19-1919; Rev 1; 01/02 24V Internal Switch, 100% Duty Cycle, Step-Down Converters ____________________________Features ♦ 4.5V to 24V Input Voltage Range The unique current-limited control scheme, operating with duty cycles up to 100%, minimizes the dropout voltage (120mV at 100mA). Additionally, this control scheme reduces supply current under light loads to 12µA. High switching frequencies allow the use of tiny surface-mount inductors and output capacitors. The MAX1836/MAX1837 step-down converters with internal switching MOSFETs are available in a 6-pin SOT23 package, making them ideal for low-cost, lowpower, space-sensitive applications. For increased output drive capability, use the MAX1776 step-down converter that uses an internal 24V switch to deliver up to 500mA. For even higher currents, use the MAX1626/ MAX1627 step-down controllers that drive an external P-channel MOSFET to deliver up to 20W. ♦ Internal P-Channel MOSFET ♦ Preset 3.3V or 5V Output ♦ Adjustable Output from 1.25V to VIN ♦ Output Currents Up to 125mA (MAX1836) or 250mA (MAX1837) ♦ Efficiency Over 90% ♦ 12µA Quiescent Current ♦ 3µA Shutdown Current ♦ 100% Maximum Duty Cycle for Low Dropout ♦ Current-Limiting and Overtemperature Protection ♦ Small 6-Pin SOT23 Package Ordering Information TOP MARK PART TEMP RANGE PINPACKAGE MAX1836EUT50-T -40°C to +85°C 6 SOT23-6 AANW MAX1836EUT33-T -40°C to +85°C 6 SOT23-6 AANY MAX1837EUT50-T -40°C to +85°C 6 SOT23-6 AANX 9V Battery Systems MAX1837EUT33-T -40°C to +85°C 6 SOT23-6 AANZ Notebook Computers Note: The MAX1836/MAX1837 require special solder temperature profile described in the Absolute Maximum Ratings. ________________________Applications Distributed Power Systems Selector Guide Backup Supplies 4mA to 20mA Loop Power Supplies PART Industrial Control Supplies Hand-Held Devices Typical Operating Circuit INPUT 4.5V TO 24V PRESET OUTPUT VOLTAGE (V) MAX1836EUT50 5 125 MAX1836EUT33 3.3 125 MAX1837EUT50 5 250 MAX1837EUT33 3.3 250 OUTPUT 3.3V OR 5V IN LOAD CURRENT (mA) Pin Configuration LX TOP VIEW SHDN OUT MAX1836 MAX1837 GND FB 1 GND 2 MAX1836 MAX1837 6 OUT 5 SHDN 4 LX FB IN 3 NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. SOT23 ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1836/MAX1837 General Description The MAX1836/MAX1837 high-efficiency step-down converters provide a preset 3.3V or 5V output voltage from supply voltages as high as 24V. Using external feedback resistors, the output voltage may be adjusted from 1.25V to VIN. An internal current-limited switching MOSFET delivers load currents up to 125mA (MAX1836) or 250mA (MAX1837). MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters ABSOLUTE MAXIMUM RATINGS IN, SHDN to GND ...................................................-0.3V to +25V LX to GND.......................................................-2V to (VIN + 0.3V) OUT, FB to GND.......................................................-0.3V to +6V Continuous Power Dissipation (TA = +70°C) (Note 1) 6-Pin SOT23 (derate 8.7mW/°C above +70°C)............696mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Note 1: Thermal properties are specified with product mounted on PC board with 1in2 of copper area and still air. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER Input Supply Range Input Undervoltage Lockout Threshold Input Supply Current Input Supply Current in Dropout SYMBOL CONDITIONS VIN VUVLO MIN MAX UNITS 24 V VIN rising 3.55 4.0 4.4 VIN falling 3.45 3.9 4.3 12 25 IIN IIN(DROP) TYP 4.5 VIN = 5V 18 Input Shutdown Current SHDN = GND 3 7 5.00 5.20 Output Voltage (Preset Mode) FB = GND, ILOAD = 0 to 125mA (MAX1836) or 250mA (MAX1837) VOUT Output Voltage Range (Adjustable Mode) VOUT Feedback Set Voltage (Adjustable Mode) VFB OUT Bias Current FB Bias Current TM FB Dual Mode Threshold tOFF(MIN) LX Switch Maximum On-Time tON(MAX) RLX ILIM MAX183_EUT33 3.168 VFB = 0 or 1.25V, TA = +25°C 3.30 1.25 1.200 µA µA 1.25 2.5 -25 3.432 VIN V 1.300 V 7.4 µA +25 nA mV 50 100 150 0.2 0.4 0.6 µs 7 10 13 µs 1.1 2 Ω MAX1836 250 312 450 MAX1837 500 625 850 VFB = 1.3V VIN = 6V LX Zero-Crossing Threshold -75 Dual Mode is a trademark of Maxim Integrated Products, Inc. 2 µA V (Note 2) VFB rising or falling LX Switch Minimum Off-Time LX Current Limit 4.80 VOUT = 5V IFB LX Switch On-Resistance MAX183_EUT50 V _______________________________________________________________________________________ +75 mA mV 24V Internal Switch, 100% Duty Cycle, Step-Down Converters (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL Zero-Crossing Timeout LX Switch Leakage Current Dropout Voltage CONDITIONS MIN LX does not rise above the threshold TYP VIN = 18V, LX = GND, TA = +25°C VDROPOUT MAX 30 UNITS µs 1 µA IOUT = 100mA, VIN = 5V 120 mV Line Regulation VIN = 5V to 24V 0.05 % Load Regulation IOUT = 0 to 125mA (MAX1836) or 250mA (MAX1837) 0.3 % Shutdown Input Threshold V SHDN VIN = 4.5V to 24V (Note 3) 0.8 Shutdown Leakage Current ISHDN V SHDN = 0 or 24V -1 Thermal Shutdown 10°C hysteresis (typ) 2.4 V +1 µA °C 160 ELECTRICAL CHARACTERISTICS (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 4) PARAMETER Input Supply Range Input Undervoltage Lockout Threshold Input Supply Current SYMBOL CONDITIONS MAX UNITS 4.5 24 V VIN rising 3.55 4.4 VIN falling 3.45 4.3 VIN VUVLO MIN IIN Input Shutdown Current SHDN = GND Output Voltage (Preset Mode) FB = GND, ILOAD = 0 to 125mA (MAX1836) or 250mA (MAX1837) VOUT Output Voltage Range (Adjustable Mode) VOUT Feedback Set Voltage (Adjustable Mode) VFB (Note 2) OUT Bias Current VOUT = 5V FB Dual Mode Threshold VFB rising or falling LX Switch Minimum Off-Time tOFF(MIN) LX Switch Maximum On-Time tON(MAX) LX Switch On-Resistance RLX LX Current Limit ILIM VFB = 1.3V MAX183_EUT50 4.80 TYP V 25 µA 7 µA 5.20 V MAX183_EUT33 3.168 3.432 1.25 VIN V 1.200 1.300 V 7.4 µA 50 150 mV 0.2 0.6 µs 7 13 µs 2 Ω VIN = 6V MAX1836 250 450 MAX1837 500 900 mA _______________________________________________________________________________________ 3 MAX1836/MAX1837 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 4) PARAMETER SYMBOL CONDITIONS MIN LX Zero-Crossing Threshold TYP MAX UNITS mV -75 +75 Shutdown Input Threshold V SHDN VIN = 4.5V to 24V (Note 3) 0.8 2.4 V Shutdown Leakage Current ISHDN V SHDN = 0 or 24V -1 +1 µA Note 2: When using the shutdown input, the maximum output voltage allowed with external feedback is 5.5V. If the output voltage is set above 5.5V, connect shutdown to the input. Note 3: Shutdown input minimum slew rate (rising or falling) is 10V/ms. Note 4: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.) MAX1836EUT33 EFFICIENCY vs. LOAD CURRENT 3.31 3.30 VIN = 9V to 12V 3.29 VIN = 9V 90 85 80 MAX1836/7 toc03 VIN = 5V EFFICIENCY (%) OUTPUT VOLTAGE (V) VIN = 5V FIGURE 1 VOUT = 3.3V 95 3.33 MAX1836/7 toc02 FIGURE 1 3.32 100 MAX1836/7 toc01 3.33 MAX1837EUT33 OUTPUT VOLTAGE vs. LOAD CURRENT FIGURE 2 3.32 OUTPUT VOLTAGE (V) MAX1836EUT33 OUTPUT VOLTAGE vs. LOAD CURRENT 3.31 VIN = 5V VIN = 9V 3.30 VIN = 12V 3.29 VIN = 12V 70 3.27 50 100 150 3.27 0.1 200 1 10 100 1000 0 150 200 250 300 350 MAX1837EUT33 EFFICIENCY vs. LOAD CURRENT MAX1837EUT33 SWITCHING FREQUENCY vs. LOAD CURRENT MAX1837EUT33 OUTPUT VOLTAGE vs. INPUT VOLTAGE 85 80 VIN = 9V 140 VIN = 12V 120 3.33 IOUT = 10mA 3.32 OUTPUT VOLTAGE (V) VIN = 9V FIGURE 2 VOUT = 3.3V 160 FREQUENCY (kHz) VIN = 5V 90 180 MAX1836/7 toc04 FIGURE 2 VOUT = 3.3V 100 80 60 3.31 75 IOUT = 200mA 3.30 3.29 70 1 10 100 LOAD CURRENT (mA) 0 1000 FIGURE 2 VOUT = 3.3V L1 = 47µH 3.28 VIN = 5V 20 VIN = 12V 3.27 0 50 100 150 200 250 LOAD CURRENT (mA) 300 350 MAX1836/7 toc06 LOAD CURRENT (mA) 40 4 100 LOAD CURRENT (mA) 100 0.1 50 LOAD CURRENT (mA) MAX1836/7 toc05 0 95 3.28 75 3.28 EFFICIENCY (%) MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters 0 4 8 12 16 INPUT VOLTAGE (V) _______________________________________________________________________________________ 20 24 24V Internal Switch, 100% Duty Cycle, Step-Down Converters (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.) 80 FIGURE 2 VOUT = 3.3V L1 = 47µH 10 IOUT = 10mA 75 IOUT = 10mA 70 1000 800 IOUT = 10mA 400 200 8 12 16 20 24 LIMITED BY tON(MIN) 0 0 4 INPUT VOLTAGE (V) 8 12 16 20 0 24 4 VIN = 12V TO 24V VIN = 9V 100 5.00 VIN = 7V VIN = 9V VIN = 7V 4.98 FIGURE 6 VOUT = 5V 95 EFFICIENCY (%) 5.02 8 12 16 20 24 MAX1837EUT50 EFFICIENCY vs. LOAD CURRENT MAX1836/7 toc10 5.04 LIMITED BY ILIM INPUT VOLTAGE (V) INPUT VOLTAGE (V) MAX1837EUT50 OUTPUT VOLTAGE vs. LOAD CURRENT OUTPUT VOLTAGE (V) VIN = 12V 90 85 80 75 VIN = 18V VIN = 24V FIGURE 6 4.96 70 0 50 100 150 200 250 300 0.1 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) MAX1837EUT50 DROPOUT VOLTAGE vs. LOAD CURRENT NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE FIGURE 6 VOUT = 5V 350 15 14 SUPPLY CURRENT (µA) 300 250 200 150 100 MAX1836/7 toc13 400 MAX1836/7 toc12 4 IOUT = 200mA 600 1 0 FIGURE 2 VOUT = 3.3V L1 = 47µH MAX1836/7 toc09 MAX1836/7 toc08 IOUT = 200mA PEAK INDUCTOR CURRENT (mA) IOUT = 200mA DROPOUT VOLTAGE (mV) EFFICIENCY (%) 90 85 100 FREQUENCY (kHz) FIGURE 2 VOUT = 3.3V L1 = 47µH MAX1836/7 toc07 100 95 MAX1837EUT33 PEAK INDUCTOR CURRENT vs. INPUT VOLTAGE MAX1837EUT33 SWITCHING FREQUENCY vs. INPUT VOLTAGE MAX1836/7 toc11 MAX1837EUT33 EFFICIENCY vs. INPUT VOLTAGE 13 12 11 50 0 10 0 100 200 LOAD CURRENT (mA) 300 0 4 8 12 16 20 24 INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1836/MAX1837 Typical Operating Characteristics (continued) MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters Typical Operating Characteristics (continued) (Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.) MAX1837EUT50 LINE TRANSIENT MAX1837EUT50 LOAD TRANSIENT MAX1836/7 toc15 MAX1836/7 toc14 20V 400mA 200mA A 0 0 5.02V 5.1V 5.00V B 4.9V 750mA 500mA C B 5.0V 4.98V 250mA 0 A 10V C 0 400µs/div 100µs/div A: VIN = 9V to 18V, 10V/div B: VOUT = 5V, ROUT = 100Ω, 100mV/div C: IL, 500mA/div FIGURE 6 A: IOUT = 10mA to 250mA, 200mA/div B: VOUT = 5V, 20mV/div C: IL, 500mA/div VIN = 12V, FIGURE 6 MAX1837EUT50 STARTUP WAVEFORM MAX1837EUT50 LINE TRANSIENT NEAR DROPOUT MAX1836/7 toc17 MAX1836/7 toc16 15V 10V A 2V A 0 5V 4V 5.1V B 5.0V 2V B 0 4.9V 500mA 500mA C C 0 0 400µs/div A: VIN = 5V to 12V, 5V/div B: VOUT = 5V, ROUT = 100Ω, 100mV/div C: IL, 500mA/div FIGURE 6 6 200µs/div A: VSHDN = 0 to 2V, 2V/div B: VOUT = 5V, ROUT = 100Ω, 2V/div C: IL, 500mA/div VIN = 12V, FIGURE 6 _______________________________________________________________________________________ 24V Internal Switch, 100% Duty Cycle, Step-Down Converters PIN NAME FUNCTION Dual-Mode Feedback Input. Connect to GND for the preset 3.3V (MAX183_EUT33) or 5.0V (MAX183_EUT50) output. Connect to a resistive divider between the output and FB to adjust the output voltage between 1.25V and VIN, and connect the OUT pin to GND. When setting output voltages above 5.5V, permanently connect SHDN to IN. 1 FB 2 GND 3 IN Input Voltage. 4.5V to 24V input range. Connected to the internal P-channel power MOSFET’s source. 4 LX Inductor Connection. Connected to the internal P-channel power MOSFET’s drain. 5 SHDN Shutdown Input. A logic low shuts down the MAX1836/MAX1837 and reduces supply current to 3µA. LX is high impedance in shutdown. Connect to IN for normal operation. When setting output voltages above 5.5V, permanently connect SHDN to IN. 6 OUT Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive divider. Connect to the output when using the preset output voltage. Connect to GND when using an external resistive divider to adjust the output voltage. INPUT 4.5V OR 12V CIN 10µF 25V Ground L1 47µH IN OUTPUT 3.3V OR 5V LX D1 SHDN COUT 100µF 6.3V INPUT 4.5V OR 12V CIN 10µF 25V L1 22µH IN LX D1 SHDN OUT COUT 150µF 6.3V OUT MAX1836 GND OUTPUT 3.3V OR 5V MAX1837 FB GND FB CIN = TAIYO YUDEN TMK432BJ106KM L1 = SUMIDA CDRH5D28-470 COUT = SANYO POSCAP 6TPC100M (SMALLER CAPACITORS CAN BE USED FOR 5V) D1 = NIHON EP05Q03L CIN = TAIYO YUDEN TMK432BJ106KM L1 = SUMIDA CDRH5D28-220 COUT = SANYO OS-CON 6SA150M (SMALLER CAPACITORS CAN BE USED FOR 5V) D1 = NIHON ED05Q03L NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. Figure 1. Typical MAX1836 Application Circuit Figure 2. Typical MAX1837 Application Circuit Detailed Description The MAX1836/MAX1837 step-down converters are designed primarily for battery-powered devices, notebook computers, and industrial control applications. A unique current-limited control scheme provides high efficiency over a wide load range. Operation up to 100% duty cycle allows the lowest possible dropout voltage, increasing the useable supply voltage range. Under no-load, the MAX1836/MAX1837 draw only 12µA, and in shutdown mode, they draw only 3µA to further reduce power consumption and extend battery life. Additionally, an internal 24V switching MOSFET, internal current sensing, and a high switching frequency minimize PC board space and component cost. Current-Limited Control Architecture The MAX1836/MAX1837 use a proprietary current-limited control scheme that operates with duty cycles up to 100%. These DC-DC converters pulse as needed to maintain regulation, resulting in a variable switching frequency that increases with the load. This eliminates the high supply currents associated with conventional con- _______________________________________________________________________________________ 7 MAX1836/MAX1837 Pin Description MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters INPUT 4.5V OR 24V CIN L1 LX IN SHDN OUTPUT 3.3V OR 5V D1 COUT VSENSE OUT R Q FB MAXIMUM OFF-TIME DELAY S Q TRIG 100mV Q TRIG MAXIMUM ON-TIME DELAY VSET 1.25V GND MAX1836 MAX1837 Figure 3. Functional Diagram stant-frequency pulse-width-modulation (PWM) controllers that switch the MOSFET unnecessarily. When the output voltage is too low, an error comparator sets a flip-flop, which turns on the internal P-channel MOSFET and begins a switching cycle (Figure 3). As shown in Figure 4, the inductor current ramps up linearly, charging the output capacitor and servicing the load. The MOSFET turns off when the current limit is reached, or when the maximum on-time is exceeded while the output voltage is in regulation. Otherwise, the MOSFET remains on, allowing a duty cycle up to 100% to ensure the lowest possible dropout voltage. Once the MOSFET turns off, the flip-flop resets, diode D1 turns on, and the current through the inductor ramps back down, transferring the stored energy to the output capacitor and load. The MOSFET remains off until the 0.5µs minimum off-time expires and the inductor current ramps down to zero, and the output voltage drops back below the set point. 8 10V A 0 B 3.3V 500mA C 0 4µs/div CIRCUIT OF FIGURE 2, VIN = 12V A. VLX, 5V/div B. VOUT = 3.3V, 20mV/div, 200mA LOAD C. INDUCTOR CURRENT, 500mA/div Figure 4. Discontinuous-Conduction Operation _______________________________________________________________________________________ 24V Internal Switch, 100% Duty Cycle, Step-Down Converters Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. For a step-down converter with 100% duty cycle, the dropout voltage depends on the MOSFET drain-to-source onresistance (RDS(ON)) and inductor series resistance; therefore, it is proportional to the load current: ( VDROPOUT = IOUT × RDS(ON) + RINDUCTOR ) Shutdown (SHDN) A logic-level low voltage on SHDN shuts down the MAX1836/MAX1837. When shut down, the supply current drops to 3µA to maximize battery life, and the internal P-channel MOSFET turns off to isolate the output from the input. The output capacitance and load current determine the rate at which the output voltage decays. A logic-level high voltage on SHDN activates the MAX1836/MAX1837. Do not leave SHDN floating. If unused, connect SHDN to IN. When setting output voltages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN. The SHDN input voltage slew rate must be greater than 10V/ms. Thermal-Overload Protection Thermal-overload protection limits total power dissipation in the MAX1836/MAX1837. When the junction temperature exceeds TJ = +160°C, a thermal sensor turns off the pass transistor, allowing the IC to cool. The thermal sensor turns the pass transistor on again after the IC’s junction temperature cools by 10°C, resulting in a pulsed output during continuous thermal-overload conditions. (see the Selector Guide). For example, the MAX1836EUT33 has a preset 3.3V output voltage. The MAX1836/MAX1837 output voltage may be adjusted by connecting a voltage divider from the output to FB (Figure 5). When externally adjusting the output voltage, connect OUT to GND. Select R2 in the 10kΩ to 100kΩ range. Calculate R1 with the following equation: V R1 = R2 OUT - 1 VFB where VFB = 1.25V, and VOUT may range from 1.25V to VIN. When setting output voltages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN. Inductor Selection When selecting the inductor, consider these four parameters: inductance value, saturation current rating, series resistance, and size. The MAX1836/MAX1837 operate with a wide range of inductance values. For most applications, values between 10µH and 100µH work best with the controller’s switching frequency. Calculate the minimum inductance value as follows: L(MIN) = The feedback input features dual-mode operation. Connect the output to OUT and FB to GND for the preset output voltage. The MAX1836/MAX1837 are supplied with factory-set output voltages of 3.3V or 5V. The two-digit part number suffix identifies the output voltage ILIM where tON(MIN) = 1.0µs. Inductor values up to six times L(MIN) are acceptable. Low-value inductors may be smaller in physical size and less expensive, but they result in higher peak-current overshoot due to currentsense comparator propagation delay (300ns). Peakcurrent overshoot reduces efficiency and could exceed the current ratings of the internal switching MOSFET and external components. INPUT 4.5V OR 24V IN CIN OUTPUT 1.25V TO VIN L1 LX D1 SHDN R1 COUT FB Design Information Output Voltage Selection (VIN(MAX) - VOUT ) t ON(MIN) MAX1836 MAX1837 GND R2 OUT NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. Figure 5. Adjustable Output Voltage _______________________________________________________________________________________ 9 MAX1836/MAX1837 Input-Output (Dropout) Voltage A step-down converter’s minimum input-to-output voltage differential (dropout voltage) determines the lowest useable input supply voltage. In battery-powered systems, this limits the useful end-of-life battery voltage. To maximize battery life, the MAX1836/MAX1837 operate with duty cycles up to 100%, which minimizes the inputto-output voltage differential. When the supply voltage approaches the output voltage, the P-channel MOSFET remains on continuously to supply the load. MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters The inductor’s saturation current rating must be greater than the peak switching current, which is determined by the switch current limit plus the overshoot due to the 300ns current-sense comparator propagation delay: IPEAK = ILIM + (VIN - VOUT ) 300ns L where the switch current-limit (ILIM) is typically 312mA (MAX1836) or 625mA (MAX1837). Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support, and the inductance starts to fall. Inductor series resistance affects both efficiency and dropout voltage (see the Input-Output Voltage section). High series resistance limits the maximum current available at lower input voltages and increases the dropout voltage. For optimum performance, select an inductor with the lowest possible DC resistance that fits in the allotted dimensions. Typically, the inductor’s series resistance should be significantly less than that of the internal P-channel MOSFET’s on-resistance (1.1Ω typ). Inductors with a ferrite core, or equivalent, are recommended. The maximum output current of the MAX1836/MAX1837 current-limited converter is limited by the peak inductor current. For the typical application, the maximum output current is approximately: IOUT(MAX) = 1 IPEAK 2 Output Capacitor Choose the output capacitor to supply the maximum load current with acceptable voltage ripple. The output ripple has two components: variations in the charge stored in the output capacitor with each LX pulse, and the voltage drop across the capacitor’s equivalent series resistance (ESR) caused by the current into and out of the capacitor: VRIPPLE ≈ VRIPPLE(ESR) + VRIPPLE(C) The output voltage ripple as a consequence of the ESR and output capacitance is: VRIPPLE(ESR) = IPEAKESR L(IPEAK - IOUT ) VIN 2COUT VOUT VIN - VOUT 2 VRIPPLE(C) = where I PEAK is the peak inductor current (see the Inductor Selection section). These equations are suit10 able for initial capacitor selection, but final values should be set by testing a prototype or evaluation circuit. As a general rule, a smaller amount of charge delivered in each pulse results in less output ripple. Since the amount of charge delivered in each oscillator pulse is determined by the inductor value and input voltage, the voltage ripple increases with larger inductance but decreases with lower input voltages. With low-cost aluminum electrolytic capacitors, the ESR-induced ripple can be larger than that caused by the current into and out of the capacitor. Consequently, high-quality low-ESR aluminum-electrolytic, tantalum, polymer, or ceramic filter capacitors are required to minimize output ripple. Best results at reasonable cost are typically achieved with an aluminum-electrolytic capacitor in the 100µF range, in parallel with a 0.1µF ceramic capacitor. Input Capacitor The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input capacitor must meet the ripple-current requirement (IRMS) imposed by the switching currents defined by the following equation: IRMS = ILOAD VOUT (VIN - VOUT ) VIN For most applications, nontantalum chemistries (ceramic, aluminum, polymer, or OS-CON) are preferred due to their robustness with high inrush currents typical of systems with low-impedance battery inputs. Alternatively, two (or more) smaller-value low-ESR capacitors can be connected in parallel for lower cost. Choose an input capacitor that exhibits <+10°C temperature rise at the RMS input current for optimal circuit longevity. Diode Selection The current in the external diode (D1) changes abruptly from zero to its peak value each time the LX switch turns off. To avoid excessive losses, the diode must have a fast turn-on time and a low forward voltage. Use a diode with an RMS current rating of 0.5A or greater, and with a breakdown voltage >VIN. Schottky diodes are preferred. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-high-speed silicon rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance. ______________________________________________________________________________________ 24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837 Table 1. Component Suppliers SUPPLIER PHONE FAX WEBSITE Coilcraft 847-639-6400 847-639-1469 Coiltronics 561-241-7876 561-241-9339 www.coiltronics.com Sumida USA 847-956-0666 847-956-0702 www.sumida.com Toko 847-297-0070 847-699-1194 www.tokoam.com INDUCTORS www.coilcraft.com CAPACITORS AVX 803-946-0690 803-626-3123 www.avxcorp.com Kemet 408-986-0424 408-986-1442 www.kemet.com Panasonic 847-468-5624 847-468-5815 www.panasonic.com Sanyo 619-661-6835 619-661-1055 www.secc.co.jp Taiyo Yuden 408-573-4150 408-573-4159 www.t-yuden.com Central Semiconductor 516-435-1110 516-435-1824 www.centralsemi.com International 310-322-3331 310-322-3332 www.irf.com Nihon 847-843-7500 847-843-2798 www.niec.co.jp On Semiconductor 602-303-5454 602-994-6430 www.onsemi.com Zetex 516-543-7100 516-864-7630 www.zetex.com DIODES MAX1836/MAX1837 Stability Commonly, instability is caused by excessive noise on the feedback signal or ground due to poor layout or improper component selection. When seen, instability typically manifests itself as “motorboating,” which is characterized by grouped switching pulses with large gaps and excessive low-frequency output ripple during no-load or light-load conditions. PC Board Layout and Grounding High switching frequencies and large peak currents make PC board layout an important part of the design. Poor layout may introduce switching noise into the feedback path, resulting in jitter, instability, or degraded performance. High-power traces, bolded in the typical application circuits (Figures 1 and 2), should be as short and wide as possible. Additionally, the current loops formed by the power components (CIN, COUT, L1, and D1) should be as tight as possible to avoid radiated noise. Connect the ground pins of these power components at a common node in a star-ground configuration. Separate the noisy traces, such as the LX node, from the feedback network with grounded copper. Furthermore, keep the extra copper on the board, and integrate it into a pseudoground plane. When using external feedback, place the resistors as close to the feedback pin as possible to minimize noise coupling. The MAX1837 evaluation kit shows the recommended layout. Applications Information High-Voltage Step-Down Converter The typical application circuits’ (Figures 1 and 2) components were selected for 9V battery applications. However, the MAX1836/MAX1837 input voltage range allows supply voltages up to 24V. Figure 6 shows a modified application circuit for high-voltage applications. When using higher input voltages, verify that the input capacitor’s voltage rating exceeds VIN(MAX) and that the inductor value exceeds the minimum inductance recommended in the Inductor Selection section. Inverter Configuration Figure 7 shows the MAX1836/MAX1837 in a floating ground configuration. By connecting what would normally be the output to the supply-voltage ground, the IC’s ground pin is forced to regulate to -5V (MAX183_EUT50) or -3.3V (MAX183_EUT33). Avoid exceeding the maximum ratings of 24V between IN and GND, and 5.5V between OUT and GND. Other negative voltages may be generated by placing a resistive divider across the output capacitor and connecting the tap to FB in the same manner as the normal step-down configuration. ______________________________________________________________________________________ 11 MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters INPUT 4.5V TO 24V CIN 10µF 25V L1 47µH IN OUTPUT 5V LX D1 SHDN COUT 68µF 10V INPUT 3.6V TO 18V CIN 10µF L1 47µH IN LX OUT SHDN MAX1836 MAX1837 MAX1837 GND GND FB CIN = TAIYO YUDEN TMK432BJ106KM L1 = SUMIDA CDRH5D28-470 COUT = SANYO POSCAP 10TPC68M D1 = NIHON EP05Q03L COUT 100µF D1 OUT OUTPUT -3.3V OR -5V FB NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. Figure 7. MAX1836/MAX1837 Inverter Configuration NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. Chip Information Figure 6. High-Voltage Application TRANSISTOR COUNT: 731 PROCESS: BiCMOS 12 ______________________________________________________________________________________ 24V Internal Switch, 100% Duty Cycle, Step-Down Converters 6LSOT.EPS Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1836/MAX1837 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)