LTC3851 LINER [Linear Technology], LTC3851 Datasheet
LTC3851
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LTC3851 Summary of contents
Page 1
... OPTI-LOOP compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The LTC3851 features a precision 0.8V reference that is compatible with a wide 4V to 38V input supply range. OUT The TK/SS pin ramps the output voltage during start-up ...
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... TAPE AND REEL LTC3851EGN#PBF LTC3851EGN#TRPBF LTC3851EUD#PBF LTC3851EUD#TRPBF Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based fi nish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/ ...
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... LOAD C = 3300pF LOAD (Note 3300pF LOAD C = 3300pF LOAD C = 3300pF Each Driver LOAD C = 3300pF Each Driver LOAD (Note 7) 6V < V < 38V 0mA to 50mA CC LTC3851 MIN TYP MAX 0.792 0.800 0.808 –10 –50 0.002 0.02 l 0.01 0.1 l –0.01 –0 1 ...
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... MODE OSC Note 4: The LTC3851 is tested in a feedback loop that servos V specified voltage and measures the resultant V Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 6: Rise and fall times are measured using 10% and 90% levels. Delay times are measured using 50% levels. Note 7: The minimum on-time condition is specifi ...
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... V = 1.5V 1μs/DIV OUT V = 12V 1mA LOAD FIGURE 11 CIRCUIT Ratiometric Tracking with Master Supply V MASTER 0.5V/DIV V OUT 2A LOAD 0.5V/DIV 10ms/DIV LTC3851 Load Step (Forced Continuous Mode) I LOAD 5A/DIV 0. 5A/DIV V OUT 100mV/DIV AC COUPLED 3851 G05 V = 1.5V 100μs/DIV OUT V = 12V ...
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... LTC3851 TYPICAL PERFORMANCE CHARACTERISTICS INTV Line Regulation CC 5 0mA 5.1 LOAD I = 25mA 4.9 LOAD 4.7 4.5 4.3 4.1 3.9 3.7 3 INPUT VOLTAGE (V) 3851 G13 Burst Mode Peak Current Sense Threshold vs I Voltage TH 60 MAXIMUIM MINIMUIM FLOAT LIM BURST COMPARATOR FALLING THESHOLD 0.4V ITH 0 0 ...
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... INPUT VOLTAGE ( 3851 G25 50 75 100 125 3851 G27 LTC3851 Undervoltage Lockout Threshold (INTV ) vs Temperature INTV RAMPING INTV RAMPING DOWN –50 – TEMPERATURE (°C) ...
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... LTC3851 PIN FUNCTIONS (GN/UD) MODE/PLLIN (Pin 1/Pin 15): Force Continuous Mode, Burst Mode or Pulse-Skipping Mode Selection Pin and External Synchronization Input to Phase Detector Pin. Connect this pin to INTV to force continuous conduction CC mode of operation. Connect to GND to enable pulse-skip- ping mode of operation. To select Burst Mode operation, ...
Page 9
... SWITCH LOGIC – AND ANTI- I REV SHOOT + THROUGH RUN OV SLOPE COMPENSATION UVLO RUN SS – – 1μA 0.64V 1.25V I RUN TK/ LTC3851 REG BOOST SENSE – SENSE INTV VCC GND + 0.72V R2 UV ...
Page 10
... MOSFETs are held off. Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Continuous Conduction) The LTC3851 can be enabled to enter high effi ciency Burst Mode operation, constant frequency pulse-skipping mode or forced continuous conduction mode. To select forced continuous operation, tie the MODE/PLLIN pin to INTV To select pulse-skipping mode of operation, fl ...
Page 11
... If the MODE/PLLIN pin is not being driven by an ex- ternal clock source, the FREQ/PLLFLTR pin can be used to program the controller’s operating frequency from 250kHz to 750kHz. A phase-locked loop (PLL) is available on the LTC3851 pin, just synchronize the internal oscillator to an external clock TH source that is connected to the MODE/PLLIN pin ...
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... SENSE current. 12 LTC3851 Figure 1. Using a Resistor to Sense Current with the LTC3851 The current comparator has a maximum threshold, V determined by the I threshold sets the maximum peak of the inductor current, yielding a maximum average output current, I the maximum peak value less half the peak-to-peak ripple is grounded, the current, Δ ...
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... Normally, this results in a reduction of maximum inductor peak cur rent for duty cycles >40%. However, the LTC3851 uses a novel scheme that allows the maximum inductor peak current to remain unaffected throughout all duty cycles. Inductor Value Calculation ...
Page 14
... When the LTC3851 is confi gured to soft-start by itself, a capacitor should be connected to ) • the TK/SS pin. The LTC3851 is in the shutdown state if the RUN pin voltage is below 1.25V. TK/SS pin is actively ⎤ pulled to ground in this shutdown state. 1 ⎥ Once the RUN pin voltage is above 1.25V, the LTC3851 ⎥ ...
Page 15
... slave supply. To implement the coincident tracking in Figure 3a, connect a resistor divider to V connect its midpoint to the TK/SS pin of the LTC3851. The ratio of this divider should be selected the same as that of the LTC3851’s feedback divider as shown in Figure 4a. In this tracking mode, V ...
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... For better output regulation, use the coincident tracking mode instead of ratiometric. INTV Regulator CC The LTC3851 features a PMOS low dropout linear regula tor (LDO) that supplies power to INTV CC INTV powers the gate drivers and much of the LTC3851 CC ’ ...
Page 17
... The reverse break down of the external Schottky diode must be greater than V . IN(MAX) Undervoltage Lockout The LTC3851 has two functions that help protect the controller in case of undervoltage conditions. A precision UVLO comparator constantly monitors the INTV to ensure that an adequate gate-drive voltage is present. It locks out the switching action when INTV 3 ...
Page 18
... Any inductance present in the circuit board traces negates their usefulness. Setting Output Voltage The LTC3851 output voltage is set by an external feed- back resistive divider carefully placed across the output, as shown in Figure 6. The regulated output volt age is determined by: ⎛ ...
Page 19
... The output of the phase detector is a pair of complemen tary current sources that charge or discharge the external fi lter ON(MIN) network connected to the FREQ/PLLFLTR pin. Note that the LTC3851 can only be synchronized to an external clock whose frequency is within range of the LTC3851’s internal V .This is guaranteed to be between 250kHz and 750kHz. ...
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... Minimum On-Time Considerations Minimum on-time t is the smallest time duration ON(MIN) that the LTC3851 is capable of turning on the top MOSFET determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time ...
Page 21
... If the ratio LOAD OUT should be controlled so that the load rise time is limited to approximately 25 • C require a 250μs rise time, limiting the charging current to about 200mA. LTC3851 -C fi lter sets the dominant pole-zero C C pin waveforms that will TH pin signal which ...
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... Figure 9. Check the following in your layout: 1. Are the board signal and power grounds segregated? The LTC3851 GND pin should tie to the ground plane close to the input capacitor(s). The low current or signal ground lines should make a single point tie directly to the GND pin ...
Page 23
... All of these nodes have very large and fast moving signals and therefore should be kept on the “output side” (Pin 9 to Pin 16) of the LTC3851EGN and occupy minimum PC trace area. PC Board Layout Debugging It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit ...
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... LTC3851 APPLICATIONS INFORMATION A 4.7μH inductor will produce 28% ripple current and a 3.3μH will result in 40%. The peak inductor current will be the maximum DC value plus one-half the ripple current, or 6A, for the 3.3μH value. Increasing the ripple current will also help ensure that the minimum on-time of 90ns is not violated ...
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... Si4412DY C B 0.1μF BOOST LTC3851 L1 3.3μ CMDSH-3 INTV CC + 4.7μ Si4410DY MBRS140T3 – GND + I LIM LTC3851 C IN 180μF L1 0.68μH V OUT 3.3V R27 15A R2 C15 10k 62.5k 47pF + 1% C OUT 330μ 20k SANYO 2R5TPE330M9 OUT C : SANYO 80UK150K ...
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... GND C5 0.22μ SENSE I LIM 26 2.5V/15A Synchronized at 350kHz 14V RJK0305DPB C B 0.1μF BOOST LTC3851 L1 0.68μ CMDSH-3 INTV CC + 4.7μ RJK0301DPB B34OLA – GND + I LIM R22 100Ω R20 100Ω 40V Input to 12V Flyback Converter ...
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... Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.75 0.05 3.00 0.10 (4 SIDES) 0.70 0.05 PIN 1 TOP MARK (NOTE 6) PACKAGE OUTLINE 0.25 0.05 0.50 BSC LTC3851 .189 – .196* (4.801 – 4.978) .009 (0.229 REF .150 – .157** (3.810 – 3.988 .004 – ...
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... LTC3851 TYPICAL APPLICATION MODE/PLLIN FREQ 80k FREQ/PLLFLTR TG RUN 0.1μF BOOST C SS LTC3851 0.1μF TK/ 4700pF C2 1k 100pF I INTV TH CC 47pF – SENSE GND 1000pF + SENSE I LIM RELATED PARTS PART NUMBER DESCRIPTION TM LTC1625/LTC1775 No R Current Mode Synchronous Step-Down Controllers ...