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| LTC1266 LTC1266-3.3/LTC1266-5 Synchronous Regulator Controller for N- or P-Channel MOSFETs FEATURES s s DESCRIPTIO s s s s s s s s s s s s Ultrahigh Efficiency: Over 95% Possible Drives N-Channel MOSFET for High Current or P-Channel MOSFET for Low Dropout Pin Selectable Burst Mode Operation 1% Output Accuracy (LTC1266A) Pin Selectable Phase of Topside Driver for Boost or Step-Down Operation Wide VIN Range: 3.5V to 20V On-Chip Low-Battery Detector High Efficiency Maintained Over Large Current Range Low 170A Standby Current at Light Loads Current Mode Operation for Excellent Line and Load Transient Response Logic Controlled Micropower Shutdown: IQ < 40A Short-Circuit Protection Synchronous Switching with Nonoverlaping Gate Drives Available in 16-Pin Narrow SO Package The LTC(R)1266 series is a family of synchronous switching regulator controllers featuring automatic Burst ModeTM operation to maintain high efficiencies at low output currents. These devices drive external power MOSFETs at switching frequencies up to 400kHz using a constant offtime current mode architecture providing constant ripple current in the inductor. They can drive either an N-channel or a P-channel topside MOSFET. The operating current level is user-programmable via an external current sense resistor. Wide input supply range allows operation from 3.5V to 18V (20V maximum). Constant off-time architecture provides low dropout regulation limited only by the RDS(ON) of the topside MOSFET (when using the P-channel) and the resistance of the inductor and current sense resistor. The LTC1266 series combines synchronous switching for maximum efficiency at high currents with an automatic low current operating mode, called Burst Mode operation, which reduces switching losses. Standby power is reduced to only 1mW at VIN = 5V (at IOUT = 0). Load currents in Burst Mode operation are typically 0mA to 500mA. , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation. APPLICATIO S s s s s s Notebook and Palmtop Computers Portable Instruments Cellular Telephones DC Power Distribution Systems GPS Systems TYPICAL APPLICATIO VIN 4V TO 9V D2 MBR0530T1 + CIN 100F x2 LOW BAT OUT VIN LBIN PINV PWR VIN LBOUT TDRIVE SENSE + CB 0.1F N-CHANNEL Si9410 L* 5H 100k LOW BAT IN LTC1266-3.3 0V = NORMAL >1.5V = SHUTDOWN RC 470 CC 3300pF SHDN ITH CT BINH SGND 1000pF N-CHANNEL Si9410 VOUT 3.3V 5A EFFICIENCY (%) RSENSE 0.02 SENSE - BDRIVE PGND CT 180pF D1 MBRS130LT3 1266 TA01 + COUT 330F x2 *TOKO 919AS-4R7M Figure 1. High Efficiency Step-Down Converter U LTC1266-3.3 Efficiency 100 VIN = 5V 95 90 85 80 0.01 0.1 1 LOAD CURRENT (A) 5 1266 TA02 U U 1 LTC1266 LTC1266-3.3/LTC1266-5 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW TDRIVE 1 PWR VIN 2 PINV 3 BINH 4 VIN 5 CT 6 ITH 7 SENSE - 8 16 BDRIVE 15 PGND 14 LBOUT 13 LBIN 12 SGND 11 SHDN 10 VFB (NC*) 9 SENSE + Input Supply Voltage (Pins 2, 5) ............... 20V to - 0.3V Continuous Output Current (Pins 1, 16) .............. 50mA Sense Voltages (Pins 8, 9) ....................... 13V to - 0.3V SHDN Voltage (Pin 11) ............................. 12V to - 0.3V PINV, BINH, LBIN (Pins 3, 4, 13)................20V to - 0.3V LBOUT Output Current ........................................... 12mA Operating Ambient Temperature Range ...... 0C to 70C Industrial Temperature Range ................ - 40C to 85C Extended Commercial Temperature Range ........................... - 40C to 85C Junction Temperature (Note 2) ............................ 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C ORDER PART NUMBER LTC1266CS LTC1266CS-3.3 LTC1266CS-5 LTC1266ACS LTC1266IS LTC1266IS-3.3 LTC1266IS-5 LTC1266AIS S PACKAGE 16-LEAD PLASTIC SO *FIXED OUTPUT VERSIONS TJMAX = 125C, JA = 110C/ W Consult factory for Industrial and Military grade parts. The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VSHDN = VBINH = 0V unless otherwise noted. SYMBOL VFB PARAMETER Feedback Voltage LTC1266ACS LTC1266CS IFB VOUT Feedback Current (LTC1266 Only) Regulated Output Voltage LTC1266CS-3.3 LTC1266CS-5 VOUT Output Ripple (Burst Mode Operation) ILOAD = 150mA Output Voltage Line Regulation ILOAD = 50mA VPINV = 0V, Topside Switch = P-Ch, VIN = 7V to 12V VPINV = VPWR, Topside Switch = N-Ch, VIN = 7V to 12V Output Voltage Load Regulation LTC1266-3.3 LTC1266-3.3 LTC1266-5 LTC1266-5 IQ1 VIN Pin DC Supply Current (Note 3) Normal Mode Sleep Mode Shutdown PWR VIN DC Supply Current (Note 3) Normal Mode Sleep Mode Shutdown 5mA < ILOAD < 2A, RSENSE = 0.05 Burst Mode Operation Enabled, VBINH = 0V Burst Mode Operation Inhibited, VBINH = 2V Burst Mode Operation Enabled, VBINH = 0V Burst Mode Operation Inhibited, VBINH = 2V 3.5V < VIN < 18V 3.5V < VIN < 18V VSHDN = 2.1V, 3.5V < VIN < 18V 3.5V < PWR VIN < 18V 3.5V < PWR VIN < 18V VSHDN = 2.1V, 3.5V < PWR VIN < 18V q q q q ELECTRICAL CHARACTERISTICS CONDITIONS VIN = 9V, ILOAD = 700mA, VPINV = VPWR = 14V, Topside Switch = N-Ch q q q MIN TYP MAX UNITS 1.252 1.210 1.265 1.265 0.2 1.278 1.290 1 VIN = 9V, ILOAD = 700mA, VPINV = VPWR = 14V, Topside Switch = N-Ch q q 3.23 4.90 3.33 5.05 50 3.43 5.20 mVP-P 40 40 65 25 100 40 3.0 250 50 40 5 5 mV mV mV mV mV mV mA A A A A A - 40 - 40 0 0 40 15 60 25 2.1 170 25 20 1 1 IQ2 2 U V V A V V W U U WW W LTC1266 LTC1266-3.3/LTC1266-5 The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VSHDN = VBINH = 0V unless otherwise noted. SYMBOL VSENSE 1 PARAMETER Current Sense Threshold (Burst Mode Operation Enabled) LTC1266 LTC1266-3.3 LTC1266-5 VSENSE 2 Current Sense Threshold (Burst Mode Operation Disabled) LTC1266 LTC1266-3.3 LTC1266-5 VSHDN ISHDN IPINV VBINH IBINH ICT tOFF tMAX tr, tf VCLAMP Shutdown Pin Threshold Shutdown Pin Input Current Phase Invert Pin Input Current Burst Mode Operation Inhibit Pin Threshold Burst Mode Operation Inhibit Pin Input Current CT Pin Discharge Current Off-Time (Note 4) Max On-Time Driver Output Transition Times (Note 7) Output Voltage Clamp in Burst Mode Operation Inhibit LTC1266 LTC1266-3.3 LTC1266-5 Low-Battery Trip Point Max Leakage Current Into Pin 14 Max Sink Current Into Pin 14 Max Leakage Current Into Pin 13 0V < VSHDN < 8V, VIN = 16V 0V < VPINV < 18V, VIN = 18V VIN = 7V 0V < VBINH < 18V, VIN = 18V VSENSE + = VOUT - 100mV, VSENSE - = VOUT - 300mV VOUT = 0V CT = 390pF, ILOAD = 700mA VOUT = 0V, VIN = 18V CL = 3000pF (Pins 1, 16), VIN = 6V VBINH = 2.1V Measured at VFB Measured at VSENSE - Measured at VSENSE - VIN = 5V VIN = 12V VLBOUT = 18V, VLBIN = 2V VLBOUT = 1V, VLBIN = 0V, 2.5V < VIN < 18V VLBIN = 18V 1 1.30 3.43 5.20 1.14 1.17 1.25 1.30 25 8 0.2 1 1.35 1.42 200 V V V V V nA mA A 50 4 0.8 CONDITIONS VBINH = 0V VSENSE - = 3.3V, VFB = VOUT/2.64 + 25mV (Forced) VSENSE - = 3.3V, VFB = VOUT/2.64 - 25mV (Forced) VSENSE - = VOUT + 100mV (Forced) VSENSE - = VOUT - 100mV (Forced) VSENSE - = VOUT + 100mV (Forced) VSENSE - = VOUT - 100mV (Forced) VBINH = 2.1V VSENSE - = 3.3V, VFB = VOUT/2.64 + 25mV (Forced) VSENSE - = 3.3V, VFB = VOUT/2.64 - 25mV (Forced) VSENSE - = VOUT + 100mV (Forced) VSENSE - = VOUT - 100mV (Forced) VSENSE - = VOUT + 100mV (Forced) VSENSE - = VOUT - 100mV (Forced) q q q q q q ELECTRICAL CHARACTERISTICS MIN TYP MAX UNITS 135 135 135 25 155 25 155 25 155 175 175 175 mV mV mV mV mV mV 135 135 135 0.6 - 20 155 - 20 155 - 20 155 0.8 1.2 0.2 1.2 0.2 70 2 5 60 100 175 175 175 2 5 1 2 1 90 10 6 200 mV mV mV mV mV mV V A A V A A A s s ns VLBTRIP ILBLEAK ILBSINK ILBIN - 40C < TA < 85C (Note 5), VIN = 10V, unless otherwise noted. SYMBOL VFB PARAMETER Feedback Voltage LTC1266AIS LTC1266CS, LTC1266IS VOUT Regulated Output Voltage LTC1266CS-3.3, LTC1266IS-3.3 LTC1266CS-5, LTC1266IS-5 VIN = 9V, ILOAD = 700mA, VPINV = VPWR = 14, Topside Switch = N-Ch 3.23 4.90 3.33 5.05 3.43 5.20 V V CONDITIONS VIN = 9V, ILOAD = 700mA, VPINV = VPWR = 14V, Topside Switch = N-Ch 1.246 1.210 1.265 1.265 1.290 1.290 V V MIN TYP MAX UNITS 3 LTC1266 LTC1266-3.3/LTC1266-5 ELECTRICAL CHARACTERISTICS SYMBOL IQ1 PARAMETER VIN Pin DC Supply Current (Note 3) Normal Mode Sleep Mode Shutdown PWR VIN DC Supply Current (Note 3) Normal Mode Sleep Mode Shutdown Current Sense Threshold (Burst Mode Operation Enabled) LTC1266CS, LTC1266IS LTC1266CS LTC1266IS LTC1266-3.3, LTC1266-5 (I and C) LTC1266CS-3.3, LTC1266CS-5 LTC1266IS-3.3, LTC1266IS-5 Current Sense Threshold (Burst Mode Operation Disabled) LTC1266CS, LTC1266IS LTC1266CS LTC1266IS LTC1266-3.3, LTC1266-5 (I and C) LTC1266CS-3.3, LTC1266CS-5 LTC1266IS-3.3, LTC1266IS-5 Shutdown Pin Threshold Off-Time (Note 4) CONDITIONS - 40C < TA < 85C (Note 5), VIN = 10V, unless otherwise noted. MIN TYP 2.1 170 25 20 1 1 MAX 3.3 260 60 50 7 7 UNITS mA A A A A A 3.5V < VIN < 18V 3.5V < VIN < 18V VSHUTDOWN = 2.1V, 3.5V < VIN < 18V 3.5V < PWR VIN < 18V 3.5V < PWR VIN < 18V VSHUTDOWN = 2.1V, 3.5V < PWR VIN < 18V VBINH = 0V VSENSE- = 3.3V, VFB = VOUT/2.64 + 25mV (Forced) VSENSE- = 3.3V, VFB = VOUT/2.64 - 25mV (Forced) VSENSE- = 3.3V, VFB = VOUT/2.64 - 25mV (Forced) VSENSE- = VOUT + 100mV (Forced) VSENSE- = VOUT - 100mV (Forced) VSENSE- = VOUT - 100mV (Forced) VBINH = 2.1V VSENSE- 3.3V, VFB = VOUT/2.64 + 25mV (Forced) VSENSE- 3.3V, VFB = VOUT/2.64 - 25mV (Forced) VSENSE- 3.3V, VFB = VOUT/2.64 - 25mV (Forced) VSENSE- = VOUT + 100mV (Forced) VSENSE- = VOUT - 100mV (Forced) VSENSE- = VOUT - 100mV (Forced) C Grade I Grade CT = 390pF, ILOAD = 700mA, C Grade CT = 390pF, ILOAD = 700mA, I Grade 130 130 130 130 0.55 0.50 3.8 3.8 135 135 135 135 IQ2 VSENSE1 25 155 155 25 155 155 180 190 180 190 mV mV mV mV mV mV VSENSE2 -20 155 155 -20 155 155 0.8 0.8 5 5 185 195 185 195 2 2 6.5 7.0 mV mV mV mV mV mV V V s s VSHDN tOFF Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD x 110C/W) Note 3: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 4: In applications where RSENSE is placed at ground potential, the offtime increases approximately 40%. Note 5: The LTC1266CS, LTC1266CS-3.3, LTC1266-5 and LTC1266ACS are not tested and not quality assurance sampled at - 40C and 85C. These specifications are guaranteed by design and/or correlation. The LTC1266IS, LTC1266IS-3.3, LTC1266IS-5 and LTC1266AIS are guaranteed and tested over the - 40C to 85C operating temperature range. Note 6: Unless otherwise noted the specifications for the LTC1266A are the same as those for the LTC1266. Note 7: tr and tf are measured at 10% and 90% levels. 4 LTC1266 LTC1266-3.3/LTC1266-5 TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Input Voltage 100 95 ILOAD = 2.5A EFFICIENCY (%) 90 ILOAD = 5A 85 80 75 -30 FIGURE 1 CIRCUIT VOUT (mV) VOUT (mV) ILOAD = 100mA 70 3 4 5 6 7 INPUT VOLTAGE (V) Efficiency vs Input Voltage 100 95 EFFICIENCY (%) FIGURE 11 CIRCUIT ILOAD = 1A ILOAD = 2.5A VOUT (mV) VOUT (mV) 90 85 80 75 70 ILOAD = 100mA 0 4 12 8 INPUT VOLTAGE (V) VIN DC Supply Current 3.0 2.5 25 SUPPLY CURRENT (mA) SUPPLY CURRENT (A) 2.0 1.5 1.0 0.5 0 ACTIVE MODE SUPPLY CURRENT (A) SLEEP MODE SLEEP MODE 0 0 4 12 8 INPUT VOLTAGE (V) 16 20 1266 G07 UW 8 1266 G01 Line Regulation 40 30 20 10 0 -10 -20 Load Regulation 20 FIGURE 1 CIRCUIT VIN = 9V (Burst Mode OPERATION ENABLED) VIN = 5V 10 0 -10 -20 -30 -40 -50 VIN = 5V (Burst Mode OPERATION INHIBITED) FIGURE 1 CIRCUIT ILOAD = 1A 9 - 40 3 4 5 6 7 INPUT VOLTAGE (V) 8 9 1266 G02 0 1 3 2 LOAD CURRENT (A) 4 5 1266 G03 Line Regulation 40 30 20 10 0 -10 -20 -30 - 40 Load Regulation 30 FIGURE 11 CIRCUIT FIGURE 11 CIRCUIT ILOAD = 1A 20 10 0 -10 VIN = 5V -20 -30 -40 VIN = 5V (Burst Mode OPERATION INHIBITED) VIN = 12V (Burst Mode OPERATION ENABLED) 16 20 1266 G04 0 4 8 12 16 1266 G05 0 0.5 INPUT VOLTAGE (V) 2.0 1.5 1.0 LOAD CURRENT (A) 2.5 3.0 1266 G06 Power VIN DC Supply Current 50 Supply Current in Shutdown 20 ACTIVE MODE 40 VIN 15 30 10 20 5 10 PWR VIN 16 20 1266 G08 0 4 12 8 INPUT VOLTAGE (V) 0 0 5 15 10 INPUT VOLTAGE (V) 20 1266 G09 5 LTC1266 LTC1266-3.3/LTC1266-5 TYPICAL PERFOR A CE CHARACTERISTICS Off-Time vs Output Voltage 100 VSENSE- = VOUT 3.0 2.5 NORMALIZED FREQUENCY 80 OFF-TIME (s) 2.0 1.5 1.0 0.5 0 25C 70C 60 SENSE VOLTAGE (mV) 40 20 LTC1266-3.3 0 0 1 LTC1266-5 3 4 2 OUTPUT VOLTAGE (V) PI FU CTIO S TDRIVE (Pin 1): High Current Drive for Topside MOSFET. This MOSFET can be either P-channel or N-channel, user selectable by Pin 3. Voltage swing at this pin is from PWR VIN to ground. PWR VIN (Pin 2): Power Suppy for Drive Signals. Must be closely decoupled to power ground (Pin 15). PINV (Pin 3): Phase Invert. Sets the phase of the topside driver to drive either a P-channel or an N-channel MOSFET as follows: P-channel: Pin 3 = 0V N-channel: Pin 3 = PWR VIN BINH (Pin 4): Burst Mode Operation Inhibit. A CMOS logic high on this pin will disable the Burst Mode operation feature forcing continuous operation down to zero load. VIN (Pin 5): Main Supply Pin. CT (Pin 6): External Capacitor. CT from Pin 4 to ground sets the operating frequency. The actual frequency is also dependent on the input voltage. ITH (Pin 7): Gain Amplifier Decoupling Point. The current comparator threshold increases with the Pin 7 voltage. SENSE - (Pin 8): Connects to internal resistive divider which sets the output voltage in LTC1266-3.3 and LTC1266-5 versions. Pin 8 is also the (-) input for the current comparator. SENSE + (Pin 9): The (+) Input to the Current Comparator. A built-in offset between Pins 8 and 9 in conjunction with RSENSE sets the current trip threshold. VFB (Pin 10): For the LTC1266 adjustable version, Pin 10 serves as the feedback pin from an external resistive divider used to set the output voltage. On LTC1266-3.3 and LTC1266-5 versions this pin is not used. SHDN (Pin 11): When grounded, the LTC1266 series operates normally. Pulling Pin 11 high holds both MOSFETs off and puts the LTC1266 in micropower shutdown mode. Requires CMOS logic signal with tr, tf < 1s. Should not be left floating. SGND (Pin 12): Small-Signal Ground. Must be routed separately from other grounds to the (-) terminal of COUT. LBIN (Pin 13): Input to the Low-Battery Comparator. This input is compared to an internal 1.25V reference. LBOUT (Pin 14): Open Drain Output of the Low-Battery Comparator. This pin will sink current when Pin 13 is below 1.25V. PGND (Pin 15): Driver Power Ground. Connects to source of N-channel MOSFET and the (-) terminal of CIN. BDRIVE (Pin 16): High Current Drive for Bottom N-Channel MOSFET. Voltage swing at Pin 16 is from ground to PWR VIN. 6 UW 1266 G10 Operating Frequency vs (VIN - VOUT) Voltage VOUT = 3.3V 0C Current Sense Threshold Voltage 200 MAX THRESHOLD 150 100 MIN THRESHOLD (Burst Mode OPERATION ENABLED) MIN THRESHOLD (Burst Mode OPERATION INHIBIT) 50 0 5 0 2 6 8 10 12 4 (VIN - VOUT) VOLTAGE (V) -50 14 16 0 20 60 40 TEMPERATURE (C) 80 100 1266 G12 1266 G11 U U U LTC1266 LTC1266-3.3/LTC1266-5 FU CTIO AL DIAGRA W R Q S VTH1 VTH2 LBIN 13 1.25V REFERENCE VIN PWR VIN 2 SIGNAL GROUND 12 15 PGND V SLEEP + S C - + VTRIP VOS 13k 5pF T MAX ON-TIME CONTROL ENABLE PINV ITH 7 G 6 CT OFF-TIME CONTROL VIN SENSE - VFB SHDN 11 OPERATIO The LTC1266 series uses a current mode, constant offtime architecture to synchronously switch an external pair of power MOSFETs. Operating frequency is set by an external capacitor at the timing capacitor Pin 6. The output voltage is sensed by an internal voltage divider connected to SENSE -, Pin 8, (LTC1266-3.3 and LTC12665) or external divider returned to VFB, Pin 10, (LTC1266). A voltage comparator V, and a gain block G, compare the divided output voltage with a reference voltage of 1.265V. To optimize efficiency, the LTC1266 automatically switches between two modes of operation, burst and continuous. The voltage comparator is the primary control element when the device is in Burst Mode operation, while the gain block controls the output voltage in continuous mode. During the switch ON cycle in continuous mode, current comparator C monitors the voltage between Pins 8 and 9 connected across an external shunt in series with the inductor. When the voltage across the shunt reaches its threshold value, the topside driver output is switched to turn off the topside MOFSET (Power VIN for P-channel or ground for N-channel). The timing capacitor connected to Pin 6 is now allowed to discharge at a rate determined by the off-time controller. The discharge current is made proportional to the output voltage (measured by Pin 8) to model the inductor current, which decays at a rate which is also proportional to the output voltage. While the timing capacitor is discharging, the bottom-side drive output is switched to power VIN to turn on the bottom-side N-channel MOSFET. - + + - - - + - + + - U U U Pin 10 Connection Shown for LTC1266-3.3 and LTC1266-5; Changes Create LTC1266 14 LBOUT LB 3 PINV 1 TDRIVE SENSE+ 9 ADJUSTABLE VERSION VFB 16 BDRIVE BINH 4 10 SENSE - 8 100k 1.265V REFERENCE 5 VIN 1266 FD 7 LTC1266 LTC1266-3.3/LTC1266-5 OPERATIO When the voltage on the timing capacitor has discharged past VTH1, comparator T trips, setting the flip-flop. This causes the bottom-side output to switch off and the topside output to switch on (ground for P-channel and Power VIN for N-channel). The cycle then repeats. As the load current increases, the output voltage decreases slightly. This causes the output of the gain stage (Pin 7) to increase the current comparator threshold, thus tracking the load current. The sequence of events for Burst Mode operation is very similar to continuous operation with the cycle interrupted by the voltage comparator. When the output voltage is at or above the desired regulated value, the topside MOSFET is held off by comparator V and the timing capacitor continues to discharge below VTH1. When the timing capacitor discharges past VTH2, voltage comparator S trips, causing the internal sleep line to go low and the bottom-side MOSFET to turn off. The circuit now enters sleep mode with both power MOSFETs turned off. In sleep mode, a majority of the circuitry is turned off, dropping the quiescent current from 2.1mA to 170A. The load current is now being supplied from the output capacitor. When the output voltage has dropped by the amount of hysteresis in comparator V, the topside MOSFET is again turned on and this process repeats. To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset VOS is incorporated in the gain stage. This prevents the current comparator threshold from increasing until the output voltage has dropped below a minimum threshold. APPLICATIO S I FOR ATIO One of the three basic LTC1266 application circuits is shown in Figure 1. This circuit uses an N-channel topside driver and a single supply. The other two circuit configurations (see Typical Applications) use an N-channel topside driver and dual supply, and a P-channel topside driver. Selections of other external components are driven by the load requirement and are the same for all three circuit configurations. The first 8 U W UU U To prevent both the external MOSFETs from ever being turned on at the same time, feedback is incorporated to sense the state of the driver output pins. Before the bottom-side drive output can turn on, the topside output must be off. Likewise, the topside output is prevented from turning on while the bottom-side drive output is still on. The LTC1266 has two select pins which provide the user with choice of topside switch and with the option of inhibiting Burst Mode operation. The phase select pin allows the user to choose whether the topside MOSFET is a P-channel or an N-channel. The phase select pin does two things: sets the proper phase of the drive signal (ON = Power VIN for N-channel and ON = 0V for P-channel) and also sets an upper limit for the on-time (60s) when set to the N-channel. The on-time limit ensures proper start-up when used in a single supply bootstrap circuit configuration (see Applications Information). In P-channel mode there is no on-time limit and thus, in dropout, the P-channel MOSFET is turned on continuously (100% duty cycle). The Burst Mode operation inhibit (BINH, Pin 4) allows the Burst Mode operation to be disabled by applying a CMOS logic high to this pin. With Burst Mode operation disabled, the LTC1266 will remain in continuous mode down to zero load. Burst Mode operation is disabled by allowing the lower current threshold limit to go below zero so that the voltage comparator will never trip. The voltage comparator trip point is also raised up so that it will not be tripped by transients. It is still active to provide a voltage clamp to prevent the output from overshooting. step is the selection of RSENSE. Once RSENSE is known, CT and L can be chosen. Next, the power MOSFETs and D1 are selected. Finally, CIN and COUT are selected and the loop is compensated. Using an N-channel topside switch, input voltages are limited to a maximum of about 15V. With a P-channel, the input voltage may be as high as 20V. LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1266 series current comparator has a threshold range which extends from a minimum of 25mV/RSENSE (when Burst Mode operation is enabled) to a maximum of 155mV/RSENSE. The current comparator threshold sets the peak of the inductor ripple current, yielding a maximum output current IMAX equal to the peak value less half the peak-to-peak ripple current. For proper Burst Mode operation, IRIPPLE(P-P) must be less than or equal to the minimum current comparator threshold. Since efficiency generally increases with ripple current, the maximum allowable ripple current is assumed, i.e., IRIPPLE(P-P) = 25mV/RSENSE (see CT and L Selection for Operating Frequency). Solving for RSENSE and allowing a margin for variations in the LTC1266 series and external component values yields: RSENSE = 100mV IMAX A graph for selecting RSENSE vs maximum output current is given in Figure 2. 100 75 RSENSE (m) 50 25 0 0 1266 F02 CAPACITANCE (pF) 4 2 8 6 MAXIMUM OUTPUT CURRENT (A) 10 Figure 2. Selecting RSENSE The load current, below which Burst Mode operation commences, (IBURST), and the peak short-circuit current, (ISC(PK)), both track IMAX. Once RSENSE has been chosen, IBURST and ISC(PK) can be predicted from the following: U IBURST 15mV RSENSE ISC(PK) = 155mV RSENSE The LTC1266 series automatically extends tOFF during a short circuit to allow sufficient time for the inductor current to decay between switch cycles. The resulting ripple current causes the average short-circuit current ISC(AVG) to be reduced to approximately IMAX. L and CT Selection for Operating Frequency The LTC1266 series uses a constant off-time architecture with tOFF determined by an external timing capacitor CT. Each time the topside MOSFET switch turns on, the voltage on CT is reset to approximately 3.3V. During the off-time, CT is discharged by a current which is proportional to VOUT. The voltage on CT is analogous to the current in inductor L, which likewise decays at a rate proportional to VOUT. Thus the inductor value must track the timing capacitor value. The value of CT is calculated from the desired continuous mode operating frequency, f: 1 2.6 * 104 * f assumes VIN = 2VOUT, (Figure 1 circuit). CT = A graph for selecting CT vs frequency including the effects of input voltage is given in Figure 3. 800 VOUT = 3.3V 600 400 VIN = 12V 200 VIN = 5V 0 0 100 200 300 FREQUENCY (kHz) 400 500 1266 F03 W UU Figure 3. Timing Capacitor Value 9 LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO As the operating frequency is increased the gate charge losses will be higher, reducing efficiency (see Efficiency Considerations). The complete expression for operating frequency of the circuit in Figure 1 is given by: f= 1 tOFF ) 1- VOUT VIN ) where: tOFF = 1.3 * 104 * CT * )) VREG VOUT VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT is the measured output voltage. Thus VREG/VOUT = 1 in regulation. Once the frequency has been set by CT, the inductor L must be chosen to provide no more than 25mV/RSENSE of peak-to-peak inductor ripple current. This results in a minimum required inductor value of: LMIN = 5.1 * 105 * RSENSE * CT * VREG As the inductor value is increased from the minimum value, the ESR requirements for the output capacitor are eased at the expense of efficiency. If too small an inductor is used, the inductor current will decrease past zero and change polarity. A consequence of this is that the LTC1266 series may not enter Burst Mode operation and efficiency will be slightly degraded at low currents. Inductor Core Selection Once the minimum value for L is known, the type of inductor must be selected. The highest efficiency will be obtained using ferrite, Kool M(R) on molypermalloy (MPP) cores. Lower cost powdered iron cores provide suitable performance but cut efficiency by 3% to 7%. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses increase. Ferrite designs have very low core loss, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design Kool M is a registered trademark of Magnetics, Inc. 10 U current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple which can cause Burst Mode operation to be falsely triggered. Do not allow the core to saturate! Kool M is a very good, low loss core material for toroids, with a "soft" saturation characteristic. Molypermalloy is slightly more efficient at high (> 200kHz) switching frequency. Toroids are very space efficient, especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more difficult. However, new designs for surface mount are available from Coiltronics and Beckman Industrial Corp. which do not increase the height significantly. Power MOSFET and D1 Selection Two external power MOSFETs must be selected for use with the LTC1266 series: either a P-channel MOSFET or an N-channel MOSFET for the main switch and an N-channel MOSFET for the synchronous switch. The main selection criteria for the power MOSFETs are the type of MOSFET, threshold voltage VGS(TH) and on-resistance RDS(ON). The cost and maximum output current determine the type of MOSFET for the topside switch. N-channel MOSFETs have the advantage of lower cost and lower RDS(ON) at the expense of slightly increased circuit complexity. For lower current applications where the losses due to RDS(ON) are small, a P-channel MOSFET is recommended due to the lower circuit complexity. However, at load currents in excess of 3A where the RDS(ON) becomes a significant portion of the total power loss, an N-channel is strongly recommended to maximize efficiency. The maximum output current IMAX determines the RDS(ON) requirement for the two MOSFETs. When the LTC1266 series is operating in continuous mode, the simplifying assumption can be made that one of the two MOSFETs is always conducting the average load current. The duty cycles for the two MOSFETs are given by: W UU Topside Duty Cycle = VOUT VIN VIN - VOUT VIN Bottom-Side Duty Cycle = LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO VIN * PT VOUT * IMAX2 * (1 + T) VIN * PB (VIN - VOUT) * IMAX2 * (1 + B) From the duty cycles, the required RDS(ON) for each MOSFET can be derived: TS RDS(ON) = BS RDS(ON) = where PT and PB are the allowable power dissipations and T and B are the temperature dependencies of RDS(ON). PT and PB will be determined by efficiency and/or thermal requirements (see Efficiency Considerations). For a MOSFET, (1 + ) is generally given in the form of a normalized RDS(ON) vs temperature curve, but PCH = 0.007/C and NCH = 0.005/C can be used as an approximation for low voltage MOSFETs. The minimum input voltage determines whether standard threshold or logic-level threshold MOSFETs must be used. For VIN > 8V, standard threshold MOSFETs (VGS(TH) < 4V) may be used. If VIN is expected to drop below 8V, logiclevel threshold MOSFETs (VGS(TH) < 2.5V) are strongly recommended. The LTC1266 series Power VIN must always be less than the absolute maximum VGS ratings for the MOSFETs. The Schottky diode D1 shown in Figure 1 only conducts during the deadtime between the conduction of the two power MOSFETs. D1's sole purpose in life is to prevent the body diode of the bottom-side MOSFET from turning on and storing charge during the deadtime, which could cost as much as 1% in efficiency (although there are no other harmful effects if D1 is omitted). Therefore, D1 should be selected for a forward voltage of less than 0.7V when conducting IMAX. CIN and COUT Selection In continuous mode, the current through the topside MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR (Effective Series Resistance) input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN Required IRMS IMAX [VOUT(VIN - VOUT)]1/2 VIN U This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer's ripple current ratings 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. Always consult the manufacturer if there is any question. An additional 0.1F to 1F ceramic capacitor is also required on Power VIN (Pin 2) for high frequency decoupling. The selection of COUT is driven by the required ESR. The ESR of COUT must be less than twice the value of RSENSE for proper operation of the LTC1266 series: COUT Required ESR < 2RSENSE Optimum efficiency is obtained by making the ESR equal to RSENSE. As the ESR is increased up to 2RSENSE, the efficiency degrades by less than 1%. If the ESR is greater than 2RSENSE, the voltage ripple on the output capacitor will prematurely trigger Burst Mode operation, resulting in disruption of continuous mode and an efficiency hit which can be several percent. If Burst Mode operation is disabled, the ESR requirement can be relaxed and is limited only by the allowable output voltage ripple. Manufacturers such as Nichicon and United Chemicon should be considered for high performance capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR/size ratio of any aluminum electrolytic at a somewhat higher price. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. In surface mount applications multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirements of the application. An excellent choice is the AVX TPS series of surface mount tantalums. At low supply voltages, a minimum capacitance at COUT is needed to prevent an abnormal low frequency operating mode (see Figure 4). When COUT is made too small, the output ripple at low frequencies will be large enough to trip the voltage comparator. This causes Burst Mode operation to be activated when the LTC1266 W UU 11 LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO 1000 L = 50H RSENSE = 0.02 800 L = 25H RSENSE = 0.02 COUT (F) 600 400 200 L = 50H RSENSE = 0.05 0 1 3 4 2 (VIN - VOUT) VOLTAGE (V) 5 1266 F04 0 Figure 4. Minimum Value of COUT series would normally be in continuous operation. The output remains in regulation at all times. This minimum capacitance requirement may be relaxed if Burst Mode operation is disabled. N-Channel vs P-Channel MOSFETs The LTC1266 has the capability to drive either an N-channel or a P-channel topside switch to give the user more flexibility. N-channel MOSFETs are superior in performance to P-channel due to their lower RDS(ON) and lower gate capacitance and are typically less expensive; however, they do have a slightly more complicated gate drive requirement and a more limited input voltage range (see following sections). Driving P-Channel Topside MOSFETs The P-channel topside switch circuit configuration is the most straightforward due to the requirement of only one supply voltage level. This is due to the negative gate threshold of the P-channel MOSFET which allows the MOSFET to be switched on and off by swinging the gate between VIN and ground. The phase invert (Pin 3) is tied to ground to choose this operating mode. Normally, the converter input (VIN) is connected to the LTC1266 supply Pins 2 and 5 and can go as high as 20V. Pin 2 supplies the high frequency current pulses to switch the MOSFETs and should be decoupled with a 0.1F to 1F ceramic capacitor. Pin 5 supplies most of the quiescent power to the rest of the chip. 12 U Driving N-Channel Topside MOSFETs Driving an N-channel topside MOSFET (PINV, Pin 3, tied to PWR VIN) is a little trickier than driving a P-channel since the gate voltage must be positive with respect to the source to turn it on, which means that the gate voltage must be higher than VIN. This requires either a second supply at least VGS(ON) above VIN or a bootstrapping circuit to boost the VIN to the proper level. The easiest method is using a higher supply (see Figure 14) but if one is not available, the bootstrap method can be used at the expense of an additional diode (see Figure 1). The bootstrap works by charging the bootstrap capacitor to VIN during the off-time. During the on-time, the bottom plate of the capacitor is pulled up to VIN so that the voltage at Pin 2 is now twice VIN (plus any ringing on the switch node). Since the maximum allowable voltage at Pin 2 is 20V, the Figure 1 bootstrap circuit limits VIN to less than 10V. A higher VIN can be achieved if the bootstrap capacitor is charged to a voltage less than VIN, in which case VIN(MAX) = 20 - VCAP. N-channel mode, internal circuitry limits the maximum on-time to 60s to guarantee start-up of the bootstrap circuit. This maximum on-time reduces the maximum duty cycle to: Max Duty Cycle = 60s 60s + tOFF W UU which slightly increases the minimum input voltage at which dropout occurs. However, because of the superior on-conductance of the N-channel, the dropout performance of an all N-channel regulator is still better (see Figure 5) even with the duty cycle limitation, except at light loads. Low-Battery Comparator The LTC1266 has an on-chip low-battery comparator which can be used to sense a low-battery condition when implemented as shown in Figure 6. The resistor divider R1, R2 sets the comparator trip point as follows: VTRIP = 1.25 1 + R2 R1 ) ) LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO 600 500 400 300 200 100 0 TOPSIDE N-CHANNEL WITH POWER VIN = 12V TOPSIDE P-CHANNEL VOUT = 3.3V VIN-V0UT (mV) AT DROPOUT TOPSIDE N-CHANNEL WITH CHARGE PUMP EFFICIENCY (%) 0 1 3 2 LOAD CURRENT 4 5 1266 F05 Figure 5. Comparison of Dropout Performance VIN R2 LTC1266 LBOUT R1 1.25V REFERENCE 1266 F06 Figure 6. Low-Battery Comparator The divided down voltage at the "-" input to the comparator is compared to an internal 1.25V reference. This reference is separate from the 1.265V reference used by the voltage comparator and current comparator for regulation and is not disabled by the shutdown pin, therefore the low-battery detection is operational even when the rest of the chip is shut down. The comparator is functional down to an input voltage of 2.5V. Thus, the output will provide a valid state even when the rest of the chip does not have sufficient voltage to operate. For best performance, the value of the pull-up resistor should be high enough that the output is pulled down to ground when sinking 200A or less. Suppressing Burst Mode Operation Normally, enabling Burst Mode operation is desired due to its superior efficiency at low load currents (see Figure 7). However, in certain applications it may be desirable to inhibit this feature. Some reasons for doing so are: 1. To eliminate audible noise from certain types of inductors at light loads. U 100 Burst Mode OPERATION ENABLED 90 80 Burst Mode OPERATION INHIBITED 70 60 0.01 0.1 1 LOAD CURRENT (A) 5 1266 F07 W UU + - Figure 7. Effect of Disabling Burst Mode Operation on Efficiency 2. If the load is never expected to drop low enough to benefit from the efficiency advantages of Burst Mode operation, the output capacitor ESR and minimum capacitance requirements (which may falsely trigger Burst Mode operation if not met) can be relaxed if Burst Mode operation is disabled. 3. If an auxiliary winding is used. Disabling Burst Mode operation guarantees switching independent of the load on the primary. This allows power to be taken from the auxiliary winding independently. 4. Tighter load regulation (< 1%). Burst Mode operation is disabled by applying a CMOS logic high voltage (> 2.1V) to Pin 4. When it is disabled, the voltage comparator limit is raised high enough so that it no longer is involved in regulation; however it is still active and is useful as a voltage clamp to keep the output from overshooting. Note that since the inductor current must reverse to regulate the output at zero load when Burst Mode operation is disabled, the minimum inductance (LMIN) specified during Inductor Core Selection is no longer applicable. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ILOAD (ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or 13 LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO discharge COUT until the regulator loop adapts to the current change and returns VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing which would indicate a stability problem. The Pin 7 external components shown in the Figure 1 circuit will prove adequate compensation for most applications. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: % Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc., are the individual losses as a percentage of input power. (For high efficiency circuits, only small errors are incurred by expressing losses as a percentage of output power). Although all dissipative elements in the circuit produce losses, three main sources usually account for most of the losses in LTC1266 series circuits: 1) LTC1266 DC bias current, 2) MOSFET gate charge current and 3) I2R losses. 1. The DC supply current is the current which flows into VIN (Pin 2). For VIN = 10V the LTC1266 DC supply current is 170A for no load, and increases proportionally with load up to a constant 2.1mA after the LTC1266 series has entered continuous mode. Because the DC bias current is drawn from VIN, the resulting loss increases with input voltage. For VIN = 5V the DC bias losses are generally less than 1% for load currents over 30mA. However, at very low load currents the DC bias current accounts for nearly all of the loss. 2. MOSFET gate charge current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from Power VIN to ground. The resulting dQ/dt is a current flowing into Power VIN (Pin 5) which is typically much larger than the DC supply current. In continuous mode, IGATECHG = f (QN + QP). The typical gate charge for a 0.05 N-channel power MOSFET is EFFICIENCY/LOSS (%) 14 U 15nC. This results in IGATECHG = 6mA in 200kHz continuous operation for a 2% to 3% typical mid-current loss with VIN = 5V. Note that the gate charge loss increases directly with both input voltage and operating frequency. This is the principal reason why the highest efficiency circuits operate at moderate frequencies. Furthermore, it argues against using larger MOSFETs than necessary to control I2R losses, since overkill can cost efficiency as well as money! 3. I2R losses are easily predicted from the DC resistances of the MOSFET, inductor and current shunt. In continuous mode the average output current flows through L and RSENSE, but is "chopped" between the topside and bottom-side MOSFETs. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L and RSENSE to obtain I2R losses. For example, if each RDS(ON) = 0.05, RL = 0.05 and RSENSE = 0.02, then the total resistance is 0.12. This results in losses ranging from 3.5% to 15% as the output current increases from 1A to 5A. I2R losses cause the efficiency to roll off at high output currents. Figure 8 shows how the efficiency losses in a typical LTC1266 series regulator end up being apportioned. The gate charge loss is responsible for the majority of the efficiency lost in the mid-current region. If Burst Mode operation was not employed at low currents, the gate charge loss alone would cause efficiency to drop to unacceptable levels (see Figure 7). With Burst Mode 100 I2R GATE CHARGE 95 LTC1266 IQ 90 85 80 0.01 0.03 0.3 0.1 IOUT (A) 1 5 1266 F08 W UU Figure 8. Efficiency Loss LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO operation, the DC supply current represents the lone (and unavoidable) loss component which continues to become a higher percentage as output current is reduced. As expected the I2R losses dominate at high load currents. Other losses including CIN and COUT ESR dissipative losses, MOSFET switching losses, Schottky conduction losses during deadtime and inductor core losses, generally account for less than 2% total additional loss. Design Example As a design example, assume VIN = 5V (nominal), VOUT = 3.3V, IMAX = 5A and f = 200kHz; RSENSE, CT and L can immediately be calculated: RSENSE = 100mV/5 = 0.02 tOFF = (1/200kHz) * [1 - (3.3/5)] = 1.7s CT = 1.7s/(1.3 * 104) = 130pF LMIN = 5.1 * 105 * 0.02 * 130pF * 3.3V = 5H Assume that the MOSFET dissipations are to be limited to PT = PB = 2W. If TA = 40C and the thermal resistance of each MOSFET is 50C/ W, then the junction temperatures will be 140C and T = B = 0.60. The required RDS(ON) for each MOSFET can now be calculated: TS RDS(ON) = BS RDS(ON) = 5(2) = 0.076 3.3(5)2 (1.60) 5(2) = 0.147 1.7(5)2 (1.60) The topside FET requirement can be met by an N-channel Si9410DY which has an RDS(ON) of about 0.04 at VGS = 5V. The bottom-side FET requirement is exceeded by an Si9410DY. Note that the most stringent requirement for the bottom-side MOSFET is with VOUT = 0 (i.e., short circuit). During a continuous short circuit, the worst-case dissipation rises to: PB = ISC(AVG)2 * RDS(ON) * (1 + B) With the 0.02 sense resistor, ISC(AVG) 6A will result, increasing the 0.04 bottom-side FET dissipation to 2.3W. U CIN will require an RMS current rating of at least 2.5A at temperature and COUT will require an ESR of 0.02 for optimum efficiency. Now allow VIN to drop to its minimum value. The minimum VIN can be calculated from the maximum duty cycle and voltage drop across the topside FET, W UU VMIN = VOUT + ILOAD * (RDS(ON) + RL + RSENSE) DMAX = 4.0V At this lower input voltage, the operating frequency decreases and the topside FET will be conducting most of the time, causing the power dissipation to increase. At dropout, fMIN = 1 = 16kHz tON (MAX) + tOFF PT = I2LOAD * RDS(ON) * (1 + T) * DMAX This last step is necessary to assure that the power dissipation and junction temperature of the topside FET are not exceeded. These last calculations assume that Power VIN is high enough to keep the topside FET fully turned on at dropout, as would be the case with the Figure 11circuit. If this isn't true (as with the Figure 1 circuit) the RDS(ON) will increase which in turn increases VMIN and PT. Adjustable Applications When an output voltage other than 3.3V or 5V is required, the LTC1266 adjustable version is used with an external resistive divider from VOUT to VFB, Pin 10. The regulated voltage is determined by: VOUT = 1.265 1 + R2 R1 ) ) To prevent stray pickup a 100pF capacitor is suggested across R1 located close to the LTC1266. For Figure 1 applications with VOUT below 2V, or when RSENSE is moved to ground, the current sense comparator inputs operate near ground. When the current comparator is operated at less than 2V common mode, the off-time increases approximately 40%, requiring the use of a smaller timing capacitor CT. 15 LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO Troubleshooting Hints Since efficiency is critical to LTC1266 series applications, it is very important to verify that the circuit is functioning correctly in both continuous and Burst Mode operation. The waveform to monitor is the voltage on the timing capacitor, Pin 6. In continuous mode (ILOAD > IBURST) the voltage on the CT pin should be a sawtooth with a 0.9VP-P swing. This voltage should never dip below 2V as shown in Figure 9a. When load currents are low (ILOAD < IBURST) Burst Mode operation should occur with the CT pin waveform periodically falling to ground for periods of time as shown in Figure 9b. 3.3V (a) Continuous Mode Operation 3.3V (b) Burst Mode Operation Figure 9. CT Waveforms CB CIN 1 2 3 TDRIVE PWR VIN PINV BDRIVE PGND LBOUT LTC1266 4 LBIN BINH 5 6 7 VIN CT ITH SENSE - SGND SHDN VFB SENSE + 1000pF CT 3300pF 8 470 Figure 10. LTC1266 Layout Diagram (See Layout Checklist) 16 U If Pin 6 is observed falling to ground at high output currents, it indicates poor decoupling or improper grounding. Refer to the Board Layout Checklist. Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1266 series. These items are also illustrated graphically in the layout diagram of Figure 10. Check the following in your layout: 1. Are the signal and power grounds segregated? The LTC1266 signal ground (Pin 12) must return to the (-) plate of COUT. The power ground returns to the source of the bottom-side MOSFET, anode of the Schottky diode and (-) plate of CIN, which should have as short lead lengths as possible. 2. Does the LTC1266 SENSE - (Pin 8) connect to a point close to RSENSE and the (+) plate of COUT? In adjustable applications, the resistive divider R1 and R2 must be connected between the (+) plate of COUT and signal ground. 0V 0V 1266 F09 + W UU + BOLD LINES INDICATE HIGH CURRENT PATHS VIN 16 15 14 13 12 11 10 9 L - - SHUTDOWN R1 + R2 OUTPUT DIVIDER REQUIRED WITH ADJUSTABLE VERSION ONLY COUT RSENSE VOUT + 1266 F10 LTC1266 LTC1266-3.3/LTC1266-5 APPLICATIO S I FOR ATIO 3. Are the SENSE- and SENSE+ leads routed together with minimum PC trace spacing? The 1000pF capacitor between Pins 8 and 9 should be as close as possible to the LTC1266. 4. Does the (+) plate of CIN connect to the source of the topside MOSFET as closely as possible? This capacitor provides the AC current to the topside MOSFET. 5. A 0.1F to 1F decoupling capacitor connected between VIN (Pin 5) and ground is optional, but is sometimes TYPICAL APPLICATIO S (Layout Assist Schematics) 1F 1 2 3 TDRIVE PWR VIN PINV BINH 4 5 6 7 LBIN BINH LTC1266-3.3 VIN SGND CT ITH SENSE - SHDN NC SENSE + RC 1k 1000pF RSENSE 0.033 VOUT 3.3V 3A 1266 F11 *DALE LPT4545-A001 COILTRONICS CTX10-4 Figure 11. Low Dropout, 3.3V/3A High Efficiency Regulator + CT 220pF CC 3300pF 8 U helpful in eliminating instabilities at high input voltage and high output loads. 6. Is the shutdown (Pin 11) actively pulled to ground during normal operation? The shutdown pin is high impedance and must not be allowed to float. The select (Pins 3 and 4) are also high impedance and must be tied high or low depending on the application. VIN 3.9V TO 18V (VIN(MIN) = 3.5V IF ILOAD < 0.8A) Si9430DY W U + UU + D1 MBRS140T3 BDRIVE PGND LBOUT 16 15 14 13 12 11 10 9 CIN 100F 25V Si9410DY SHUTDOWN L* 10H COUT 220F 10V 2x 17 LTC1266 LTC1266-3.3/LTC1266-5 TYPICAL APPLICATIO S VIN 4.3V TO 10V (VIN (MIN) = 3.5V IF ILOAD < 100mA BINH CT 200pF CC 3300pF RC 1k *DALE LPT4545-A002 COILTRONICS CTX20-4 **MMBT2222ALT1 Figure 12. 5V to 12V/500mA High Efficiency Boost Regulator PWR VIN VIN + 4.5V TO 18V BINH SENSE - SENSE + RC 470 1000pF RSENSE 0.02 1266 F13 *TOKO 919AS-4R7M Figure 13. All N-Channel 5V to 3.3V/5A Converter with Drivers Powered from External PWR VIN Supply 18 + CT 180pF CC 3300pF + U 1 2 3 4 5 6 7 8 (Layout Assist Schematics) 0.068 L* 20H D1 MBRS130LT3 VOUT 12V/500mA 127k 1% Si9410DY 100pF 15k 1% 0.1F + CIN 100F 20V 1M 16 1M 15 14 13 12 11 10 9 180k 1N4148 SHUTDOWN 100k 1266 F12 + C0UT 100F 20V TDRIVE PWR VIN PINV BINH BDRIVE PGND LBOUT LBIN Q1** LTC1266 VIN CT ITH SENSE - SGND SHDN VFB SENSE + 1000pF VIN 4V TO PWR VIN - 4.5V (VIN(MIN) = 3.5V IF ILOAD < 2.5A) Si9410DY + D1 MBRS140T3 1F 1 2 3 4 5 6 7 8 TDRIVE PWR VIN PINV BDRIVE PGND LBOUT 16 15 14 13 12 11 10 9 Si9410DY CIN 100F 20V 2x BINH LBIN LTC1266-3.3 VIN SGND CT ITH SHDN NC SHUTDOWN L* 5H COUT 220F 10V 2x VOUT 3.3V 5A LTC1266 LTC1266-3.3/LTC1266-5 TYPICAL APPLICATIO S BINH SENSE - SENSE + RC 470 1000pF RSENSE 0.01 1266 F14 *MAGNETICS Kool M 77120-A7 Figure 14. All N-Channel 5V to 3.3V/10A High Efficiency Regulator VIN 4V TO 9V (VIN(MIN) = 3.5V IF ILOAD < 1A) 0.1F MBR0530T1 Si9410DY 1 2 3 BINH 4 5 6 7 CT 180pF CC 3300pF RC 470 8 TDRIVE PWR VIN PINV BINH BDRIVE PGND LBOUT LBIN 16 15 14 13 12 11 10 SHUTDOWN Si9410DY D1 MBRS130T3 LTC1266 VIN CT ITH SENSE - SGND SHDN VFB SENSE + L* 5H 100pF 1000pF RSENSE 0.02 97.6k 1% *TOKO 919AS-4R7M Figure 15. All N-Channel 5V to 2.5V/5A High Efficiency Regulator Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. + 9 + CT 220pF CC 3300pF U (Layout Assist Schematics) VIN 4V TO 9V 0.1F MBR0530T1 Si4410DY + D1 MBRS340T3 1 2 3 4 5 6 7 8 TDRIVE PWR VIN PINV BDRIVE PGND LBOUT 16 15 14 13 12 11 10 9 SHUTDOWN 47F 10V OS-CON 3x Si4410DY BINH LBIN LTC1266-3.3 VIN SGND CT ITH SHDN NC L* 5H COUT 330F 10V 3x VOUT 3.3V 10A + 100F 10V OS-CON 2x 100k 1% COUT 330F 10V 2x 1266 F15 VOUT 2.5V 5A 19 LTC1266 LTC1266-3.3/LTC1266-5 PACKAGE DESCRIPTIO 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0 - 8 TYP 0.016 - 0.050 (0.406 - 1.270) *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE RELATED PARTS PART NUMBER LTC1530 LTC1625 LTC1628 LTC1735 LTC1772 LTC1929 DESCRIPTION Synchronous Step-Down Controller in SO-8 97% Efficiency Synchronous Step-Down Controller 2-Phase, Dual Synchronous Controller High Efficiency Synchronous Controller SOT-23 P-Channel Controller 42A 2-Phase Synchronous Controller for Single Output COMMENTS No RSENSETM Voltage Mode, IOUT Up to 15A No RSENSE Current Mode, Low Dropout, IOUT Up to 20A, VOUT Up to 36V Minimizes CIN and COUT, Two Outputs 4V VIN 36V, IOUT Up to 20A Wide Input Range 3.5V VIN 36V, 0.8V VOUT 6V OPTI-LOOPTM Compensation Minimizes COUT Tiny Design, 550kHz, 2.5V VIN 9.8V, IOUT Up to 4.5A IOUT Up to 42A with Single Controller, Minimizes CIN and COUT, Up to 200A Out No RSENSE and OPTI-LOOP are trademarks of Linear Technology Corporation. 20 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com U Dimensions in inches (millimeters) unless otherwise noted. S Package 16-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.386 - 0.394* (9.804 - 10.008) 16 15 14 13 12 11 10 9 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 1 0.053 - 0.069 (1.346 - 1.752) 2 3 4 5 6 7 8 0.004 - 0.010 (0.101 - 0.254) 0.014 - 0.019 (0.355 - 0.483) TYP 0.050 (1.270) BSC S16 1098 1266fa LT/TP 1000 2K REV A * PRINTED IN USA (c) LINEAR TECHNOLOGY CORPORATION 1995 |
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