LTC3833 Fast Accurate Step-Down DC/DC Controller with Differential Output Sensing DESCRIPTION FEATURES n n n n n n n n n n n n n n Wide VIN Range: 4.5V to 38V VOUT Range: 0.6V to 5.5V Output Accuracy: ±0.25% at 25°C and ±0.
LTC3833 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN Voltage ................................................. –0.3V to 40V BOOST Voltage........................................... –0.3V to 46V SW Voltage.................................................... –5V to 40V INTVCC, EXTVCC, (BOOST-SW), PGOOD, RUN, MODE/PLLIN, VRNG Voltages........................ –0.3V to 6V VOUT, SENSE+, SENSE– Voltages................... –0.6V to 6V VOSNS+, VOSNS – Voltages......... –0.6V to (INTVCC + 0.3V) RT, ITH Voltages.............
LTC3833 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VFB = VOSNS+ – VOSNS–, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS General VIN Input Voltage Operating Range 4.5 38 V VOUT Output Voltage Operating Range 0.6 5.
LTC3833 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VFB = VOSNS+ – VOSNS–, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS RTG(HI) TG Driver Pull-Up On-Resistance TG High 2.5 Ω RTG(LO) TG Driver Pull-Down On-Resistance TG Low 1.2 Ω RBG(HI) BG Driver Pull-Up On-Resistance BG High 2.
LTC3833 TYPICAL PERFORMANCE CHARACTERISTICS Transient Response: Forced Continuous Mode TA = 25°C unless otherwise noted Load Release: Forced Continuous Mode Load Step: Forced Continuous Mode ILOAD 20A/DIV ILOAD 20A/DIV VOUT 50mV/DIV ILOAD 20A/DIV VOUT 50mV/DIV VOUT 50mV/DIV IL 20A/DIV IL 20A/DIV 50µs/DIV LOAD TRANSIENT = 0A TO 20A VIN = 12V, VOUT = 1.5V FIGURE 10 CIRCUIT IL 20A/DIV 5µs/DIV LOAD STEP = 0A TO 20A VIN = 12V, VOUT = 1.
LTC3833 TYPICAL PERFORMANCE CHARACTERISTICS Overcurrent Protection Short-Circuit Protection CURRENT LIMIT (25A) 7.5A ILOAD 12A ILOAD 12A NOTE 7 BG 5V/DIV Output Regulation vs Load Current 0.2 NORMALIZED ∆VOUT (%) 0 –0.1 0 5 10 15 20 25 VIN (V) 30 35 0.2 VIN = 15V VOUT = 0.6V VOUT NORMALIZED AT ILOAD = 4A 0.1 0 0 2 6 4 ILOAD (A) 8 3833 G13 0.2 2.0 NORMALIZED ∆f (%) NORMALIZED ∆f (%) 0 VOUT = 0.6V ILOAD = 5A f = 500kHz FREQUENCY NORMALIZED AT VIN = 15V –0.
LTC3833 TYPICAL PERFORMANCE CHARACTERISTICS tON(MIN) and tOFF(MIN) vs Voltage on VOUT Pin 100 90 90 tOFF(MIN) tON(MIN) and tOFF(MIN) vs Switching Frequency 100 tOFF(MIN) 80 70 60 60 60 tON(MIN) 40 50 40 30 30 20 20 VIN = 38V f ≈ 2000kHz 10 0 1 tON(MIN) 2 3 VOUT (V) 4 6 5 0 5 10 10 15 20 25 VIN (V) 35 30 MAXIMUM CURRENT SENSE VOLTAGE (mV) 100 CURRENT SENSE VOLTAGE (mV) 1.60 1.55 1.50 –50 –25 0 80 60 40 20 0 –40 –60 25 50 75 100 125 150 TEMPERATURE (°C) VRNG = 0.
LTC3833 PIN FUNCTIONS (FE/UDC) PGOOD (Pin 1/Pin 17): Power Good Indicator Output. This open-drain logic output is pulled to ground when the output voltage is outside of a ±7.5% window around the regulation point. ITH (Pin 8/Pin 4): Current Control Voltage and Switching Regulator Compensation Point. The current sense threshold increases with this control voltage which ranges from 0V to 2.4V. SENSE+ (Pin 2/Pin 18): Differential Current Sensing (+) Input.
LTC3833 PIN FUNCTIONS VIN (Pin 14/Pin 10): Main Supply Input. The supply voltage can range from 4.5V to 38V. For increased noise immunity decouple this pin to signal ground with an RC filter. The voltage on this pin is also used to adjust the TG on-time in order to maintain constant frequency operation. INTVCC (Pin 15/Pin 11): Internal 5.3V Regulator Output. The driver and control circuits are powered from this voltage. Decouple this pin to power ground with a minimum of 4.7μF ceramic capacitor (CVCC).
LTC3833 FUNCTIONAL DIAGRAM VIN CIN VIN UVLO RUN IN LDO OUT EN + – – BO0ST 3.65V 4.2V VOUT MT L – START LOGIC CONTROL 4.6V VOUT COUT INTVCC INTVCC STOP ONE-SHOT TIMER RSENSE EXTVCC + + – – CB DB SW 1.3µA 0.75V 1.2V TG TG DRV RFB2 CVCC BG DRV TIME ADJUST BG MB RFB1 PGND CLOCK MODE/PLLIN CLOCK DETECT – PLL SYSTEM ICMP – + IREV + SENSE+ SENSE– RT OSCILLATOR RT INTVCC RPGD PGOOD 1µA + – OV + – UV + + – 0.645V EA (gm(EA) = 1.7mS) 0.
LTC3833 OPERATION (Refer to Functional Diagram) The voltage on the ITH pin sets the ICMP valley threshold point. The error amplifier, EA, adjusts this ITH voltage by comparing the differential feedback signal, VOSNS+ − VOSNS–, to a 0.6V internal reference voltage. Consequently, the LTC3833 regulates the output voltage by forcing the differential feedback voltage to be equal to the 0.6V internal reference.
LTC3833 OPERATION (Refer to Functional Diagram) When the RUN pin is pulled low to disable the controller or when INTVCC drops below its undervoltage lockout threshold of 3.65V, the TRACK/SS pin is pulled low internally. Light Load Current Operation When the DC load current is less than 1/2 of the peakto-peak inductor current ripple, the inductor current can drop to zero or become negative.
LTC3833 APPLICATIONS INFORMATION The Typical Application on the first page of this data sheet is a basic LTC3833 application circuit. The LTC3833 can be configured to sense the inductor current either through a series sense resistor, RSENSE, or through an RC filter across the inductor (DCR). The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption and accuracy.
LTC3833 APPLICATIONS INFORMATION CIN MT LTC3833 VOSNS+ RFB2 VOSNS– + – VIN POWER TRACE PARASITICS L ±VDROP(PWR) MB RFB1 ILOAD COUT1 COUT2 I LOAD GROUND TRACE PARASITICS ±VDROP(GND) OTHER CURRENTS FLOWING IN SHARED GROUND PLANE 3833 F02 Figure 2: Differential Output Sensing Used to Correct Line Loss Variations in a High Power Distributed System with a Shared Ground Plane point that is to be accurately regulated through remote differential sensing. for maximum synchronization margin.
LTC3833 APPLICATIONS INFORMATION ripple current does not exceed a specified maximum, the inductance should be chosen according to: L= VOUT V • 1− OUT f • ∆IL(MAX) VIN(MAX) Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot tolerate the core loss of low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or Kool Mμ cores.
LTC3833 APPLICATIONS INFORMATION Because of possible PCB noise in the current sensing loop, the current ripple of ∆VSENSE = ∆IL • RSENSE also needs to be checked in the design to get a good signal-to-noise ratio. In general, for a reasonably good PCB layout, a 10mV ∆VSENSE voltage is recommended as a conservative number to start with, either for RSENSE or DCR sensing applications.
LTC3833 APPLICATIONS INFORMATION Ensure that R1 has a power rating higher than this value. If high efficiency is necessary at light loads, consider this power loss when deciding whether to use DCR sensing or RSENSE sensing. Light load power loss can be modestly higher with a DCR network than with a sense resistor due to the extra switching losses incurred through R1. However, DCR sensing eliminates a sense resistor, reduces conduction losses and provides higher efficiency at heavy loads.
LTC3833 APPLICATIONS INFORMATION do not offer much relief. Note that capacitor manufacturers’ ripple current ratings for electrolytic and conductive polymer capacitors are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. The selection of COUT is primarily determined by the effective series resistance, ESR, to minimize voltage ripple.
LTC3833 APPLICATIONS INFORMATION the high transient currents required by the MOSFET gate drivers. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maximum junction temperature rating for the LTC3833 to be exceeded, especially if the LDO is active and provides INTVCC. Power dissipation for the IC in this case is highest and is approximately equal to VIN • IINTVCC.
LTC3833 APPLICATIONS INFORMATION VIN Undervoltage Lockout (UVLO) The LTC3833 has two functions that help protect the controller in case of input undervoltage conditions. A precision UVLO comparator constantly monitors the INTVCC voltage to ensure that an adequate gate-drive voltage is present. The comparator enables UVLO and locks out the switching action until INTVCC rises above 4.2V. Once UVLO is released, the comparator does not retrigger UVLO until INTVCC falls below 3.65V.
LTC3833 APPLICATIONS INFORMATION VOUT EXTERNAL SUPPLY VOLTAGE VOLTAGE EXTERNAL SUPPLY VOUT TIME TIME Coincident Tracking Ratiometric Tracking 3833 F06 Figure 6a. Two Different Modes of Output Tracking EXT. V TO TRACK/SS VOUT RFB2 RFB2 TO VOSNS+ RFB1 RFB1 VOUT EXT. V TO TRACK/SS R1 R2 0.6V ≥ R1+ R2 EXT. V R2 TO VOSNS– RFB2 TO VOSNS+ RFB1 TO VOSNS– 3833 F06b Coincident Tracking Setup Ratiometric Tracking Setup Figure 6b.
LTC3833 APPLICATIONS INFORMATION ILOAD CLOCK INPUT PHASE LOCKED LOSES PHASE LOCK DUE TO FAST LOAD STEP ESTABLISHES FREQUENCY LOCK SOON ESTABLISHES PHASE LOCK AFTER ~600µs LOSES PHASE LOCK DUE TO FAST LOAD RELEASE ESTABLISHES FREQUENCY LOCK SOON SW VOUT 3833 F07 Figure 7. Phase and Frequency Locking Behavior During Transient Load Conditions For light loading conditions, the phase and frequency synchronization will be active if there is a clock input applied.
LTC3833 APPLICATIONS INFORMATION TG-SW (VGS OF TOP MOSFET) then immediately turned back on. This minimum off-time includes the time to turn on the bottom power MOSFET ’s gate and turn it back off along with the dead time delays from top MOSFET off to bottom MOSFET on and bottom MOSFET off to top MOSFET on. The minimum off-time that the LTC3833 can achieve is 90ns.
LTC3833 APPLICATIONS INFORMATION Fault Conditions: Current Limiting and Overvoltage The maximum inductor current is inherently limited in a current mode controller by the maximum sense voltage. In the LTC3833, the maximum sense voltage is controlled by the voltage on the VRNG pin. With valley current mode control, the maximum sense voltage and the sense resistance determine the maximum allowed inductor valley current.
LTC3833 APPLICATIONS INFORMATION The gain of the loop increases with RITH and the bandwidth of the loop increases with decreasing CITH1. If RITH is increased by the same factor that CITH1 is decreased, the zero frequency will be kept the same, thereby keeping the phase the same in the most critical frequency range of the feedback loop. In addition, a feedforward capacitor, CFF, can be added to improve the high frequency response, as shown in Figure 1.
LTC3833 APPLICATIONS INFORMATION When making adjustments to improve efficiency, the input current is the best indicator of changes in efficiency. If you make a change and the input current decreases, then the efficiency has increased. If there is no change in input current there is no change in efficiency. The frequency is programmed by: R T [kΩ ] = Select the nearest standard value of 115k. Power losses in the switching regulator will reflect as a longer than ideal on-time.
LTC3833 APPLICATIONS INFORMATION The resulting maximum ripple current is: ∆IL = 1.2V 1.2V • 1– ≈ 5.8A 350kHz • 0.56µH 24V Often in high power applications, DCR current sensing is preferred over RSENSE in order to maximize efficiency. In order to determine the DCR filter values, first the inductor manufacturer has to be chosen. For this design, the Vishay IHLP-4040DZ-01 model is chosen with a value of 0.56μH and DCRMAX =1.8mΩ. This implies that: VSENSE(MAX) = DRCMAX at 25°C • [1 + 0.
LTC3833 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3833. • Multilayer boards with dedicated ground layers are preferable for reduced noise and for heat sinking purposes. Use wide rails and/or entire planes for VIN, VOUT and PGND nodes for good filtering and minimal copper loss.
LTC3833 APPLICATIONS INFORMATION RVIN 2.2Ω INTVCC VIN RPGD 100k LTC3833 VOUT PGOOD TG MODE/PLLIN SW EXTVCC L1 0.47µH DB RITH 84.5k RT 137k TRACK/SS RSENSE 1.5mΩ RFB2 15k CB 0.1µF INTVCC INTVCC RFB1 10k CVCC 4.7µF MB BG ITH COUT2 100µF ×2 + VOUT 1.5V 20A COUT1 330µF 2.5V ×2 PGND VOSNS+ VOSNS– RT SGND 3833 F10a CIN1: SANYO 16SVP180M COUT1: SANYO 2R5TPE330M9 DB: CENTRAL CMDSH-3 L1: PULSE PA0515.
LTC3833 TYPICAL APPLICATIONS 5V, 8A, 200kHz High Efficient Step-Down Converter RVIN 2.2Ω VIN INTVCC RPGD 100k EXTVCC PGOOD VOUT SENSE– SENSE+ VRNG LTC3833 TRACK/SS CITH1 220pF RITH 86.6k RT 205k ITH RT VIN 7V TO 38V L1 6µH SW INTVCC CIN1 100µF 50V MT TG BOOST + CDCR RDCR 0.22µF 5.9k RUN CSS 0.1µF CIN2 10µF ×3 CVIN 0.1µF RB 10Ω CVCC 4.7µF RFB2 147k DB CB INTVCC 0.1µF RFB1 20k MB BG VOUT 5V 8A COUT2 100µF ×2 + COUT1 330µF 6.
LTC3833 TYPICAL APPLICATIONS 0.6V, 10A, 200kHz Low Output Step-Down Converter INTVCC RPGD 100k RVIN 2.2Ω VIN CVIN 0.1µF LTC3833 PGOOD VOUT CITH1 220pF RUN RITH 51k MT TG SW BOOST RSENSE 3mΩ + DB ITH RT INTVCC CVCC 4.7µF VOUT 0.6V COUT1 10A 330µF 2.5V ×2 COUT2 100µF ×2 MB BG EXTVCC SGND L1 1µH CB 0.1µF INTVCC RT 205k VIN CIN1 4.
LTC3833 TYPICAL APPLICATIONS Area Compact 2.5V, 5A, 1.2MHz Step-Down Converter RVIN 2.2Ω INTVCC VIN VRNG RPGD 100k LTC3833 VOUT SENSE– SENSE+ PGOOD RUN EXTVCC RT 33.2k SW TRACK/SS CIN2 10µF CIN1 47µF 35V L1 1µH CB 0.1µF RFB2 31.6k DB INTVCC CVCC 4.7µF RT COUT1 100µF RFB1 10k MB BG VIN 6V TO 28V VOUT 2.5V 5A BOOST INTVCC ITH + CDCR 0.1µF RDCR MT 1.1k TG MODE/PLLIN CSS 0.01µF CITH1 220pF RITH 20k CVIN 0.
LTC3833 TYPICAL APPLICATIONS 3.3V, 15A, 200kHz High Power Step-Down Converter VIN CVIN RVIN 0.1µF 2.2Ω INTVCC VOUT RUN PGOOD VRNG SENSE– CSS 0.1µF TRACK/SS CIN1 4.5V TO 24V 100µF 35V RDCR2 RDCR1 17.4k 3.92k MT TG L1 2µH SW VOUT 3.3V 15A BOOST DB INTVCC INTVCC RFB2 90.9k CB 0.1µF CVCC 4.7µF ITH RFB1 20k MB BG CITH2 47pF RT 205k CDCR 0.22µF SENSE+ MODE/PLLIN EXTVCC CITH1 680pF RITH 18.
LTC3833 PACKAGE DESCRIPTION UDC Package 20-Lead Plastic QFN (3mm × 4mm) (Reference LTC DWG # 05-08-1742 Rev Ø) 0.70 ±0.05 3.50 ± 0.05 2.10 ± 0.05 1.50 REF 2.65 ± 0.05 1.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.50 REF 3.10 ± 0.05 4.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 ± 0.10 0.75 ± 0.05 1.50 REF 19 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER 20 0.40 ± 0.10 1 PIN 1 TOP MARK (NOTE 6) 4.00 ± 0.10 2 2.65 ± 0.
LTC3833 PACKAGE DESCRIPTION FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev H) Exposed Pad Variation CB 6.40 – 6.60* (.252 – .260) 3.86 (.152) 3.86 (.152) 20 1918 17 16 15 14 13 12 11 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 0.50 – 0.75 (.020 – .030) NOTE: 1.
LTC3833 TYPICAL APPLICATION High Frequency 5.5V, 4A, 2MHz Step-Down Converter RDIV1 100k VIN PGOOD MODE/PLLIN LTC3833 VRNG RDIV2 26.1k SENSE– CSS 0.1µF TRACK/SS SENSE+ CIN2 4.7µF ×2 CVIN 0.1µF EXTVCC VOUT RUN CITH1 220pF RITH 20k RVIN 2.2Ω SW BOOST RT INTVCC SGND BG Efficiency 90 CF RF1 1000pF 10Ω 70 MT L1 1.2µH CB 0.1µF INTVCC CVCC 4.7µF RSENSE 10mΩ CFF 22pF MB PGND RFB2 165k RFB1 20k VOSNS+ VOSNS– CIN1: KEMET T521X476M035ATE070 DB: DIODES, INC.
