19-1926; Rev 0; 2/01 KIT ATION EVALU E L B A AVAIL Simple SOT23 Boost Controllers The MAX1522/MAX1523/MAX1524 are simple, compact boost controllers designed for a wide range of DC-DC conversion topologies, including step-up, SEPIC, and flyback applications. They are for applications where extremely low cost and small size are top priorities. These devices are designed specifically to provide a simple application circuit and minimize the size and number of external components, making them ideal for PDAs, digital cameras, and other low-cost consumer electronics applications. These devices use a unique fixed on-time, minimum offtime architecture, which provides excellent efficiency over a wide-range of input/output voltage combinations and load currents. The fixed on-time is pin selectable to either 0.5µs (50% max duty cycle) or 3µs (85% max duty cycle), permitting optimization of external component size and ease of design for a wide range of output voltages. The MAX1522/MAX1523 operate from a +2.5V to +5.5V input voltage range and are capable of generating a wide range of outputs. The MAX1524 is intended for bootstrapped operation, permitting startup with lower input voltage. All devices have internal soft-start and short-circuit protection to prevent excessive switching current during startup and under output fault conditions. The MAX1522/MAX1524 have a latched fault mode, which shuts down the controller when a shortcircuit event occurs, whereas the MAX1523 reenters soft-start mode during output fault conditions. The MAX1522/MAX1523/MAX1524 are available in a spacesaving 6-pin SOT23 package. ________________________Applications Low-Cost, High-Current, or High-Voltage Boost Conversion LCD Bias Supplies Industrial +24V and +28V Power Supplies ____________________________Features ♦ Simple, Flexible Application Circuit ♦ 2-Cell NiMH or Alkaline Operation (MAX1524) ♦ Low Quiescent Current (25µA typ) ♦ Output Fault Protection and Soft-Start ♦ High Efficiency Over 1000:1 IOUT Range ♦ Pin-Selectable Maximum Duty Factor ♦ Micropower Shutdown Mode ♦ Small 6-Pin SOT23 Package ♦ No Current-Sense Resistor Ordering Information PART TEMP. RANGE PINPACKAGE TOP MARK MAX1522EUT-T -40°C to +85°C 6 SOT23-6 AAOX MAX1523EUT-T -40°C to +85°C 6 SOT23-6 AAOY MAX1524EUT-T -40°C to +85°C 6 SOT23-6 AAOZ __________Typical Operating Circuit Low-Cost, Multi-Output Flyback Converters SEPIC Converters Low-Cost BatteryPowered Applications INPUT OUTPUT Pin Configurations VCC 6 V CC TOP VIEW GND 1 6 VCC 50% 85% FB 2 MAX1522 MAX1523 MAX1524 SET 3 5 EXT 4 SHDN OFF ON 3 4 MAX1522 SET MAX1523 MAX1524 SHDN EXT FB GND 5 N 2 1 SOT23-6 ________________________________________________________________ Maxim Integrated Products 1 For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. MAX1522/MAX1523/MAX1524 General Description MAX1522/MAX1523/MAX1524 Simple SOT23 Boost Controllers ABSOLUTE MAXIMUM RATINGS VCC, FB, SHDN, SET to GND ...................................-0.3V to +6V EXT to GND ................................................-0.3V to (VCC + 0.3V) Continuous Power Dissipation (TA = +70°C) 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 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 (VCC = SHDN = 3.3V, SET = GND , TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS VCC Operating Voltage Range VCC Minimum Startup Voltage Undervoltage Lockout Threshold 2.5 UNITS 5.5 V fEXT > 100kHz, MAX1524 (Note 1), bootstrap required 1.5 VCC rising 2.37 VCC falling No load, nonbootstrapped SHDN = GND Fixed tON Time VFB =1.2V 2.20 2.47 2.30 V V 25 50 µA 0.001 1 µA SET = GND 0.4 0.5 0.6 SET = VCC 2.4 3.0 3.6 VFB > 0.675V 0.5 VFB < 0.525V 1.0 µs µs SET = GND 45 50 55 SET = VCC 80 85 90 % FB Regulation Threshold (Note 2) VCC = +2.5V to +5.5V 1.23 1.25 1.27 V FB Undervoltage Fault Threshold (Note 2) FB falling 525 575 625 mV FB Input Bias Current VFB = 1.3V nA EXT Resistance IEXT = 20mA 6 50 EXT high 2 4 EXT low 1.5 3 3.2 4.2 Soft-Start Ramp Time Logic Input High 2.2 VCC = +2.5V to +5.5V, SET, SHDN Logic Input Low VCC = +2.5V to +5.5V, SET, SHDN Logic Input Leakage Current SET, SHDN = VCC or GND 1.6 -1 Note 1: Actual startup voltage is dependent on the external MOSFET’s VGS(TH). Note 2: Specification applies after soft-start mode is completed. 2 MAX 2.5 VCC Shutdown Current Maximum Duty Factor TYP MAX1522/MAX1523 VCC Supply Current Minimum tOFF Time MIN _______________________________________________________________________________________ Ω ms V 0.4 V +1 µA Simple SOT23 Boost Controllers EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 2) 90 70 60 80 VIN = +3.6V VIN = +2.7V 70 60 1000 10 1 100 1000 0.1 1 10 100 LOAD CURRENT (mA) LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 4) EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 5) STARTUP INPUT VOLTAGE vs. OUTPUT CURRENT VIN = +3.6V 80 70 VIN = +2.4V VIN = +2.7V 60 VOUT = +24V 50 50 0.1 1 100 10 1.25 1 0 100 10 SWITCHING WAVEFORM (CONTINUOUS CONDUCTION) BOOTSTRAPPED 1 50 75 100 SWITCHING WAVEFORM (DISCONTINUOUS CONDUCTION) MAX1522/3/4 toc08 MAX1522/3/4 toc07 10 25 LOAD CURRENT (mA) LOAD CURRENT (mA) NO-LOAD INPUT CURRENT vs. INPUT VOLTAGE VOUT = +3.3V BOOTSTRAPPED RESISTIVE LOADS 0.75 0.1 LOAD CURRENT (mA) 1.50 1.00 VIN = +1.8V MAX1524 VOUT = +3.3V 60 MAX1522/3/4 toc06 VIN = +3.0V 1000 1.75 STARTUP VOLTAGE (V) 80 MAX1522/3/4 toc05 90 EFFICIENCY (%) VIN = +4.2V 70 100 MAX1522/3/4 toc04 90 EFFICIENCY (%) 50 0.1 LOAD CURRENT (mA) 100 INPUT CURRENT (mA) MAX1524 VOUT = +5V 50 100 10 VIN = +1.8V 70 VOUT = +12V 50 1 80 60 VOUT = +5V VIN = 3.3V 0.1 VIN = +3V VIN = +2.4V 90 EFFICIENCY (%) 80 100 MAX1522/3/4 toc02 VIN = +4.2V EFFICIENCY (%) 90 EFFICIENCY (%) 100 MAX1522/3/4 toc01 100 EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 3) MAX1522/3/4 toc03 EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 1) MAX1522/3/4 toc09 A A 0.1 B B 0.01 NONBOOTSTRAPPED C 0.001 C 0.0001 0 1 2 3 INPUT VOLTAGE (V) 4 5 400ns/div VIN = +3.3V, VOUT = +5V, IOUT = 350mA A : VOUT, 200mV/div, AC-COUPLED B : VLX, 5V/div C : IL, 0.5A/div 4µs/div VIN = +3.3V, VOUT = +24V, IOUT = 10mA A : VOUT, 200mV/div, AC-COUPLED B : VLX, 10V/div C : IL, 0.5A/div _______________________________________________________________________________________ 3 MAX1522/MAX1523/MAX1524 Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) MAX1522/MAX1523/MAX1524 Simple SOT23 Boost Controllers Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) SOFT-START RESPONSE FAULT-DETECTION RESPONSE MAX1522/3/4 toc10 MAX1522/3/4 toc11 A A B B C C MAX1522 400µs/div 400µs/div 200Ω RESISTIVE LOAD A : VOUT, 5V/div B : VSHDN, 5V/div C : IL, 1A/div A : VOUT, 10V/div B : VEXT, 5V/div C : IL, 5A/div LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE MAX1522/3/4 toc13 MAX1522/3/4 toc12 A A B B 40µs/div VIN = +3.5V TO +4.0V, VOUT = +12V, IOUT = 60mA A : VIN, 500mV/div, AC-COUPLED B : VOUT, 10mV/div, AC-COUPLED 4 100µs/div VIN = +3.3V, VOUT = +12V, IOUT = 30mA TO 120mA A : IOUT, 100mA/div B : VOUT, 100mV/div, AC-COUPLED _______________________________________________________________________________________ Simple SOT23 Boost Controllers PIN NAME 1 GND 2 FB Feedback Input. Connect FB to external resistive voltage-divider. FB regulates to 1.25V. 3 SET On-Time Control. Connect SET to VCC to set the fixed 3µs on-time (85% duty cycle). Connect SET to GND to set the fixed 0.5µs on-time (50% duty cycle). See On-Time SET Input section for more information. 4 SHDN 5 EXT External MOSFET Drive. EXT drives the gate of an external NMOS power FET and swings from VCC to GND. VCC Supply Voltage to the IC. Bypass VCC to GND with a 0.1µF capacitor. Connect VCC to a +2.5V to +5.5V supply, which may come from VIN (nonbootstrapped) or VOUT (bootstrapped) or from the output of another regulator. For bootstrapped operation, connect VCC to the output through a series 10Ω resistor. 6 FUNCTION Ground Shutdown Control Input. Drive SHDN high for normal operation. Drive SHDN low for low-power shutdown mode. Driving SHDN low clears the fault latch of the MAX1522 and MAX1524. Detailed Description The MAX1522/MAX1523/MAX1524 are simple, compact boost controllers designed for a wide range of DC-DC conversion topologies including step-up, SEPIC, and flyback applications. These devices are designed specifically to provide a simple application circuit with a minimum of external components and are ideal for PDAs, digital cameras, and other low-cost consumer electronics applications. These devices use a unique fixed on-time, minimum off-time architecture, which provides excellent efficiency over a wide range of input/output voltage combinations and load currents. The fixed on-time is pin selectable to either 0.5µs or 3µs, permitting optimization of external component size and ease of design for a wide range of output voltages. Control Scheme The MAX1522/MAX1523/MAX1524 feature a unique fixed on-time, minimum off-time architecture, which provides excellent efficiency over a wide range of input/output voltage combinations. The fixed on-time is pin selectable to either 0.5µs or 3µs for a maximum duty factor of either 45% or 80%, respectively. An inductor charging cycle is initiated by driving EXT high, turning on the external MOSFET. The MOSFET remains on for the fixed on-time, after which EXT turns off the MOSFET. EXT stays low for at least the minimum off- time, and another cycle begins when FB drops below its 1.25V regulation point. Bootstrapped vs. Nonbootstrapped The V CC supply voltage range of the MAX1522/ MAX1523/MAX1524 is +2.5V to +5.5V. The supply for V CC can come from the input voltage (nonbootstrapped), the output voltage (bootstrapped), or an independent regulator. The MAX1522/MAX1523 are usually utilized in a nonbootstrapped configuration, allowing for high or low output voltage operation. However, when both the input and output voltages fall within the +2.5V to +5.5V range, the MAX1522/MAX1523 may be operated in nonbootstrapped or bootstrapped mode. Bootstrapped mode provides higher gate-drive voltage to the MOSFET switch, reducing I2R losses in the switch, but will also increase the VCC supply current to charge and discharge the gate. Depending upon the MOSFET selected, there may be minor variation in efficiency vs. load vs. input voltage when comparing bootstrapped and nonbootstrapped configurations. The MAX1524 is always utilized in bootstrapped configuration for applications where the input voltage range extends down below 2.5V and the output voltage is between 2.5V and 5.5V. VCC is connected to the output (through a 10Ω series resistor) and receives startup voltage through the DC current path from the input through the inductor, diode, and 10Ω resistor. The MAX1524 features a low-voltage startup oscillator that _______________________________________________________________________________________ 5 MAX1522/MAX1523/MAX1524 Pin Description MAX1522/MAX1523/MAX1524 Simple SOT23 Boost Controllers guarantees startup with input voltages down to 1.5V at VCC. The startup oscillator has a fixed 25% duty cycle and will toggle the MOSFET gate and begin boosting the output voltage. Once the output voltage exceeds the UVLO threshold, the normal control circuitry is used and the startup oscillator is disabled. However, N-channel MOSFETs are rarely specified for guaranteed RDS(ON) with VGS below 2.5V; therefore, guaranteed startup down to 1.5V input will be limited by the MOSFET specifications. Nevertheless, the MAX1524 bootstrapped circuit on the MAX1524 EV kit typically starts up with input voltage below 1V and no load. The MAX1522/MAX1523 may also be utilized by connecting VCC to the output of an independent voltage regulator between 2.5V and 5.5V to allow operation with any combination of low or high input and output voltages. In this case, the independent regulator must supply enough current to satisfy the I GATE current as calculated in the Power MOSFET Selection section when considering the maximum switching frequency as calculated in the CCM or DCM design procedure. On-Time SET Input The MAX1522/MAX1523/MAX1524 feature pin-selectable fixed on-time control, allowing their operation to be optimized for various input/output voltage combinations. Connect SET to VCC for the 3µs fixed on-time. Connect SET to GND for the 0.5µs fixed on-time. The 3µs on-time setting (SET = VCC) permits higher than 80% guaranteed maximum duty factor, providing improved efficiency in applications with higher step-up ratios (such as 3.3V boosting to 12V). This setting is recommended for higher step-up ratio applications. The 0.5µs on-time setting (SET = GND) permits higher frequency operation, minimizing the size of the external inductor and capacitors. The maximum duty factor is limited to 45% guaranteed, making this setting suitable for lower step-up ratios such as 3.3V to 5V converters. Soft-Start The MAX1522/MAX1523/MAX1524 have a unique softstart mode that reduces inductor current during startup, reducing battery, input capacitor, MOSFET, and inductor stresses. The soft-start period is fixed at 3.2ms and requires no external components. Fault Detection Once the soft-start period has expired, if the output voltage falls to, or is less than, 50% of its regulation value, a fault is detected. Under this condition, the MAX1522 disables the regulator until either SHDN is toggled low or power is removed and reapplied, after which it attempts to power up again in soft-start. For the 6 MAX1523, the fault condition is not latched, and softstart is repetitively reinitiated until a valid output voltage is realized. The MAX1524 has a latched fault detection, but when bootstrapped, the latch will be cleared when VCC falls below 2.37V. Shutdown Mode Drive SHDN to GND to place the MAX1522/MAX1523/ MAX1524 in shutdown mode. In shutdown, the internal reference and control circuitry turn off, EXT is driven to GND, the supply current is reduced to less than 1µA, and the output drops to one diode drop below the input voltage. Connect SHDN to VCC for normal operation. When exiting shutdown mode, the 3.2ms soft-start is always initiated. Undervoltage Lockout The MAX1522/MAX1523 have undervoltage lockout (UVLO) circuitry, which prevents circuit operation and MOSFET switching when VCC is less than the UVLO threshold (2.37V typ). The UVLO comparator has 70mV of hysteresis to eliminate chatter due to V CC input impedance. Applications Information Setting the Output Voltage The output voltage is set by connecting FB to a resistive voltage-divider between the output and GND (Figures 1 and 2). Select feedback resistor R2 in the 30kΩ to 100kΩ range. R1 is then given by: V R1 = R2 OUT − 1 VFB where VFB = 1.25V. Design Procedure Continuous vs. Discontinuous Conduction A switching regulator is operating in continuous conduction mode (CCM) when the inductor current is not allowed to decay to zero. This is accomplished by selecting an inductor value large enough that the inductor ripple current becomes less than one half of the input current. The advantage of this mode is that peak current is lower, reducing I2R losses and output ripple. In general, the best transient performance and most of the ripple reduction and efficiency increase of CCM are realized when the inductance is large enough to reduce the ripple current to 30% of the input current at maximum load. It is important to note that CCM circuits operate in discontinuous conduction mode (DCM) _______________________________________________________________________________________ Simple SOT23 Boost Controllers 2) Small output current. If the maximum output current is very small, the inductor required for CCM may be disproportionally large and expensive. Since I2R losses are not a concern, it may make sense to use a smaller inductor and run in DCM. This typically occurs when the load current times the output-to-input voltage ratio drops below a few hundred milliamps, although this also depends on the external components. Calculate the Maximum Duty Cycle The maximum duty cycle of the application is given by: DutyCycle(MAX ) = VOUT + VD − VIN(MIN) VOUT + VD nect SET to GND for 0.5µs on-time to get fast switching and a smaller inductor. For applications up to 80% duty cycle, it is necessary to connect SET to VCC for 3.0µs on-time. For applications greater than 80% duty cycle, CCM operation is not guaranteed; see the Design Procedure for DCM section. Switching Frequency A benefit of CCM is that the switching frequency remains high as the load is reduced, whereas in DCM the switching frequency varies directly with load. This is important in applications where switching noise needs to stay above the audio band. The medium- and heavyload switching frequency in CCM circuits is given by: ƒ SWITCHING = V + VD − VIN × OUT t ON VOUT + VD 1 Note that f SWITCHING is not a function of load and varies primarily with input voltage. However, when the load is reduced, a CCM circuit drops into DCM, and the frequency becomes load dependent: ƒ SWITCHING(LIGHT−LOAD) ≈ × 100% where VD is the forward voltage drop of the Schottky diode (about 0.5V). Design Procedure for CCM On-Time Selection For CCM to occur, the MAX1522/MAX1523/MAX1524 must be able to exceed the application’s maximum duty cycle. For applications up to 45% duty cycle, con- 1 × t ON VOUT + VD − VIN ILOAD × 0.18 × ILOAD(MAX) VOUT + VD Calculate the Peak Inductor Current For CCM, the peak inductor current is given by: V + VD IPEAK = 1.15 × OUT × ILOAD(MAX) VIN(MIN) INPUT 2.7V TO 4.2V C1 10µF 6.3V C3 0.1µF 6 3 OFF ON 4 VCC SET EXT MAX1522 MAX1523 SHDN FB GND 5 L1 33µH CDR74B-330 D1 MBR0530T3 Q1 R1 FDC633N 130kΩ 1% OUTPUT 12V CFF 220pF C2 33µF TPSD336M020R0200 2 1 R1 CFB 220pF 15.0kΩ 1% Figure 1. MAX1522/MAX1523 Standard Operating Circuit _______________________________________________________________________________________ 7 MAX1522/MAX1523/MAX1524 under light loads. The selection of 30% ripple current causes this to happen at loads less than approximately 1/6th of maximum load. There are two common reasons not to run in CCM: 1) High output voltage. In this case, the output-toinput voltage ratio exceeds the level obtainable by the MAX1522/MAX1523/MAX1524s’ maximum duty factor. Calculate the application’s maximum duty cycle using the equation in the Calculate the Maximum Duty Cycle section. If this number exceeds 80%, you will have to design for DCM. MAX1522/MAX1523/MAX1524 Simple SOT23 Boost Controllers INPUT 3.3V ±10% C1 10µF 6.3V R3 10Ω L1 33µH CR43-3R3 C3 0.1µF 6 V CC 3 OFF OUTPUT 5V D1 CRS01 4 ON SET EXT MAX1524 SHDN FB GND 5 Q1 FDC633N R1 100kΩ 1% C2 33µF 10TPA33M CFF 100pF 2 1 R2 33.2kΩ 1% Figure 2. MAX1524 Standard Operating Circuit Inductor Selection For CCM, the ideal inductor value is given by: LIDEAL = VIN(TYP) × t ON(TYP) 0.3 × IPEAK If LIDEAL is not a standard value, choose the next-closest value, either higher or lower. Nominal values within 50% are acceptable. Values lower than ideal will have slightly higher peak inductor current; values greater than ideal will have slightly lower peak inductor current. Due to the MAX1522/MAX1523/MAX1524s’ high switching frequencies, inductors with a ferrite core or equivalent are recommended. Powdered iron cores are not recommended due to their high losses at frequencies over 50kHz. The saturation rating of the selected inductor should meet or exceed the calculated value for I PEAK , although most coil types can be operated up to 20% over their saturation rating without difficulty. In addition to the saturation criteria, the inductor should have as low a series resistance as possible. The power loss in the inductor resistance is approximately given by: 2 I × (VOUT + VD ) PLR ≅ LOAD × RL VIN Output Capacitor Selection In CCM, to provide stable operation and to control output sag to less than 0.5%, the output bulk capacitance should be greater than: 8 COUT(MIN) = ILOAD(MAX) × t ON 0.005 × VOUT To properly control peak inductor current during the 3.2ms soft-start, the output bulk capacitance should be less than: ILOAD(MAX) × t SS COUT(MAX) = VOUT where tSS = 3.2ms. Because the MAX1522/MAX1523/MAX1524 are voltage-mode devices (and therefore do not require an expensive current-sense resistor), cycle-to-cycle stability is obtained from the output capacitor’s equivalent series resistance (ESR). Choose an output capacitor with actual ESR greater than: ESRCOUT > L COUT × ILOAD(MAX) VIN(MIN) Additionally, to control peak inductor current during softstart, the output capacitor’s ESR should be greater than: V ESRCOUT > 60 × 10−3 × FB IPEAK Usually, this prevents the use of ceramic capacitors in CCM applications. Alternatives include tantalum, electrolytic, and organic types such as Sanyo’s POSCAP. The output capacitor must also be rated to withstand the output voltage and the output ripple current, which is equivalent to IPEAK. Since output ripple in boost DCDC designs is dominated by capacitor ESR, a capaci- _______________________________________________________________________________________ Simple SOT23 Boost Controllers VRIPPLE(ESR) ≅ 0.3 × IPEAK × ESRCOUT at light and medium loads, and three times as great at peak load. Continue the CCM design procedure by going to the Optional Feed-Forward Capacitor Selection section. Design Procedure for DCM On-Time Selection The MAX1522/MAX1523/MAX1524 may operate in DCM at any duty cycle as required by the application’s input and output voltages. However, best performance is achieved when the maximum duty cycle of the application is similar to the MAX1522/MAX1523/MAX1524s’ maximum duty factor as set using the SET input. Connect SET to GND for applications with maximum duty cycles less than 67%. Connect SET to VCC for applications with maximum duty cycles between 67% and 99%. Inductor Selection For DCM, the ideal inductor value is given by: LIDEAL = (VIN(MIN) )2 × t ON(MIN) 3 × (VOUT + VD ) × ILOAD(MAX) If LIDEAL is not a standard value, choose the next lower nominal value. The above formula already includes a factor for ±30% inductor tolerance. Values higher than ideal may not supply the maximum load when the input voltage is low, while values much lower than ideal will have poorer efficiency. Calculate the Peak Inductor Current For DCM, the peak inductor current is given by: IPEAK = VIN(MAX) × t ON(MAX) L The saturation rating of the selected inductor should meet or exceed the calculated value for I PEAK , although most coil types can be operated up to 20% over their saturation rating without difficulty. In addition to the saturation criteria, the inductor should have as low a series resistance as possible. The power loss in the inductor resistance is approximately given by: PLR ≅ V + VD 2 IPEAK × IOUT × OUT RL VIN 3 Due to the MAX1522/MAX1523/MAX1524s’ high switching frequencies, inductors with a ferrite core or equivalent are recommended. Powdered iron cores are not recommended due to their high losses at frequencies over 50kHz. Switching Frequency In DCM, the switching frequency is proportional to the load current and is approximately given by: ƒ SWITCHING ≈ 0.7IOUT × (VOUT + VD − VIN ) × 2L t ON2 × VIN2 Note that fSWITCHING is a function of load and input voltage. Output Capacitor Selection In DCM, the MAX1522/MAX1523/MAX1524 may use either a ceramic output capacitor (with very low ESR) or other capacitors, such as tantalum or organic, with higher ESR. For less than 2% output ripple, the minimum value for ceramic output capacitors should be greater than: COUT(MIN) = t ON2 × VIN2 1 1 × × 2L (VOUT + VD − VIN ) 0.02VOUT To control inductor current during soft-start, the maximum value for any type of output capacitors should be less than: COUT(MAX) = ILOAD(MAX) × t SS VOUT where tSS = 3.2ms. The capacitor should be chosen to provide an output ripple between 25mV minimum and 2% of VOUT maximum. The output ripple due to capacitance ripple and ESR ripple can be approximated by: 1 t ON2 × VIN2 1 VRIPPLE(COUT+ESR) ≅ × × 2L (VOUT + VD − VIN ) COUT V × t + IN ON × ESRCOUT L For output ripple close to 2% of VOUT, the optional feed-forward capacitor may not be required. For lower output ripple, a feed-forward capacitor is necessary for stability and to control inductor current during soft-start. _______________________________________________________________________________________ 9 MAX1522/MAX1523/MAX1524 tance value two or three times larger than COUT(MIN) is typically needed. Output ripple due to ESR is: MAX1522/MAX1523/MAX1524 Simple SOT23 Boost Controllers Optional Feed-Forward Capacitor Selection For proper control of peak inductor current during softstart and for stable switching, the ripple at FB should be greater than 25mV. Without a feed-forward capacitor connected between the output and FB, the output’s ripple must be at least 2% of VOUT in order to meet this requirement. Alternatively, if a low-ESR output capacitor is chosen to obtain small output ripple, then a feed-forward capacitor should be used, and the output ripple may be as low as 25mV. The approximate value of the feed-forward capacitor is given by: 1 1 CFF ≅ 3 × 10−6 + R1 R2 Do not use a feed-forward capacitor that is much larger than this because line-transient performance will degrade. Do not use a feed-forward capacitor at all if the output ripple is large enough without it to provide stable switching because load regulation will degrade. Optional Feedback Capacitor Selection When using a feed-forward capacitor, it is possible to achieve too much ripple at FB. The symptoms of this include excessive line and load regulation and possibly high output ripple at light loads in the form of pulse groupings or “bursts.” Fortunately, this is easy to correct by either choosing a lower-ESR output capacitor or by adding a feedback capacitor between FB and ground. This feedback capacitor (CFB), along with the feed-forward capacitor, form an AC-coupled ripple voltage-divider from the output to FB: CFF RippleFB = RippleOUTPUT× + C C FB FF It is relatively simple to determine a good value for CFB experimentally. Start with CFB = CFF to cut the FB ripple in half; then increase or decrease CFB as needed. The ideal ripple at FB is from 25mV to 40mV, which will provide stable switching, low output ripple at light and medium loads, and reasonable line and load regulation. Never use a feedback capacitor without also using a feed-forward capacitor. Input Capacitor Selection The input capacitor (CIN) in boost designs reduces the current peaks drawn from the input supply, increases efficiency, and reduces noise injection. The source impedance of the input supply largely determines the value of CIN. High source impedance requires high input capacitance, particularly as the input voltage 10 falls. Since step-up DC-DC converters act as “constantpower” loads to their input supply, input current rises as input voltage falls. Consequently, in low-input-voltage designs, increasing CIN and/or lowering its ESR can add as many as five percentage points to conversion efficiency. A good starting point is to use the same capacitance value for C IN as for C OUT . The input capacitor must also meet the ripple current requirement imposed by the switching currents, which is about 30% of IPEAK in CCM designs and 100% of IPEAK in DCM designs. In addition to the bulk input capacitor, a ceramic 0.1µF bypass capacitor at VCC is recommended. This capacitor should be located as close to VCC and GND as possible. In bootstrapped configuration, it is recommended to isolate the bypass capacitor from the output capacitor with a series 10Ω resistor between the output and VCC. Power MOSFET Selection The MAX1522/MAX1523/MAX1524 drive a wide variety of N-channel power MOSFETs (NFETs). Since the output gate drive is limited to VCC, a logic-level NFET is required. Best performance, especially when VCC is less than 4.5V, is achieved with low-threshold NFETs that specify on-resistance with a gate-source voltage (VGS) of 2.7V or less. When selecting an NFET, key parameters include: 1) Total gate charge (Qg) 2) Reverse transfer capacitance or charge (CRSS) 3) On-resistance (RDS(ON)) 4) Maximum drain-to-source voltage (VDS(MAX)) 5) Minimum threshold voltage (VTH(MIN)) At high switching rates, dynamic characteristics (parameters 1 and 2 above) that predict switching losses may have more impact on efficiency than R DS(ON), which predicts I2R losses. Qg includes all capacitances associated with charging the gate. In addition, this parameter helps predict the current needed to drive the gate when switching at high frequency. The continuous VCC current due to gate drive is: IGATE = Qg × ƒ SWITCHING Use the FET manufacturer’s typical value for Qg (see manufacturer’s graph of Qg vs. Vgs) in the above equation since a maximum value (if supplied) is usually too conservative to be of any use in estimating IGATE. ______________________________________________________________________________________ Simple SOT23 Boost Controllers IDIODE(RMS) < IOUT × IPEAK Also, the diode reverse breakdown voltage must exceed VOUT. For high output voltages (50V or above), Schottky diodes may not be practical because of this voltage requirement. In these cases, use a high-speed silicon rectifier with adequate reverse voltage. Another consideration for high input voltages is reverse leakage of the diode. This should be considered using the manufacturer’s specification due to its direct influence on system efficiency. Layout Considerations High switching frequencies and large peak currents make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star-ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces (such as the trace from the junction of the inductor and MOSFET) away from the voltagefeedback network; also keep them separate, using grounded copper. The MAX1522/MAX1523/ MAX1524 evaluation kit manual shows an example PC board layout and routing scheme. Generating Resistance with PC Board Traces If the output capacitor’s ESR is too low for proper regulation, it can be increased artificially directly on the PC board. For example, an additional 50mΩ of ESR added to the output capacitor provides best regulation. The resistivity of a 10mil trace using 1oz copper is about 50mΩ per inch. Therefore, a 10mil trace 1in long generates the required resistance. ______________________________________________________________________________________ 11 MAX1522/MAX1523/MAX1524 Diode Selection The MAX1522/MAX1523/MAX1524s’ high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. Ensure that the diode’s current rating is adequate to withstand the diode’s RMS current: MAX1522/MAX1523/MAX1524 Simple SOT23 Boost Controllers Table 1. Design Examples Using CCM PARAMETER EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 VIN 3.3V ±10% 2.7V to 4.2V 1.8V to 3.0V VOUT 5V 12V 5V IOUT(MAX) 700mA 200mA 1.0A R1, R2 274kΩ, 90.9kΩ 866kΩ, 100kΩ 274kΩ, 90.9kΩ Duty Cycle (max) 45.5% 78.4% 67.3% tON 0.5µs (SET = GND) 3µs (SET = VCC) 3µs (SET = VCC) fSWITCHING 691kHz to 909kHz when IOUT > 120mA 221kHz to 261kHz when IOUT > 35mA 152kHz to 224kHz when IOUT > 167mA IPEAK 1.48A 1.06A 3.51A LIDEAL 3.73µH 33.8µH 6.83µH LACTUAL Sumida CR43-3R3 3.3µH, 86mΩ, 1.44A Sumida CDR74B-330 33µH, 180mΩ, 0.97A Sumida CDRH125-5R8 5.8µH, 17mΩ, 4.4A PLR 29mW at IOUT = 350mA 22mW at IOUT = 100mA 22mW at IOUT = 500mA COUT(MIN) to COUT(MAX) 14µF to 448µF 10µF to 53µF 120µF to 640µF COUT 33µF 33µF 150µF 23mΩ for stability, 51mΩ for soft-start Sanyo POSCAP 10TPA33M 33µF, 10V, 60mΩ, 100mΩ max 74mΩ for stability, 70mΩ for soft-start AVX TPSD336M020R0200 33µF, 20V, 150mΩ, 200mΩ max 21mΩ for stability, 21mΩ for soft-start Sanyo POSCAP 6TPB150M 150µF, 6.3V, 40mΩ, 55mΩ max VRIPPLE(ESR) 27mVp-p at light loads, 81mVp-p at full load 48mVp-p at light loads, 144mVp-p at full load 42mVp-p at light loads, 126mVp-p at full load CFF 100pF 100pF 100pF CFB 100pF 330pF 220pF CIN 10µF, 6.3V ceramic 10µF, 6.3V ceramic 10µF, 6.3V ceramic MOSFET Fairchild FDC633N Fairchild FDC633N Vishay Si3446DV Qg 8nC at Vgs = 3V 12nC at Vgs = 5V 9nC at Vgs = 3.6V 10nC at Vgs = 5V IGATE 7.3mA nonbootstrapped, 10.9mA bootstrapped 2.4mA nonbootstrapped 2.2mA bootstrapped IDIODE(RMS) 0.96A 0.49A 1.84A Diode Nihon EP10QY03, 1A Nihon EP10QY03, 1A Nihon EC21QS03L, 2A ESRCOUT(MIN) COUT(ACTUAL) 12 ______________________________________________________________________________________ Simple SOT23 Boost Controllers PARAMETER VIN EXAMPLE 4 2.7V to 4.2V EXAMPLE 5 1.8V to 3.0V VOUT 24V 3.3V IOUT(MAX) 30mA 100mA R1, R2 909kΩ, 49.9kΩ 150kΩ, 93.1kΩ Duty Cycle (max) 89.0% 52.6% tON 3µs (SET = VCC) 0.5µs (SET = GND) LIDEAL 11.9µH 1.14µH LACTUAL Sumida CDRH5D28-100 10µH, 65mΩ, 1.3A Sumida CDRH4D18-1R0 1µH, 45mΩ, 1.72A IPEAK 1.51A 1.80A PLR 4.5mW at IOUT = 10mA 5.7mW IOUT = 50mA fSWITCHING 208kHz when IOUT = 20mA 737kHz when IOUT = 100mA COUT(MIN) to COUT(MAX) 0.8µF to 2.7µF 3µF to 97µF COUT(ACTUAL) Taiyo Yuden GMK325BJ225K 2.2µF, X5R, 35V, 1210 Taiyo Yuden TMK316BT106ML 10µF, X7R, 6.3V, 1206 ESRCOUT(ACTUAL) 10mΩ 10mΩ VRIPPLE(COUT+ESR) 126mVp-p 40mVp-p CFF 100pF 220pF CFB 220pF 100pF optional CIN 10µF, 6.3V 10µF, 6.3V MOSFET Fairchild FDC633N Vishay Si2302DS Qg 8nC at Vgs = 3V 5nC at Vgs = 3.3V IGATE 1.7mA nonbootstrapped 3.7mA bootstrapped IDIODE(RMS) 0.17A 0.42A Diode Nihon EP10QY03, 1A Nihon EP10QY03, 1A Table 3. Component Manufacturers MANUFACTURER Coilcraft Fairchild International Rectifier Kemet NIC Components Panasonic Sumida Taiyo Yuden PHONE 847-639-6400 800-341-0392 WEB www.coilcraft.com www.fairchildsemi.com 310-322-3331 www.irf.com 408-986-0424 408-954-8470 847-468-5624 847-956-0666 408-573-4150 www.kemet.com www.niccomp.com www.panasonic.com www.sumida.com www.t-yuden.com Chip Information TRANSISTOR COUNT: 1302 ______________________________________________________________________________________ 13 MAX1522/MAX1523/MAX1524 Table 2. Design Examples Using DCM Simple SOT23 Boost Controllers 6LSOT.EPS MAX1522/MAX1523/MAX1524 Package Information 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. 14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.