LTC1872BES6#TRPBF

LTC1872B Constant Frequency Current Mode Step-Up DC/DC Controller in ThinSOT Features Description Burst Mode™ Operation Disabled for Lower Output Ripple at Light Loads nn High Efficiency: Over 90% nn High Output Currents Easily Achieved nn Wide V Range: 2.5V to 9.8V IN nn V OUT Limited Only by External Components nn Constant Frequency 550kHz Operation nn Current Mode Operation for Excellent Line and Load Transient Response nn Shutdown Mode Draws Only 8µA Supply Current nn Low Profile (1mm) ThinSOT™ Package The LTC®1872B is a constant frequency current mode step-up DC/DC controller providing excellent AC and DC load and line regulation. The device incorporates an accurate undervoltage lockout feature that shuts down the LTC1872B when the input voltage falls below 2.0V. nn Applications Optical Communications Lithium-Ion-Powered Applications nn Cellular Telephones nn Wireless Devices nn Portable Computers nn Scanners nn The LTC1872B provides a ± 2.5% output voltage accuracy and consumes only 270µA of quiescent current. In shutdown, the device draws a mere 8µA. High constant operating frequency of 550kHz allows the use of a small external inductor. The constant frequency operation is maintained down to very light loads, resulting in less low frequency noise generation over a wide load current range. The LTC1872B is available in a 6-lead low profile (1mm) ThinSOT package. For a Burst Mode operation enabled version of the LTC1872B, please refer to the LTC1872 data sheet. nn L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application C1 10µF 10V 147k 220pF 80.6k 1 2 3 ITH/RUN VIN LTC1872B GND SENSE – VFB NGATE 5 M1 D1 90 C2 4× 10µF 10V VOUT 5V 1A 422k C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT C2: MURATA GRM42-2X5R106K010AL D1: IR10BQ015 L1: MURATA LQN6C4R7M04 M1: Si2302DS R1: DALE 0.25W VIN = 3.3V VOUT = 5V 95 L1 4.7µH 4 6 100 EFFICIENCY (%) R1 0.03Ω Typical Efficiency vs Load Current* VIN 3.3V 85 80 75 70 65 1872B TA01 1 10 100 LOAD CURRENT (mA) 1000 1872B TA01b Figure 1. LTC1872B High Output Current 3.3V to 5V Boost Converter *Output ripple waveforms for the circuit of Figure 1 appear in Figure 2. 1872bfa For more information www.linear.com/LTC1872B 1 LTC1872B Absolute Maximum Ratings Pin Configuration (Note 1) Input Supply Voltage (VIN).......................... – 0.3V to 10V SENSE–, NGATE Voltages.............. –0.3V to (VIN + 0.3V) VFB, ITH/RUN Voltages............................... –0.3V to 2.4V NGATE Peak Output Current (< 10µs) ......................... 1A Storage Ambient Temperature Range.....– 65°C to 150°C Operating Temperature Range (Note 2)....– 40°C to 85°C Junction Temperature (Note 3).............................. 150°C Lead Temperature (Soldering, 10 sec).................... 300°C TOP VIEW ITH/RUN 1 6 NGATE GND 2 5 VIN 4 SENSE – VFB 3 S6 PACKAGE 6-LEAD PLASTIC SOT-23 TJMAX = 150°C, θJA = 230°C/W Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC1872BES6#PBF LTC1872BES6#TRPBF LTXY 16-Lead Plastic SOT-23 –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on nonstandard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 4.2V unless otherwise specified. (Note 2) PARAMETER CONDITIONS Input DC Supply Current Normal Operation Sleep Mode Shutdown UVLO Undervoltage Lockout Threshold Typicals at VIN = 4.2V (Note 4) 2.4V ≤ VIN ≤ 9.8V 2.4V ≤ VIN ≤ 9.8V 2.4V ≤ VIN ≤ 9.8V, VITH/RUN = 0V VIN < UVLO Threshold VIN Falling VIN Rising Shutdown Threshold (at ITH/RUN) Start-Up Current Source Regulated Feedback Voltage VFB Input Current Oscillator Frequency Gate Drive Rise Time Gate Drive Fall Time Peak Current Sense Voltage VITH/RUN = 0V 0°C to 70°C(Note 5) – 40°C to 85°C(Note 5) (Note 5) VFB = 0.8V CLOAD = 3000pF CLOAD = 3000pF (Note 6) Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC1872BE is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the – 40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD • θJA°C/W) 2 MIN TYP MAX UNITS l 1.55 1.85 270 230 8 6 2.00 2.10 420 370 22 10 2.35 2.40 µA µA µA µA V V l 0.15 0.25 0.780 0.770 0.35 0.5 0.800 0.800 10 550 40 40 120 0.55 0.85 0.820 0.830 50 650 V µA V V nA kHz ns ns mV l l l 500 114 Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: The LTC1872B is tested in a feedback loop that servos VFB to the output of the error amplifier. Note 6: Guaranteed by design at duty cycle = 30%. Peak current sense voltage is VREF/6.67 at duty cycle 7.5% by turning off the external N-channel power MOSFET and keeping it off until the fault is removed. Low Load Current Operation Under very light load current conditions, the ITH/RUN pin voltage will be very close to the zero current level of 0.85V. As the load current decreases further, an internal offset at the current comparator input will assure that the current comparator remains tripped (even at zero load current) and the regulator will start to skip cycles, as it must, in order to maintain regulation. This behavior allows the regulator to maintain constant frequency down to very light loads, resulting in less low frequency noise generation over a wide load current range. For more information www.linear.com/LTC1872B 1872bfa LTC1872B Operation (Refer to Functional Diagram) Figure 2 illustrates this result for the circuit of Figure 1 using both an LTC1872 in Burst Mode operation and an LTC1872B (non-Burst Mode operation). At an output current of 50mA, the Burst Mode operation part exhibits an output ripple of approximately 80mVP-P, whereas the non-Burst Mode operation part has an output ripple of ≈45mVP-P. At lower output current levels, the improvement is even greater. This comes at a trade off of slightly lower efficiency for the non-Burst Mode operation part. Also notice the constant frequency operation of the LTC1872B, even at 5% of maximum output current. Slope Compensation and Inductor’s Peak Current The inductor’s peak current is determined by: IPK = VITH −0.7 10 (RSENSE ) when the LTC1872B is operating below 40% duty cycle. However, once the duty cycle exceeds 40%, slope compensation begins and effectively reduces the peak inductor current. The amount of reduction is given by the curves in Figure 3. 110 Undervoltage Lockout 100 To prevent operation of the N-channel MOSFET below safe input voltage levels, an undervoltage lockout is incorporated into the LTC1872B. When the input supply voltage drops below approximately 2.0V, the N-channel MOSFET and all circuitry is turned off except the undervoltage block, which draws only several microamperes. SF = IOUT/IOUT(MAX) (%) 90 80 70 60 50 IRIPPLE = 0.4IPK AT 5% DUTY CYCLE IRIPPLE = 0.2IPK AT 5% DUTY CYCLE 40 30 Overvoltage Protection 20 The overvoltage comparator in the LTC1872B will turn the external MOSFET off when the feedback voltage has risen 7.5% above the reference voltage of 0.8V. This comparator has a typical hysteresis of 20mV. 20mV AC/DIV 10 VIN = 4.2V 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 1872B F03 Figure 3. Maximum Output Current vs Duty Cycle 20mV AC/DIV VIN = 3.3V VOUT = 5V IOUT = 50mA 5µs/DIV 1872B F02a (2a) VOUT Ripple for Figure 1 Circuit Using LTC1872 Burst Mode Operation VIN = 3.3V VOUT = 5V IOUT = 50mA 5µs/DIV 1872B F02b (2b) VOUT Ripple for Figure 1 Circuit Using LTC1872B Non-Burst Mode Operation Figure 2. Output Ripple Waveforms for the Circuit of Figure 1 1872bfa For more information www.linear.com/LTC1872B 5 LTC1872B Operation (Refer to Functional Diagram) Short-Circuit Protection Since the power switch in a boost converter is not in series with the power path from input to load, turning off the switch provides no protection from a short-circuit at the output. External means such as a fuse in series with the boost inductor must be employed to handle this fault condition. Applications Information The basic LTC1872B application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L1 and RSENSE (= R1). Next, the power MOSFET and the output diode D1 is selected followed by CIN(= C1) and COUT(= C2). RSENSE Selection for Output Current RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage developed across RSENSE, the threshold of the comparator determines the inductor’s peak current. The output current the LTC1872B can provide is given by: ⎛ 0.12 I ⎞ V IN IOUT = ⎜ − RIPPLE ⎟ 2 ⎠ VOUT + VD ⎝ R SENSE where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation section) and VD is the forward drop of the output diode at the full rated output current. A reasonable starting point for setting ripple current is: IRIPPLE = (O.4) (IOUT ) VOUT + VD VIN ⎛ V ⎞ IN ⎜ ⎟ (10) (IOUT ) (100) ⎝ VOUT + VD ⎠ SF Inductor Value Calculation The operating frequency and inductor selection are interrelated in that higher operating frequencies permit the use of a smaller inductor for the same amount of inductor ripple current. However, this is at the expense of efficiency due to an increase in MOSFET gate charge losses. The inductance value also has a direct effect on ripple current. The ripple current, IRIPPLE, decreases with higher inductance or frequency and increases with higher VOUT. The inductor’s peak-to-peak ripple current is given by: V ⎛ V + V − V ⎞ IRIPPLE = IN ⎜ OUT D IN ⎟ f (L ) ⎝ VOUT + VD ⎠ where f is the operating frequency. Accepting larger values of IRIPPLE allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is: ⎛ V + V ⎞ IRIPPLE = 0.4 IOUT(MAX ) ⎜ OUT D ⎟ ⎝ VIN ⎠ ( Rearranging the above equation, it becomes: ⎛ V ⎞ 1 IN RSENSE = ⎜ ⎟ (10) ( IOUT) ⎝ VOUT + VD ⎠ for Duty Cycle