LT1913 25V, 3.5A, 2.4MHz Step-Down Switching Regulator U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LT®1913 is an adjustable frequency (200kHz to 2.4MHz) monolithic buck switching regulator that accepts input voltages up to 25V. A high efficiency 95m switch is included on the die along with a boost Schottky diode and the necessary oscillator, control, and logic circuitry. Current mode topology is used for fast transient response and good loop stability.
LT1913 W W U W ABSOLUTE AXI U RATI GS (Note 1) VIN, RUN/SS Voltage .................................................25V BOOST Pin Voltage ...................................................50V BOOST Pin Above SW Pin.........................................25V FB, RT, VC Voltage .......................................................5V PG, BD Voltage .........................................................25V SYNC Voltage ............................................................
LT1913 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS Feedback Voltage ● FB Pin Bias Current (Note 3) VFB = 0.8V, VC = 0.4V FB Voltage Line Regulation 4V < VIN < 25V MIN TYP MAX UNITS 780 775 790 790 800 805 mV mV 10 40 nA 0.002 0.
LT1913 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency Efficiency Efficiency 100 100 VIN = 12V EFFICIENCY (%) 80 VIN = 24V 70 60 50 0 0.5 80 VIN = 24V 70 60 VOUT = 5V L = 4.7μH f = 600kHz 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 3.5 EFFICIENCY (%) VIN = 12V 90 50 0.5 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 1913 G01 90 2.5 80 2.0 70 1.5 VIN = 12V VOUT = 5V L = 4.7μH f = 600kHz 50 3.5 0 LOAD CURRENT (A) 3.5 VOUT = 3.3V TA = 25°C L = 4.7μH f = 600kHz 3.0 5 10 4.5 MINIMUM 4.
LT1913 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Switching Frequency Feedback Voltage 840 Frequency Foldback 1200 1.20 RT = 34.0k RT = 34.0k SWITCHING FREQUENCY (kHz) 820 1.10 FREQUENCY (MHz) FEEDBACK VOLTAGE (mV) 1.15 800 780 1.05 1.00 0.95 0.90 1000 0.85 760 –50 –25 0 0.
LT1913 U W TYPICAL PERFOR A CE CHARACTERISTICS VC Voltages 6.5 2.50 6.0 2.00 Power Good Threshold 95 THRESHOLD VOLTAGE (%) Minimum Input Voltage CURRENT LIMIT CLAMP VC VOLTAGE (V) INPUT VOLTAGE (V) TA = 25°C unless otherwise noted. 5.5 5.0 VOUT = 5V TA = 25 °C L = 4.7μH f = 600kHz 4.5 1.00 SWITCHING THRESHOLD 85 80 0.50 4.0 1 1.
LT1913 U U U PI FU CTIO S BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BD also supplies current to the internal regulator. BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 3): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor.
LT1913 BLOCK DIAGRAM VIN 4 VIN – + C1 INTERNAL 0.79V REF 5 10 RUN/SS ∑ SLOPE COMP BD SWITCH LATCH BOOST 2 C3 R RT OSCILLATOR 200kHz TO 2.4MHz RT Q S SW 6 1 SYNC L1 VOUT 3 C2 D1 SOFT-START 7 PG ERROR AMP + – + – 0.
LT1913 OPERATION The LT1913 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin.
LT1913 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: V R1= R2 OUT – 1 0.79V Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT1913 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground.
LT1913 APPLICATIONS INFORMATION operating input voltage. Conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage transients of up to 25V are acceptable regardless of the switching frequency. In this mode, the LT1913 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation.
LT1913 APPLICATIONS INFORMATION load is lower than 3.5A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages.
LT1913 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, COMMANDS Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum Sanyo (408) 749-9714 www.sanyovideo.com T494, T495 Ceramic, Polymer, POSCAP Tantalum Murata (408) 436-1300 AVX www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com rating, may be required.
LT1913 APPLICATIONS INFORMATION Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.47μF capacitor will work well. Figure 2 shows three ways to arrange the boost circuit. The BOOST pin must be LT1913 CURRENT MODE POWER STAGE gm = 5.3mho SW ERROR AMPLIFIER OUTPUT R1 CPL FB gm = 525μmho + Loop compensation determines the stability and transient performance.
LT1913 APPLICATIONS INFORMATION VOUT BD BOOST VIN VIN LT1913 GND 4.7μF C3 SW (4a) For VOUT > 2.8V VOUT D2 BD BOOST VIN VIN LT1913 GND 4.7μF is more efficient because the BOOST pin current and BD pin quiescent current comes from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded. The minimum operating voltage of an LT1913 application is limited by the minimum input voltage (3.
LT1913 APPLICATIONS INFORMATION charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 5 shows a plot of minimum load to start and to run as a function of input voltage.
LT1913 APPLICATIONS INFORMATION the output is held high, then parasitic diodes inside the LT1913 can pull large currents from the output through the SW pin and the VIN pin. Figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. The Exposed Pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink.
LT1913 APPLICATIONS INFORMATION to prevent this overshoot. Figure 9 shows the waveforms that result when an LT1913 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 4.7μF ceramic capacitor at the input. The input voltage rings as high as 50V and the input current peaks at 26A. A good solution is shown in Figure 9b. A 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current).
LT1913 APPLICATIONS INFORMATION sistance. Because of the large output current capability of the LT1913, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125°C. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125°C.
LT1913 TYPICAL APPLICATIONS 3.3V Step-Down Converter VOUT 3.3V 3.5A VIN 4.8V TO 25V VIN BD RUN/SS ON OFF BOOST L 3.3μH 0.47μF VC 4.7μF SW LT1913 D RT 19k PG SYNC 63.4k 316k FB GND 680pF 47μF 100k f = 600kHz 1913 TA03 D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 2.5V Step-Down Converter VOUT 2.5V 3.5A VIN 4V TO 25V VIN BD RUN/SS ON OFF D2 BOOST 1μF VC 4.7μF LT1913 SW D1 RT 15.4k L 3.3μH PG 215k SYNC 63.
LT1913 TYPICAL APPLICATIONS 5V, 2MHz Step-Down Converter VOUT 5V 2.5A VIN 8.6V TO 22V VIN BD RUN/SS ON OFF BOOST L 2.2μH 0.47μF VC 4.7μF LT1913 SW D RT 15k PG 536k SYNC 12.7k FB GND 680pF 22μF 100k f = 2MHz 1913 TA05 D: ON SEMI MBRA340 L: NEC MPLC0730L2R2 12V Step-Down Converter VOUT 12V 3.5A VIN 15V TO 25V VIN BD RUN/SS ON OFF BOOST 0.47μF VC 10μF LT1913 SW D RT 17.4k L 8.2μH PG 715k SYNC 63.
LT1913 TYPICAL APPLICATIONS 1.8V Step-Down Converter VOUT 1.8V 3.5A VIN 3.6V TO 25V VIN BD RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT1913 SW D RT 16.9k L 3.3μH PG SYNC 78.
LT1913 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 ±0.05 3.50 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.115 TYP 6 0.38 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 0.200 REF 1 0.25 ± 0.05 0.50 BSC 0.75 ±0.05 0.00 – 0.05 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
LT1913 U TYPICAL APPLICATIO 1.2V Step-Down Converter VOUT 1.2V 3.5A VIN 3.6V TO 25V VIN BD RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT1913 SW D RT 17k L 3.3μH PG 52.3k SYNC 78.7k GND 470pF FB 100k 100μF f = 500kHz 1913 TA09 D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.