LT3433IFE

LT3433 High Voltage Step-Up/Step-Down DC/DC Converter FEATURES s s s s s s s s s s s s s s DESCRIPTIO Automatic Step-Up and Step-Down Conversion Uses a Single Inductor Wide 4V to 60V Input Voltage Range VOUT from 3.3V to 20V Dual Internal 500mA Switches 100µA No-Load Quiescent Current Low Current Shutdown ±1% Output Voltage Accuracy 200kHz Operating Frequency Boosted Supply Pin to Saturate High Side Switch Frequency Foldback Protection Current Limit Foldback Protection Current Limit Unaffected by Duty Cycle 16-lead Thermally Enhanced TSSOP Package The LT®3433 is a 200kHz fixed-frequency current mode switching regulator that provides both step-up and stepdown regulation using a single inductor. The IC operates over a 4V to 60V input voltage range making it suitable for use in various wide input voltage range applications such as automotive electronics that must withstand both load dump and cold crank conditions. Internal control circuitry monitors system conditions and converts from single switch buck operation to dual switch bridged operation when required, seamlessly changing between step-down and step-up voltage conversion. Optional Burst Mode® operation reduces no-load quiescent current to 100µA and maintains high efficiencies with light loads. Current limit foldback and frequency foldback help prevent inductor current runaway during start-up. Programmable soft-start helps prevent output overshoot at start-up. The LT3433 is available in a 16-lead thermally enhanced TSSOP package. APPLICATIO S s s s 12V Automotive Systems Wall Adapter Powered Systems Battery Power Voltage Buffering , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. U.S. patent number: 5731694 TYPICAL APPLICATIO 4V to 60V to 5V DC/DC Converter with Burst Mode Operation 1N4148 VIN 2.2µF 0.01µF 330pF VIN SHDN SS VC 68k 1nF BURST_EN SWL VOUT 1N4148 VBIAS VFB 100k 3433 TA01 Maximum Output Current vs VIN 500 VOUT = 5V VBST SWH MAXIMUM OUTPUT CURRENT (mA) 0.1µF B160A 100µH B120A BUCK 400 300 EFFICIENCY (%) LT3433 + 0.1µF 309k VOUT 47µF 200 BRIDGED 100 SGND PGND 0 0 10 20 30 VIN (V) 40 50 60 3433 TA01c U Efficiency 90 80 VIN = 13.8V 70 60 50 40 30 20 0.1 VIN = 4V 1 10 100 OUTPUT CURRENT (mA) 1000 3433 TA01b U U 3433f 1 LT3433 ABSOLUTE MAXIMUM RATINGS (Note 1) PACKAGE/ORDER INFORMATION TOP VIEW SGND VBST SW_H VIN BURST_EN VC VFB SGND 1 2 3 4 5 6 7 8 17 16 SGND 15 SW_L 14 PWRGND 13 VOUT 12 VBIAS 11 SHDN 10 SS 9 SGND Input Supply (VIN) .................................... –0.3V to 60V Boosted Supply (VBST) .............. –0.3V to VSW_H + 30V (VBST(MAX) = 80V) Internal Supply (VBIAS) ............................. – 0.3V to 30V SW_H Switch Voltage .................................. – 2V to 60V SW_L Switch Voltage ............................... – 0.3V to 30V Feedback Voltage (VFB) ............................... – 0.3V to 5V Burst Enable Pin (VBURST_EN) ................... – 0.3V to 30V Shutdown Pin (VSHDN) ............................. – 0.3V to 60V Operating Junction Temperature Range (Note 5) LT3433E (Note 6) ............................ – 40°C to 125°C LT3433I ........................................... – 40°C to 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER LT3433EFE LT3433IFE FE PART MARKING 3433EFE 3433IFE FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W EXPOSED PAD (PIN 17) MUST BE SOLDERED TO SGND Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS SYMBOL VIN VIN(UVLO) VOUT VBST IVIN PARAMETER Operating Voltage Range Undervoltage Lockout Undervoltage Lockout Hysteresis Operating Voltage Range Operating Voltage Range Normal Operation Burst Mode Operation Shutdown Internal Supply Output Voltage Operating Voltage Range IVBIAS Normal Operation Burst Mode Operation Shutdown Short-Circuit Current Limit Boost Supply Switch On-Resistance Output Supply Switch On-Resistance Shutdown Pin Thresholds Boost Supply Switch Drive Current Output Supply Switch Drive Current Switch Current Limit Foldback Current Limit ISS Soft-Start Output Current The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 13.8V, VFB = 1.25V, VOUT = 5V, VBURST_EN = 0V, VBST – VIN = 5V, unless otherwise noted. CONDITIONS q MIN 4 q TYP 3.4 160 MAX 60 3.95 20 75 20 UNITS V V mV V V V µA µA µA V V µA µA µA mA Ω Ω V V mA/A mA/A A A µA 3433f Enable Threshold q 3.3 3.3 580 100 10 2.6 660 0.1 0.1 4.5 0.8 0.6 0.4 VBST < VSW_H + 20V VBST – VSW_H (Notes 2, 3) VVC < 0.6V VSHDN < 0.4V q q q q q q q q 940 190 25 2.9 20 990 VBIAS VVC < 0.6V VSHDN < 0.4V ISW = 500mA ISW = 500mA Disable Enable High Side Switch On, ISW = 500mA Low Side Switch On, ISW = 500mA VFB = 0V q q q q q q q q RSWH(ON) RSWL(ON) VSHDN IVBST/ISW IVOUT/ISW ILIM 1.2 1 1 30 30 0.5 3 0.7 0.35 5 50 50 0.9 9 2 U W U U WW W LT3433 ELECTRICAL CHARACTERISTICS SYMBOL VFB ∆VFB IFB gm AV ISW/VVC fO PARAMETER Feedback Reference Voltage The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 13.8V, VFB = 1.25V, VOUT = 5V, VBURST_EN = 0V, VBST – VIN = 5V, unless otherwise noted. CONDITIONS q MIN 1.224 1.215 TYP 1.231 0.002 35 MAX 1.238 1.245 0.01 100 330 UNITS V V %/V nA umhos dB A/V Feedback Reference Line Regulation VFB Pin Input Bias Current Error Amplifier Transconductance Error Amplifier Voltage Gain Control Voltage to Switch Transconductance Operating Frequency Foldback Frequency 5.5V ≤ VIN ≤ 60V q q q 200 270 66 0.6 VFB > 1V q 185 170 200 50 0.8 35 215 230 kHz kHz kHz V µA VFB = 0V VBURST_EN ≥ 2V RL = 35Ω (Note 4) RL = 35Ω (Note 4) q q VBURST_EN IBURST_EN tON(MIN) tOFF(MIN) Burst Enable Threshold Input Bias Current Minimum Switch On Time Minimum Switch Off Time 250 500 450 800 ns ns Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Supply current specification does not include switch drive currents. Actual supply currents will be higher. Note 3: “Normal Operation” supply current specification does not include IBIAS currents. Powering the VBIAS pin externally reduces ICC supply current. Note 4: Minimum times are tested using the high side switch with a 35Ω load to ground. Note 5: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 6: The LT3433E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the – 40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3433I is guaranteed over the full –40°C to 125°C operating junction temperature range. TYPICAL PERFOR A CE CHARACTERISTICS Maximum Output Current vs VIN 500 MAXIMUM OUTPUT CURRENT (mA) BUCK 400 VOUT = 5V TA = 25°C VBIAS OUTPUT VOLTAGE (V) IVIN (µA) 300 200 BRIDGED 100 SEE TYPICAL APPLICATION ON THE FIRST PAGE OF THIS DATA SHEET 0 10 20 30 VIN (V) 40 50 60 3433 G11 0 UW VBIAS Output Voltage vs Temperature 2.8 620 VIN Supply Current vs VIN Supply Voltage TA = 25°C 590 2.6 560 2.4 530 2.2 –50 500 0 50 TEMPERATURE (°C) 100 125 0 15 30 VIN (V) 45 60 3433 G02 3433 G01 3433f 3 LT3433 TYPICAL PERFOR A CE CHARACTERISTICS Soft-Start Current vs Temperature 7.0 6.5 6.0 1.232 SWITCH CURRENT LIMIT (mA) ERROR AMP REFERENCE (V) ISS (µA) 5.5 5.0 4.5 4.0 –50 0 50 TEMPERATURE (°C) Oscillator Frequency vs Temperature 210 200 OSCILLATOR FREQUENCY (kHz) OSCILLATOR FREQUENCY (kHz) 205 CURRENT LIMIT (A) 200 195 190 –50 0 50 TEMPERATURE (°C) Maximum Boost Supply Switch Drive Current vs Boost Supply Voltage 70 TA = 25°C 70 65 IVOUT/ISW (mA/A) IBST/ISW (mA/A) 60 55 50 45 4 5 6 9 10 VBST – VSW_H (V) 7 8 11 12 4 UW 100 3433 G03 Error Amp Reference vs Temperature 700 Switch Current Limit vs VFB TA = 25°C 1.231 600 1.230 500 1.229 400 125 1.228 –50 300 0 50 TEMPERATURE (°C) 100 125 0 0.2 0.6 0.4 VFB (V) 0.8 1.0 3433 G05 3433 G04 Oscillator Frequency vs VFB 1.0 TA = 25°C 0.9 150 0.8 Current Limit vs Temperature W/C HIGH 100 TYPICAL 0.7 50 0.6 W/C LOW 100 125 0 0 0.2 0.6 0.4 VFB (V) 0.8 1.0 3433 G07 0.5 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 3433 G06 3433 G08 Maximum Output Supply Switch Drive Current vs Output Supply Voltage TA = 25°C 65 60 55 50 45 4 5 6 7 8 VOUT (V) 3433 G09 3433 G10 9 10 11 12 3433f LT3433 TYPICAL PERFOR A CE CHARACTERISTICS Switch Resistance vs Temperature (ISW = 500mA) 1.1 1.0 40 SWITCH ON RESISTANCE (Ω) IBST/ISW (mA/A) RSWH 0.8 0.7 RSWL 0.6 0.5 0.4 –50 –25 34 IVOUT/ISW (mA/A) 50 75 0 25 TEMPERATURE (°C) 0.9 50 25 75 0 TEMPERATURE (°C) PI FU CTIO S SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference. VBST (Pin 2): Boosted Switch Supply. This “boosted” supply rail is referenced to the SW_H pin. Supply voltage is maintained by a bootstrap capacitor tied from the VBST pin to the SW_H pin. A 1µF capacitor is generally adequate for most applications. The charge on the bootstrap capacitor is refreshed through a diode, typically connected from the converter output (VOUT), during the switch-off period. Minimum off-time operation assures that the boost capacitor is refreshed each switch cycle. The LT3433 supports operational VBST supply voltages up to 75V (absolute maximum) as referenced to ground. SW_H (Pin 3): Boosted Switch Output. This is the current return for the boosted switch and corresponds to the emitter of the switch transistor. The boosted switch shorts the SW_H pin to the VIN supply when enabled. The drive circuitry for this switch is boosted above the VIN supply through the VBST pin, allowing saturation of the switch for maximum efficiency. The “ON” resistance of the boosted switch is 0.8Ω. VIN (Pin 4): Input Power Supply. This pin supplies power to the boosted switch and corresponds to the collector of the switch transistor.This pin also supplies power to most of the IC’s internal circuitry if the VBIAS pin is not driven externally. This supply will be subject to high switching transient currents so this pin requires a high quality bypass capacitor that meets whatever application-specific input ripple current requirements exist. BURST_EN (Pin 5): Burst Mode Enable/Disable. When this pin is below 0.3V, Burst Mode operation is enabled. Pin input bias current < 1µA when Burst Mode operation is enabled. If Burst Mode operation is not desired, pulling this pin above 2V will disable the burst function. When Burst Mode operation is disabled, typical pin input current = 35µA. BURST_EN should not be pulled above 20V. This pin is typically shorted to SGND for Burst Mode function, or connected to either VBIAS or VOUT to disable Burst Mode operation. VC (Pin 6): Error Amplifier Output. The voltage on the VC pin corresponds to the maximum switch current per oscillator cycle. The error amplifier is typically configured as an integrator circuit by connecting an RC network from this pin to ground. This circuit typically creates the dominant pole for the converter regulation feedback loop. Specific integrator characteristics can be configured to optimize transient response. See Applications Information. 3433f UW 100 3433 G12 VBST Supply Switch Drive Current vs Temperature (ISW = 500mA) 40 VOUT Supply Switch Drive Current vs Temperature (ISW = 500mA) 37 37 34 31 31 28 28 125 25 –50 –25 100 125 25 –50 –25 50 75 0 25 TEMPERATURE (°C) 100 125 3433 G13 3433 G14 U U U 5 LT3433 PI FU CTIO S VFB (Pin 7): Error Amplifier Inverting Input. The noninverting input of the error amplifier is connected to an internal 1.231V reference. The VFB pin is connected to a resistor divider from the converter output. Values for the resistor connected from VOUT to VFB (RFB1) and the resistor connected from VFB to ground (RFB2) can be calculated to program converter output voltage (VOUT) via the following relation: VOUT = 1.231 • (RFB1 + RFB2)/RFB2 The VFB pin input bias current is 35nA, so use of extremely high value feedback resistors could cause a converter output that is slightly higher than expected. Bias current error at the output can be estimated as: ∆VOUT(BIAS) = 35nA • RFB1 The voltage on VFB also controls the LT3433 oscillator frequency through a “frequency-foldback” function. When the VFB pin voltage is below 0.8V, the oscillator runs slower than the 200kHz typical operating frequency. The oscillator frequency slows with reduced voltage on the pin, down to 50kHz when VFB = 0V. The VFB pin voltage also controls switch current limit through a “current-limit foldback” function. At VFB = 0V, the maximum switch current is reduced to half of the normal value. The current limit value increases linearly until VFB reaches 0.6V when the normal maximum switch current level is restored. The frequency and current-limit foldback functions add robustness to short-circuit protection and help prevent inductor current runaway during start-up. SS (Pin 10): Soft Start. Connect a capacitor (CSS) from this pin to ground. The output voltage of the LT3433 error amplifier corresponds to the peak current sense amplifier output detected before resetting the switch output(s). The soft-start circuit forces the error amplifier output to a zero peak current for start-up. A 5µA current is forced from the SS pin onto an external capacitor. As the SS pin voltage ramps up, so does the LT3433 internally sensed peak current limit. This forces the converter output current to ramp from zero until normal output regulation is achieved. This function reduces output overshoot on converter start-up. The time from VSS = 0V to maximum available current can be calculated given a capacitor CSS as: tSS = (2.7 • 105)CSS or 0.27s/µF SHDN (Pin 11): Shutdown. If the SHDN pin is externally pulled below 0.5V, low current shutdown mode is initiated. During shutdown mode, all internal functions are disabled, and ICC is reduced to 10µA. This pin is intended to receive a digital input, however, there is a small amount of input hysteresis built into the SHDN circuit to help assure glitchfree mode switching. If shutdown is not desired, connect the SHDN pin to VIN. VBIAS (Pin 12): Internal Local Supply. Much of the LT3433 circuitry is powered from this supply, which is internally regulated to 2.5V through an on-board linear regulator. Current drive for this regulator is sourced from the VIN pin. The VBIAS supply is short-circuit protected to 5mA. The VBIAS supply only sources current, so forcing this pin above the regulated voltage allows the use of external power for much of the LT3433 circuitry. When using external drive, this pin should be driven above 3V to assure the internal supply is completely disabled. This pin is typically diodeconnected to the converter output to maximize conversion efficiency. This pin must be bypassed with at least a 0.1µF ceramic capacitor to SGND. VOUT (Pin 13): Converter Output Pin. This pin voltage is compared with the voltage on VIN internally to control operation in single or 2-switch mode. When the ratios of the two voltages are such that a >75% duty cycle is required for regulation, the low side switch is enabled. Drive bias for the low side switch is also derived directly from this pin. PWRGND (Pin 14): High Current Ground Reference. This is the current return for the low side switch and corresponds to the emitter of the low side switch transistor. SW_L (Pin 15): Ground Referenced Switch Output. This pin is the collector of the low side switch transistor. The low side switch shorts the SW_L pin to PWRGND when enabled. The series impedance of the ground-referenced switch is 0.6Ω. Exposed Pad (Pin 17): Exposed Pad must be soldered to PCB ground for optimal thermal performance. 3433f 6 U U U LT3433 BLOCK DIAGRA W VBIAS 12 1.25V BURST CONTROL CIRCUITS BIAS BURST_EN 5 VIN 4 SENSE AMPLIFIER VBST COMPARATOR BOOSTED DRIVER SW_H 3 2 SLOPE COMP OSCILLATOR 200kHz FREQUENCY CONTROL MODE CONTROL SWITCH CONTROL LOGIC SW_L DRIVER GND 14 15 VFB 30% LOAD ERROR AMPLIFIER 1.231V VC SHDN 6 11 7 + Burst Mode CONTROL 15% LOAD 5µA SHUTDOWN – 0.7V SS 10 VOUT SGND 1, 8, 9,16,17 13 3433 BD VOUT + 3433f 7 LT3433 APPLICATIO S I FOR ATIO Overview The LT3433 is a high input voltage range, step-up/stepdown DC/DC converter IC using a 200kHz constant frequency, current mode architecture. Dual internal switches allow the full input voltage to be imposed across the switched inductor, such that both step-up and step-down modes of operation can be realized using the same single inductor topology. The LT3433 has provisions for high efficiency, low load operation for battery-powered applications. Burst Mode operation reduces average quiescent current to 100µA in no load conditions. A low current shutdown mode can also be activated, reducing total quiescent current to 10µA. Much of the LT3433’s internal circuitry is biased from an internal low voltage linear regulator. The output of this regulator is brought out to the VBIAS pin, allowing bypassing of the internal regulator. The associated internal circuitry can be powered directly from the output of the converter, increasing overall converter efficiency. Using externally derived power also eliminates the IC’s power dissipation associated with the internal VIN to VBIAS regulator. Theory of Operation (See Block Diagram) The LT3433 senses converter output voltage via the VFB pin. The difference between the voltage on this pin and an internal 1.231V reference is amplified to generate an error voltage on the VC pin which is, in turn, used as a threshold for the current sense comparator. During normal operation, the LT3433 internal oscillator runs at 200kHz. At the beginning of each oscillator cycle, the switch drive is enabled. The switch drive stays enabled until the sensed switch current exceeds the VC-derived threshold for the current sense comparator and, in turn, disables the switch driver. If the current comparator threshold is not obtained for the entire oscillator cycle, the switch driver is disabled at the end of the cycle for 250ns. This minimum off-time mode of operation assures regeneration of the VBST bootstrapped supply. If the converter input and output voltages are close together, proper operation in normal buck configuration would require high duty cycles. The LT3433 senses this 8 U condition as requiring a duty cycle greater than 75%. If such a condition exists, a second switch is enabled during the switch on time, which acts to pull the output side of the inductor to ground. This “bridged” operation allows voltage conversion to continue when VOUT approaches or exceeds VIN. Shutdown The LT3433 incorporates a low current shutdown mode where all IC functions are disabled and the VIN current is reduced to 10µA. Pulling the SHDN pin down to 0.4V or less activates shutdown mode. Burst Mode Operation The LT3433 employs low current Burst Mode functionality to maximize efficiency during no load and low load conditions. Burst Mode function is disabled by shorting the BURST_EN pin to either VBIAS or VOUT. Burst Mode function is enabled by shorting BURST_EN to SGND. In certain wide current range applications, the IC could enter burst operation during normal load conditions. If the additional output ripple and noise generated by Burst Mode operation is not desired for normal operation, BURST_EN can be biased using an external supply that is disabled during a no-load condition. This enables Burst Mode operation only when it is required. The BURST_EN pin typically draws 35µA when Burst Mode operation is disabled (VBURST_EN ≥ 2V) and will draw no more than 75µA with VBURST_EN = 2V. When the required switch current, sensed via the VC pin voltage, is below 30% of maximum, the Burst Mode function is employed. When the voltage on VC drops below the 30% load level, that level of sense current is latched into the IC. If the output load requires less than this latched current level, the converter will overdrive the output slightly during each switch cycle. This overdrive condition forces the voltage on the VC pin to continue to drop. When the voltage on VC drops below the 15% load level, switching is disabled, and the LT3433 shuts down most of its internal circuitry, reducing quiescent current to 100µA. When the voltage on the VC pin climbs back to 20% load level, the IC returns to normal operation and switching resumes. 3433f W UU LT3433 APPLICATIO S I FOR ATIO Antislope Compensation Most current mode switching controllers use slope compensation to prevent current mode instability. The LT3433 is no exception. A slope compensation circuit imposes an artificial ramp on the sensed current to increase the rising slope as duty cycle increases. Unfortunately, this additional ramp corrupts the sensed current value, reducing the achievable current limit value by the same amount as the added ramp represents. As such, current limit is typically reduced as duty cycles increase. The LT3433 contains circuitry to eliminate the current limit reduction associated with slope-compensation, or antislope compensation. As the slope compensation ramp is added to the sensed current, a similar ramp is added to the current limit threshold reference. The end result is that current limit is not compromised so the LT3433 can provide full power regardless of required duty cycle. Mode Switching The LT3433 switches between buck and buck/boost modes of operation automatically. While in buck mode, if the converter input voltage becomes close enough to the output voltage to require a duty cycle greater than 75%, the LT3433 enables a second switch which pulls the output side of the inductor to ground during the switch-on time. This “bridged” switching configuration allows voltage conversion to continue when VIN approaches or is less than VOUT. When the converter input voltage falls to where the duty cycle required for continuous buck operation is greater than 75%, the LT3433 enables its ground-referred switch, changing the converter operation to a dual-switch bridged configuration. Because the voltage available across the switched inductor is greater while bridged, operational duty cycle will decrease. Voltage drops associated with external diodes and loss terms are estimated internally so that required operating duty cycle can be calculated regardless of specific operating voltages. In the simplest terms, a buck DC/DC converter switches the VIN side of the inductor, while a boost converter U switches the VOUT side of the inductor. The LT3433 bridged topology merges the elements of buck and boost topologies, providing switches on both sides of the inductor. Operating both switches simultaneously achieves both step-up and step-down functionality. Step-Down (VIN > VOUT) VIN SW L VOUT COUT CIN D W UU Step-Up (VIN < VOUT) VIN CIN L D VOUT COUT SW Step-Up/Step-Down (VIN > VOUT or VIN < VOUT) VIN SW L D VOUT COUT 3433 F01 CIN D SW Maximum duty cycle capability (DCMAX) gates the dropout capabilities of a buck converter. As VIN – VOUT is reduced, the required duty cycle increases until DCMAX is reached, beyond which the converter loses regulation. With a second switch bridging the switched inductor between VIN and ground, the entire input voltage is imposed across the inductor during the switch-on time, which subsequently reduces the duty cycle required to maintain regulation. Using this topology, regulation is maintained as VIN approaches or drops below VOUT. Inductor Selection The primary criterion for inductor value selection in LT3433 applications is the ripple current created in that inductor. Design considerations for ripple current are converter output capabilities in bridged mode, output voltage ripple and the ability of the internal slope compensation waveform to prevent current mode instability. 3433f 9 LT3433 APPLICATIO S I FOR ATIO The requirement for avoiding current mode instability is that the rising slope of sensed inductor ripple current (S1) is greater than the falling slope (S2). At duty cycles greater than 50% this is not true. To avoid the instability condition, a false signal is added to the sensed current with a slope (SX) that is sufficient to prevent current mode instability, or S1 + SX ≥ S2. This leads to the following relations: SX ≥ S2(2DC – 1)/DC If the forward voltages of a converter’s catch and pass diodes are defined as VF1 and VF2, then: S2 = (VOUT + VF1 + VF2)/L Solving for L yields a relation for the minimum inductance that will satisfy slope compensation requirements: LMIN = (VOUT + VF1 + VF2)(2DC – 1)/(DC • SX) The LT3433 maximizes available dynamic range using a slope compensation generator that generates a continuously increasing slope as duty cycle increases. The slope compensation waveform is calibrated at 80% duty cycle to generate an equivalent slope of at least 0.05A/µs. The equation for minimum inductance then reduces to: LMIN = (VOUT + VF1 + VF2)(15e-6) For example, with VOUT = 5V and using VF1 + VF2 = 1.1V (cold): LMIN = (5 + 1.1)(15e-6) = 91.5µH Slope Compensation Requirements Typical Minimum Inductor Values vs VOUT 350 300 250 LMIN (µH) 200 150 100 50 4 6 8 10 12 14 VOUT (V) 16 18 20 3433 AI01 10 U Converter Capabilities The output current capability of an LT3433 converter is affected by a myriad of variables. The current in the switches is limited by the LT3433. Switch current is measured coming from the VIN supply, and does not directly translate to a limitation in load current. This is especially true during bridged mode operation when the converter output current is discontinuous. During bridged mode operation, the converter output current is discontinuous, or only flowing to the output while the switches are off (not to be confused with discontinuous switcher operation). As a result, the maximum output current capability of the converter is reduced from that during buck mode operation by a factor of roughly 1 – DC, not including additional losses. Most converter losses are also a function of DC, so operational duty cycle must be accurately determined to predict converter load capabilities. VIN SW_H D1 LT3433 SW_L L D2 VOUT 3433 AI02 W UU Application variables: VIN = Converter input supply voltage VOUT = Converter programmed output voltage VBST = Boosted supply voltage (VBST – VSWH) DC = Operational duty cycle fO = Switching frequency IMAX = Peak switch current limit ∆IL = Inductor ripple current ISW = Average switch current or peak switch current less half the ripple current (IMAX – ∆IL/2) RSWH = Boosted switch “on” resistance RSWL = Grounded switch “on” resistance L = Inductor value 3433f LT3433 APPLICATIO S I FOR ATIO RL = Inductor series resistance ∆BST = Boosted switch drive currents IVBST/ISW (in A/A) ∆OUT = Grounded switch drive currents IVOUT/ISW (in A/A) VF1 = Switch node catch diode forward voltage VF2 = Pass diode forward voltage IVIN = VIN quiescent input current IIN = VIN switched current IBIAS = VBIAS quiescent input current RCESR = Output capacitor ESR Operational duty cycle is a function of voltage imposed across the switched inductance and switch on/off times. Using the relation for change in current in an inductor: δI = V • δt/L and putting the application variables into the above relation yields: δION(BRIDGED) = (DC/fO • L)[VIN – ISW • (RSWH + RSWL + RL)] δION(BUCK) = (DC/fO • L)[VIN – VOUT – VF2 – ISW • (RSWH + RL + RESR)] δIOFF = [(1 – DC)/fO • L][VOUT + VF1 + VF2 – ISW • (RL + RESR)] Current conservation in an inductor dictates δION = δIOFF, so plugging in the above relations and solving for DC yields: DC(BRIDGED) = [VOUT + VF1 + VF2 – ISW • (RL + RESR)]/ [VIN – ISW • (RSWH + RSWL + 2RL + RESR) + VOUT + VF1 + VF2 ] DC(BUCK) = [VOUT + VF1 + VF2 – ISW • (RL + RESR)]/ [VIN – ISW • (RSWH + 2RL + 2RESR) + VF1] In order to solve the above equations, inductor ripple current (∆I) must be determined so ISW can be calculated. ∆I follows the relation: ∆I = (VOUT + VF1 + VF2 – ISW • RL)(1 – DC)/(L • fO) As ∆I is a function of DC and vice-versa, the solution is iterative. Seed ∆I and solve for DC. Using the resulting value for DC, solve for ∆I. Use the resulting ∆I as the new seed value and repeat. The calculated value for DC can be used once the resulting ∆I is close (