6 The LTC1574 develops a 1.25V reference voltage between the feedback terminal (Pin 10) and ground (see Figure 3). By selecting resistor R1, a constant current is caused to flow through R1 and R2 to set the overall output voltage. The regulated output voltage is determined by: VOUT 47µF* 16V ×2 + 47µF* 16V ×2 LTC1574-5 7 SHDN LBOUT 10 VOUT IPGM 3, 14 SW 50µH** GND 2, 4, 13, 15 * AVX TPSD476K016 ** COILTRONICS CTX50-4 VOUT –5V 45mA 1574 • F04 Figure 4. Positive-to-Negative 5V Converter Low Noise Regulators R2 = 1.25 1 + R1 For most applications, a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested across R1 located close to the LTC1574. VOUT R2 LTC1574 VFB LBIN + 0.1µF 10 100pF R1 In some applications it is important not to introduce any switching noise within the audio frequency range. Due to the nature of the LTC1574 during Burst ModeTM operation, there is a possibility that the regulator will introduce audio noise at some load currents. To circumvent this problem, a feed-forward capacitor can be used to shift the noise spectrum up and out of the audio band. Figure 5 shows the low noise connection with C2 being the feed-forward capacitor. The peak-to-peak output ripple is reduced to 30mV over the entire load range. A toroidal surface mount Burst Mode is a trademark of Linear Technology Corporation 1574 • F03 + 5 VIN Figure 3. LTC1574 Adjustable Configuration 12 Inverting Applications The LTC1574 can easily be set up for a negative output voltage. If – 5V is desired, the LTC1574-5 is ideal for this application as it requires the least components. Figure 4 shows the schematic for this application. Note that the output voltage is now taken off the GND pins. Therefore, the maximum input voltage is now determined by the 11 6 LTC1574 SHDN LBIN LBOUT SW IPGM VFB 7 3, 14 10 L1** 100µH 56k C2 6.8nF + GND 2, 4, 13, 15 VIN 5V 100µF* 10V 33k * AVX TPSD107K010 ** COILTRONICS CTX100-4 VOUT 3.3V 425mA 100µF* 10V 1574 • F05 Figure 5. Low Noise 5V to 3.3V Regulator 5 LTC1574 LTC1574-3.3/LTC1574-5 U W U U APPLICATIO S I FOR ATIO inductor L1 is chosen for its excellent self-shielding properties. Open magnetic structures such as drum and rod cores are to be avoided since they inject high flux levels into their surroundings. This can become a major source of noise in any converter circuit. For COUT, the RMS current rating should be at least: Design Example Absolute Maximum Ratings and Latchup Prevention As a design example, assume VIN = 9V (nominal), VOUT = 5V and IOUT = 350mA maximum. The LTC1574-5 is used for this application with IPGM (Pin 6) connected to VIN. The minimum value of L is determined by assuming the LTC1574-5 is operating in continuous mode. The absolute maximum ratings specify that SW (Pins 3, 14) can never exceed VIN (Pin 5) by more than 0.3V. Normally this situation should never occur. It could, however, if the output is held up while the supply is pulled down. A condition where this could potentially occur is when a battery is supplying power to an LTC1574 regulator and also to one or more loads in parallel with the the regulator’s VIN. If the battery is disconnected while the LTC1574 regulator is supplying a light load and one of the parallel circuits is a heavy load, the input capacitor of the LTC1574 regulator could be pulled down faster than the output capacitor, causing the absolute maximum ratings to be exceeded. The result is often a latchup which can be destructive if VIN is reapplied. Battery disconnect is possible as a result of mechanical stress, bad battery contacts or use of a lithium-ion battery with a built-in internal disconnect. The user needs to assess his/her application to determine whether this situation could occur. If so, additional protection is necessary. INDUCTOR CURRENT IPEAK AVG CURRENT = IOUT I +I = PEAK V IV 2 = 350mA TIME 1574 • F06 Figure 6. Continuous Inductor Current With IOUT = 350mA and IPEAK = 0.6A (IPGM = VIN), IV = 0.1A. The peak-to-peak ripple inductor current, IRIPPLE, is 0.5A and is also equal to: V + VD IRIPPLE = 4 • 10 −6 OUT L (AP-P ) Solving for L in the above equation and with VD = 0.5V, L = 44µH. The next higher standard value of L is 50µH (example: Coiltronics CTX50-4). The operating frequency, ignoring voltage across diode VD is: V f ≈ 2.5 • 105 1 − OUT VIN = 111kHz With the value of L determined, the requirements for CIN and COUT are calculated. For CIN, its RMS current rating should be at least: IRMS = [ = 174mA 6 ( IOUT VOUT VIN − VOUT VIN )] IRMS ≈ I PEAK 2 = 300mA (A RMS) Prevention against latchup can be accomplished by simply connecting a Schottky diode across the SW and VIN pins as shown in Figure 7. The diode will normally be reverse biased unless VIN is pulled below VOUT at which time the diode will clamp the (VOUT – VIN) potential to less than the 0.6V required for latchup. Note that a low leakage Schottky should be used to minimize the effect LATCHUP PROTECTION SCHOTTKY VIN VOUT SW LTC1574 + 1/ 2 (A RMS) 1574 F07 Figure 7. Preventing Absolute Maximum Ratings from Being Exceeded LTC1574 LTC1574-3.3/LTC1574-5 U W U U APPLICATIO S I FOR ATIO on no-load supply current. Schottky diodes such as MBR0530, BAS85 and BAT84 work well. Another more serious effect of the protection diode leakage is that at no load with nothing to provide a sink for this leakage current, the output voltage can potentially float above the maximum allowable tolerance. To prevent this from occuring, a resistor must be connected between VOUT and ground with a value low enough to sink the maximum possible leakage current. U TYPICAL APPLICATIO S Low Noise, High Efficiency 3.3V Regulator VIN 4V TO 12.5V + 5 VIN 6 IPGM SHDN LTC1574 12 11 LBIN VFB LBOUT SW 7 22µF* 25V ×2 10 3, 14 50µH† 6.8nF + GND 2, 4, 13, 15 100pF * AVX TPSD226K025 ** AVX TPSD107K010 † COILTRONICS CTX50-4 U PACKAGE DESCRIPTIO 0.1µF 100µF** 10V ×2 VOUT 3.3V 450mA 56k 33k 1574 TA03 Dimension in inches (millimeters) unless otherwise noted. S Package 16-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.386 – 0.394* (9.804 – 10.008) 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.053 – 0.069 (1.346 – 1.752) 0.004 – 0.010 (0.101 – 0.254) 16 15 14 13 12 11 10 9 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.014 – 0.019 (0.355 – 0.483) TYP *DIMENSION 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 0.050 (1.270) BSC 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) S16 1098 1 2 3 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. 4 5 6 7 8 7 LTC1574 LTC1574-3.3/LTC1574-5 U TYPICAL APPLICATIO S Low Dropout 5V Step-Down Regulator with Low-Battery Detection High Efficiency 3.3V Regulator VIN 5.5V to 12.5V VIN 4V TO 12.5V 5 4.7k VIN *LOWBATTERY INDICATOR 6 11 162k 0.1µF 12 SHDN IPGM LTC1574-5 LBOUT VOUT LBIN SW 7 3, 14 L1 100µH† + 2, 4, 13, 15 VIN 6 12 11 47µF** 16V ×2 7 SHDN LTC1574-3.3 10 LBIN VOUT IPGM LBOUT SW 3, 14 22µF* 25V ×2 0.1µF VOUT 3.3V 425mA 50µH† + GND 2, 4, 13, 15 * AVX TPSD226K025 ** AVX TPSD476K016 † COILTRONICS CTX50-4 1574 TA04 * LOW-BATTERY INDICATOR IS SET UP TO TRIP AT VIN = 5.5V ** AVX TPSD476K016 + 5 47µF** 16V ×2 VOUT 5V 365mA 10 GND 47.5k + 47µF* 16V ×2 1574 TA05 † SELECTION MANUFACTURER COILTRONICS SUMIDA GOWANDA PART NO. CTX100-4 CD75-101 GA10-103K TYPE SURFACE MOUNT SURFACE MOUNT THROUGH HOLE Positive to – 5V Converter VIN 4V TO 12.5V *LOWBATTERY INDICATOR † SELECTION 280k MANUFACTURER COILTRONICS COILCRAFT SUMIDA GOWANDA PART NO. CTX50-3 DT3316-473 CD54-470 GA10-472K 5 4.7k * LOW-BATTERY INDICATOR IS SET TO TRIP AT VIN = 4.4V ** AVX TPSD106K035 *** AVX TPSD107K010 TYPE SURFACE MOUNT SURFACE MOUNT SURFACE MOUNT THROUGH HOLE 6 11 12 43k 0.1µF VIN IPGM SHDN LTC1574-5 VOUT LBOUT LBIN SW GND + 7 10µF** 35V ×2 10 VIN (V) 4 6 8 10 12.5 IOUT (mA) 110 140 170 200 235 3, 14 L1† 50µH + 2, 4, 13, 15 100µF*** 10V VOUT –5V 1574 TA06 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT®1074/LT1076 Step-Down Switching Regulator 100kHz, 5A (LT1074) or 2A (LT1076) Monolithic LTC1147 High Efficiency Step-Down DC/DC Controller 8-Pin Controller LTC1174 High Efficiency Step-Down and Inverting DC/DC Converter 0.5A, Burst Mode Operation, SO-8 Package, VIN to 18V LTC1265 1.2A High Efficiency Step-Down DC/DC Regulator Burst Mode Operation, Monolithic LT1375/LT1376 1.5A 500kHz Step-Down Switching Regulator High Frequency Small Inductor LT1611 Inverting 1.4MHz Switching Regulator in SOT-23 – 5V at 150mA from 5V Input, 1mVP-P Output Ripple, SOT-23 Package LTC1701 1MHz Step-Down DC/DC Converter in SOT-23 VIN = 2.5V to 5.5V, IQ = 135µA, VOUT = 5V to 1.25V LTC1707 High Efficiency Synchronous Step-Down Regulator VIN = 2.85V to 8.5V, Selectable Burst Mode Operation, 600mA Output Current, SO-8 Package LTC1877/LTC1878 High Efficiency Synchronous Step-Down Regulator 8 Linear Technology Corporation 600mA at VIN = 5V, 2.65V to 10V = VIN, IQ = 10µA sn1574 1574fas LT/TP 1000 2K REV A • PRINTED IN 1630 McCarthy Blvd., Milpitas, CA 95035-7417 USA (408)432-1900 FAX: (408) 434-0507 www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 1995 ● ●