LTC1574 LTC1574-3.3/LTC1574-5 High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 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 = 12V Standby Current: 130µA Active Low Micropower Shutdown U APPLICATIONS ■ ■ ■ ■ ■ ■ 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 and LTC1265 data sheets. For detailed applications information, see the LTC1174 data sheet. U TYPICAL APPLICATION LTC1574-5 Efficiency High Efficiency Step-Down Converter 100 + 5 VIN 12 11 6 SHDN LBIN LBOUT LTC1574-5 VOUT SW IPGM 7 10 3, 14 100µH† + GND 2, 4, 13, 15 * AVX TPSD226K035 ** AVX TPSD107K010 † COILTRONICS CTX100-4 5V 175mA 100µF* 10V 1574 TA01 L = 100µH VOUT = 5V IPGM = 0V 95 22µF* 35V EFFICIENCY (%) VIN 5.5V to 16V 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 U W W U U W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (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 1) ............................ 125°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER TOP VIEW NC 1 16 NC GND 2 15 GND SW 3 14 SW GND 4 13 GND VIN 5 12 LBIN IPGM 6 SHDN 7 NC 8 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 TA = 25°C, VIN = 9V, VSHUTDOWN = 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 VIN = 6V to 12V, ILOAD = 100mA, IPGM = VIN (Note 2) Output Voltage Load Regulation LTC1574-3.3 (Note 2) LTC1574-5 (Note 2) IQ Input DC Supply Current (Note 3) Active Mode Sleep Mode Shutdown (Note 4) MIN MAX UNITS 1 µA 1.25 1.30 V 3.30 5.00 3.46 5.25 V V 10 70 mV 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN –5 – 45 – 70 – 70 mV mV 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 VSHUTDOWN = 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 Shutdown Pin High Minimum Voltage at Pin 7 for Device to Be Active VIL Shutdown Pin Low Maximum Voltage at Pin 7 for Device to Be in Shutdown IIH Shutdown Pin Input Current VSHUTDOWN = 16V 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 0.78 0.50 A A 0.9 1.55 Ω 4 5 µs ● 3 1.2 V 0.75 V 2 µA LTC1574 LTC1574-3.3/LTC1574-5 ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 9V, VSHUTDOWN = VIN, IPGM = 0V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN IIL Shutdown Pin Input Current 0 ≤ VSHUTDOWN ≤ 0.8V VF Schottky Diode Forward Voltage Forward Current = 200mA IR Schottky Reverse Current Reverse Voltage = 5V Reverse Voltage = 18.5V TYP MAX UNITS 0.5 µA 0.450 0.570 V 10 100 25 250 µA µA Note 3: Does not include Schottky reverse current. Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 4: Current into Pin 5 only, measured without electrolytic input capacitor. The ● denotes specifications which apply over the full operating temperature range. Note 1: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: TJ = TA + (PD × 110°C/W) Note 2: Guaranteed by Design. U W TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Load Current Efficiency vs Load Current EFFICIENCY (%) 90 EFFICIENCY (%) VIN = 5V VIN = 9V 80 70 L = 50µH VOUT = 3.3V IPGM = VIN COIL = CTX50-4 60 Efficiency vs Input Voltage 100 95 95 94 VIN = 6V 90 EFFICIENCY (%) 100 VIN = 9V 85 80 L = 50µH VOUT = 5V IPGM = VIN COIL = CTX50-4 75 1 10 100 LOAD CURRENT (mA) Efficiency Using Different Types of Inductor Core Material 5 1 10 100 LOAD CURRENT (mA) 500 14 TA = 25°C 1.6 1.5 140 1.4 120 100 80 60 1.3 1.2 1.1 1.0 40 0.9 20 0.8 0 50 13 1.7 RDS(ON) (Ω) LEAKAGE CURRENT (nA) VIN = 5V VOUT = 3.3V IPGM = VIN 60 8 9 10 11 12 INPUT VOLTAGE (V) 7 Switch Resistance vs Input Voltage CTX50-4 70 6 1574 • TPC03 VIN = 13.5V 160 EFFICIENCY (%) 400 180 100 CTX50-4P ILOAD = 100mA IPGM = 0V 91 Switch Leakage Current vs Temperature 80 ILOAD = 300mA IPGM = VIN 1574 • TPC02 1574 • TPC01 90 92 89 10 100 LOAD CURRENT (mA) 1 500 93 90 70 50 VOUT = 5V L = 100µH COIL = CTX100-4 0.7 0 20 60 40 TEMPERATURE (°C) 80 100 4 6 8 10 12 14 16 INPUT VOLTAGE (V) 18 20 1574 • TPC04 1574 • TPC05 1574 • TPC06 3 LTC1574 LTC1574-3.3/LTC1574-5 U U U PIN FUNCTIONS 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 APPLICATIONS INFORMATION 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 ) By choosing a smaller inductor, a low ESR (Effective Series Resistance) output filter capacitor has to be used. Core loss will increase due to higher ripple current. 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 U W U APPLICATIONS INFORMATION going into Pin 12 being negligible, the following expression is used for setting the trip limit: R4 VLBTRIP = 1.25 1 + 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 1.25V REFERENCE 12 1574 • F02 11 Figure 2. Low-Battery Comparator 6 2 × 47µF* 16V LBIN SHDN LBOUT VOUT SW 7 10 3, 14 50µH** + LTC1574-5 GND 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: 2 × 47µF* 16V VOUT –5V 45mA 2, 4, 13, 15 * AVX TPSD476K016 ** COILTRONICS CTX50-4 1574 • F04 Figure 4. Positive-to-Negative 5V Converter Low Noise Regulators R2 VOUT = 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 VFB + 0.1µF IPGM LTC1574 Adjustable Applications LTC1574 + 5 VIN 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 VIN Figure 3. LTC1574 Adjustable Configuration 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 + 5 12 11 6 LTC1574 SHDN LBIN LBOUT SW IPGM VFB 7 3, 14 10 VIN 5V 100µF* 10V L1** 100µH 56k C2 6.8nF + GND 2, 4, 13, 15 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 APPLICATIONS INFORMATION 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. Design Example 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. INDUCTOR CURRENT IPEAK AVG CURRENT = IOUT +I I = PEAK V IV 2 V + VD IRIPPLE = 4 × 10 −6 OUT L 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: = 350mA TIME IRMS = 1574 • F06 Figure 6. Continuous Inductor Current [ ( IOUT VOUT VIN − VOUT )] VIN ( IPEAK A RMS 2 = 300mA IRMS ≈ ) U TYPICAL APPLICATIONS Low Noise, High Efficiency 3.3V Regulator + 5 VIN 6 12 11 SHDN IPGM LBIN LTC1574 VFB SW LBOUT 7 * AVX TPSD226K025 ** AVX TPSD107K010 † COILTRONICS CTX50-4 6 0.1µF 50µH† GND 2, 4, 13, 15 22µF* 25V ×2 10 3, 14 1/ 2 (ARMS) = 174mA For COUT, the RMS current rating should be at least: 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: VIN 4V TO 12.5V (AP-P ) 100pF 6.8nF + 100µF** 10V ×2 56k VOUT 3.3V 450mA 33k 1574 TA03 LTC1574 LTC1574-3.3/LTC1574-5 U TYPICAL APPLICATIONS Low Dropout 5V Step-Down Regulator with Low-Battery Detection VIN 5.5V to 12.5V † SELECTION MANUFACTURER COILTRONICS SUMIDA GOWANDA 6 11 162k PART NO. CTX100-4 CD75-101 GA10-103K TYPE SURFACE MOUNT SURFACE MOUNT THROUGH HOLE 0.1µF VIN *LOWBATTERY INDICATOR * LOW-BATTERY INDICATOR IS SET UP TO TRIP AT V IN = 5.5V ** AVX TPSD476K016 + 5 4.7k 12 IPGM 7 SHDN 10 LBOUT LTC1574-5 VOUT 3, 14 SW LBIN L1 100µH† GND 47.5k 47µF** 16V ×2 + 2, 4, 13, 15 VOUT 5V 47µF** 365mA 16V ×2 1574 TA04 High Efficiency 3.3V Regulator VIN 4V TO 12.5V + 5 VIN 6 12 11 SHDN IPGM LBIN LTC1574-3.3 VOUT SW LBOUT 7 22µF* 25V ×2 0.1µF 10 50µH† 3, 14 + GND 2, 4, 13, 15 * AVX TPSD226K025 ** AVX TPSD476K016 † COILTRONICS CTX50-4 47µF* 16V ×2 VOUT 3.3V 425mA 1574 TA05 Positive to –5V Converter VIN 4V TO 12.5V † SELECTION MANUFACTURER COILTRONICS COILCRAFT SUMIDA GOWANDA *LOWBATTERY INDICATOR 280k PART NO. CTX50-3 DT3316-473 CD54-470 GA10-472K TYPE SURFACE MOUNT SURFACE MOUNT SURFACE MOUNT THROUGH HOLE 0.1µF VIN 6 11 12 43k + 5 4.7k * LOW-BATTERY INDICATOR IS SET TO TRIP AT V IN = 4.4V ** AVX TPSD106K035 *** AVX TPSD107K010 IPGM SHDN LBOUT VOUT LTC1574-5 SW LBIN GND 7 10 10µF** 35V ×2 V IN (V) I OUT (mA) 4 110 6 140 8 170 10 200 12.5 235 3, 14 L1† 50µH 2, 4, 13, 15 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. + 100µF*** 10V VOUT –5V 1574 TA06 7 LTC1574 LTC1574-3.3/LTC1574-5 U PACKAGE DESCRIPTION Dimension in inches (millimeters) unless otherwise noted. S Package 16-Lead Plastic SOIC 0.386 – 0.394* (9.804 – 10.008) 16 15 14 13 12 11 10 9 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 2 3 4 5 0.053 – 0.069 (1.346 – 1.752) 0.014 – 0.019 (0.355 – 0.483) 7 8 0.004 – 0.010 (0.101 – 0.254) 0° – 8° TYP 0.016 – 0.050 0.406 – 1.270 6 0.050 (1.270) TYP SO16 0695 *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 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT ® 1076 Step-Down Switching Regulator 2A Monolithic Bipolar Switcher for VIN to 60V LTC1174 High Efficiency Step-Down/Inverting DC/DC Converter Same as LTC1574 Without Schottky Diode in SO-8 Package LTC1265 1.2A, High Efficiency Step-Down DC/DC Converter Current Mode with 0.3Ω Switch for Higher Current LT1375/LT1376 1.5A, 500kHz Step-Down Switching Regulator High Frequency, Synchronizable in SO-8 Package 8 Linear Technology Corporation LT/GP 0795 6K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1995