LTC2927 Single Power Supply Tracking Controller DESCRIPTIO U FEATURES n n n n n n n n n The LTC®2927 provides a simple solution to power supply tracking and sequencing requirements. By selecting a few resistors, the supply can be configured to ramp-up and ramp-down with differing ramp rates, voltage offsets, or time delays relative to other supplies or a master signal. Flexible Power Supply Tracking Tracks Both Up and Down Power Supply Sequencing Supply Stability is Not Affected Low Pin Count Controls Single Supply without Series FETs Adjustable Ramp Rate Supply Shutdown Output Available in 8-Lead ThinSOT™ and 8-Lead (3mm × 2mm) DFN Packages By forcing current into a feedback node of an independent supply, the LTC2927 causes the output to track a ramp signal without inserting any pass element losses. Because the current is controlled in an open-loop manner, the LTC2927 does not affect the transient response or stability of the supply. The compact solution at point of load minimizes the trace length of the DC/DC circuit sensitive FB node. Furthermore, it presents a high impedance when power-up is complete, effectively removing it from the DC/DC circuit. U APPLICATIO S n n n n VCORE and VI/O Supply Tracking Microprocessor, DSP and FPGA Supplies Multiple Supply Systems Point-of-Load Supplies , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patents Pending. U TYPICAL APPLICATIO Track-Up and Track-Down Waveforms EARLY VIN 3.3V 0.1μF 138k VCC ON 100k RAMP SDO RUN/SS RAMP (3.3V) IN 2.5V 1.8V 1V/DIV DC/DC RAMPBUF FB 16.5k VIN 0.1μF LTC2927 FB = 1.235V OUT 1.8V TRACK GND 13k 35.7k 16.5k 10ms/DIV EARLY VIN 3.3V 2927 TA01b 0.1μF VCC ON RAMP VIN RAMP (3.3V) IN 2.5V 1.8V LTC2927 SDO RUN/SS RAMPBUF FB 887k 1V/DIV DC/DC FB = 0.8V OUT 2.5V TRACK 412k GND 412k 2927 TA01a 887k 10ms/DIV 2927 TA01c 2927fb 1 LTC2927 U W W W ABSOLUTE AXI U RATI GS (Note 1) Supply Voltage (VCC) ................................. –0.3V to 10V Input Voltages ON ......................................................... –0.3V to 10V TRACK .........................................–0.3V to VCC + 0.3V Output Voltages FB, SDO ................................................. –0.3V to 10V RAMP, RAMPBUF .........................–0.3V to VCC + 0.3V Average Current TRACK .................................................................5mA FB ........................................................................5mA RAMPBUF ............................................................5mA Operating Temperature Range LTC2927C ................................................ 0°C to 70°C LTC2927I.............................................. –40°C to 85°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C PIN CONFIGURATION TOP VIEW TOP VIEW ON 1 RAMP 2 RAMPBUF 3 TRACK 4 8 VCC 9 VCC 1 SDO 2 FB 3 GND 4 7 SDO 6 FB 5 GND 8 ON 7 RAMP 6 RAMPBUF 5 TRACK TS8 PACKAGE 8-LEAD PLASTIC TSOT-23 TJMAX = 125°C, θJA = 250°C/W DDB PACKAGE 8-LEAD (3mm × 2mm) PLASTIC DFN EXPOSED PAD (PIN 9) PCB GND, CONNECTION OPTIONAL TJMAX = 125°C, θJA = 76°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2927CDDB#PBF LTC2927CDDB#TRPBF LBQH 8-Lead (3mm × 2mm) Plastic DFN 0°C to 70°C LTC2927IDDB#PBF LTC2927IDDB#TRPBF LBQH 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C LTC2927CTS8#PBF LTC2927CTS8#TRPBF LTBQJ 8-Lead Plastic TSOT-23 0°C to 70°C LTC2927ITS8#PBF LTC2927ITS8#TRPBF LTBQJ 8-Lead Plastic TSOT-23 –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2927CDDB LTC2927CDDB#TR LBQH 8-Lead (3mm × 2mm) Plastic DFN 0°C to 70°C LTC2927IDDB LTC2927IDDB#TR LBQH 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C LTC2927CTS8 LTC2927CTS8#TR LTBQJ 8-Lead Plastic TSOT-23 0°C to 70°C LTC2927ITS8 LTC2927ITS8#TR LTBQJ 8-Lead Plastic TSOT-23 –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. 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/ 2927fb 2 LTC2927 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. 2.9V < VCC < 5.5V unless otherwise noted (Note 2). SYMBOL PARAMETER VCC Supply Voltage ICC Supply Current CONDITIONS VCC(UVLO) Supply Undervoltage Lockout ΔVCC(UVHYST) Supply Undervoltage Lockout Hysteresis VON(TH) ON Pin Threshold Voltage ΔVON(HYST) ON Pin Hysteresis ION ON Pin Input Current IRAMP RAMP Pin Input Current MIN TYP MAX UNITS l 2.9 5.5 V IFB = 0mA, ITRACK = 0mA l 0.25 0.56 1.2 mA IFB = –1mA, ITRACK = –1mA, IRAMPBUF = –1mA l 3 3.6 4.2 mA VCC Rising l 2.2 2.5 2.7 V VON Rising l 1.210 1.230 1.250 l 30 75 150 mV 0 ±100 nA 25 l VON = 1.2V, VCC = 5.5V mV V 0V < VRAMP < VCC, Ramp On –9 –10 –11 μA 0V < VRAMP < VCC, Ramp Off 9 10 11 μA VRAMPBUF(OL) RAMPBUF Output Low Voltage IRAMPBUF = 1mA l 20 100 mV VRAMPBUF(OH) RAMPBUF Output High Voltage, VRAMPBUF(OH) = VCC – VRAMPBUF IRAMPBUF = –1mA l 45 150 mV VOS Ramp Buffer Offset, VOS = VRAMPBUF – VRAMP VRAMP = VCC/2, IRAMPBUF = 0mA 0 30 mV IFB to ITRACK Current Mismatch ITRACK = –10μA l 0 ±5 % IERROR(%) = (IFB – ITRACK)/ITRACK ITRACK = –1mA l 0 ±5 % IERROR(%) TRACK Pin Voltage VTRACK –30 ITRACK = –10μA l 0.77 0.800 0.82 V ITRACK = –1mA l 0.77 0.800 0.82 V VFB = 2V, VCC = 5.5V l ±1 ±100 nA IFB(LEAK) FB Pin Leakage Current VFB(CLAMP) FB Pin Clamp Voltage 1μA < IFB < 1mA l VSDO(OL) SDO Output Low Voltage ISDO = 1mA, VCC = 2.3V l Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. 1.5 2 2.3 V 0.1 0.4 V Note 2: All currents into the device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. TYPICAL PERFORMANCE CHARACTERISTICS ITRACK = IFB = 0mA IRAMBUF = 0mA 700 4.65 650 4.60 ICC (mA) ICC (μA) ICC vs VCC 4.70 600 4.55 550 4.50 500 2.5 3.0 3.5 4.0 4.5 VCC (V) 5.0 5.5 6.0 2927 G01 4.45 VTRACK vs Temperature 820 ITRACK = IFB = –1mA IRAMBUF = –2mA 810 VTRACK (mV) ICC vs VCC 750 800 790 780 2.5 3.0 3.5 4.0 4.5 VCC (V) 5.0 5.5 6.0 2927 G02 770 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 2927 G03 2927fb 3 LTC2927 U W TYPICAL PERFOR A CE CHARACTERISTICS VON(TH) vs Temperature VRAMPBUF(OL) vs Temperature 1.240 VRAMPBUF(OH) vs Temperature 70 28 26 1.235 65 VON(TH) (V) 1.230 1.225 1.220 VRAMPBUF(OH) (mV) VRAMPBUF(OL) (mV) 24 22 20 18 16 60 55 50 14 1.215 45 12 1.210 –50 –25 0 25 50 TEMPERATURE (°C) 75 10 –50 100 VTRACK = 0V ERROR = 40 –50 100 30 20 3.0 3.5 4.0 4.5 VCC (V) 5.0 5.5 6.0 VTRACK I • FB –1 0.8V ITRACK 100 2927 G06 0.8 3 2 0 75 VSDO(OL) vs VCC 1 2.5 0 25 50 TEMPERATURE (°C) 1.0 VSDO(OL) (V) 40 –25 2927 G05 4 ERROR (%) ITRACK (mA) 75 Tracking Cell Error vs ITRACK 5 50 10 0 25 50 TEMPERATURE (°C) 2927 G04 MAX ITRACK vs VCC 60 –25 0.6 ISDO = 5mA 0.4 0.2 1 2 2927 G07 3 ITRACK (mA) 4 5 2927 G0 0.0 ISDO = 10μA 0 1 2 3 VCC (V) 4 5 2927 G0 PIN FUNCTIONS TSOT/DFN Packages VCC (Pin 1/Pin 8): Supply Voltage Input. Operating range is from 2.9V to 5.5V. An undervoltage lockout asserts SDO until VCC > 2.5V. VCC should be bypassed to GND with a 0.1μF capacitor. SDO (Pin 2/Pin 7): Slave Supply Shutdown Output. SDO is an open-drain output that holds the shutdown (RUN/SS) pin of the slave supply low until the VCC pin is pulled above 2.5V, and the ON pin is pulled above 1.23V, or RAMP is above 200mV. SDO is pulled low again when both RAMP < 200mV and ON < 1.23V. If the slave supply is capable of operating with an input supply that is lower than the LTC2927’s minimum operating voltage of 2.9V, the SDO pin can be used to hold off the slave supply. Tie the SDO pin to GND if unused. FB (Pin 3/Pin 6): Feedback Control Output. FB pulls up on the feedback node of the slave supply. Tracking is achieved by mirroring the current from TRACK into FB. A resistive divider connecting RAMPBUF and TRACK will force the output voltage of the slave supply to track RAMP. To prevent damage to the slave supply, the FB pin will not force the slave’s feedback node above 2.3V. In addition, the LTC2927 will not actively sink current from this node, even when it is unpowered. 2927fb 4 LTC2927 U U U PI FU CTIO S TSOT/DFN Packages GND (Pin 4/Pin 5): Device Ground. RAMP (Pin 7/Pin 2): Ramp Buffer Input. The RAMP pin is the input to the voltage buffer whose output drives a resistive voltage divider connected to the TRACK pin. Connect this input to a capacitor to set the ramp voltage generated from internal 10μA pull-up or pull-down currents. RAMP can also be connected to an external ramping signal for tracking. Ground RAMP if unused. TRACK (Pin 5/Pin 4): Tracking Control Input. A resistive voltage divider between RAMPBUF and TRACK determines the tracking profile of the slave supply. TRACK servos to 0.8V, and the current supplied at TRACK is mirrored at FB. The TRACK pin is capable of supplying at least 1mA when VCC = 2.9V. Under short circuit conditions, the TRACK pin is capable of supplying up to 70mA. Do not connect to GND for extended periods. Limit the capacitance at the TRACK pin to less than 25pF. ON (Pin 8/Pin 1): On Control Input. The voltage level of the ON pin relative to its 1.23V threshold (with 75mV hysteresis) controls the tracking direction of the LTC2927. An active high causes a 10μA pull-up current to flow at the RAMP pin, which charges an external capacitor. An active low at the ON pin causes a 10μA pull-down current at the RAMP pin to discharge the external capacitor relative to GND. RAMPBUF (Pin 6/Pin 3): Ramp Buffer Output. Provides a low impedance buffered version of the signal on the RAMP pin. This buffered output drives the resistive voltage divider that connects to the TRACK pin. Limit the capacitance at the RAMPBUF pin to less than 100pF. Float RAMPBUF if unused. Exposed Pad (NA/Pin 9): Exposed pad may be left open or connected to device ground. W FU CTIO AL BLOCK DIAGRA U 5 5 VCC RAMPBUF RAMP 1x 5 VCC 5 ON 1.23V 10μA + – 10μA – + 2.5V 0.2V SDO 5 + UVLO VCC – VCC + 0.8V – 5 TRACK FB 5 GND 5 2927 BD 2927fb 5 U LTC2927 U W U U APPLICATIO S I FOR ATIO Power Supply Tracking and Sequencing The LTC2927 handles a variety of power-up profiles to satisfy the requirements of digital logic circuits including FPGAs, PLDs, DSPs and microprocessors. These requirements fall into one of the four general categories illustrated in Figures 1 to 4. Some applications require that the potential difference between two power supplies must never exceed a specified voltage. This requirement applies during power-up and power-down as well as during steady-state operation, often to prevent destructive latch-up in a dual supply ASIC. Typically, this is achieved by ramping the supplies up and down together (Figure 1). In other applications it is desirable to have supplies ramp up and down with fixed voltage offsets between them (Figure 2) or to have them ramp up and down ratiometrically (Figure 3). Certain applications require one supply to come up after another. For example, a system clock may need to start before a block of logic. In this case, the supplies are sequenced as in Figure 4 where the 2.5V supply ramps up after the 1.8V supply is completely powered. Operation The LTC2927 provides a simple solution to all of the power supply tracking and sequencing profiles shown in Figures 1 to 4. A single LTC2927 controls a single supply that tracks to a “master” signal. With two resistors, a slave supply is configured to ramp up as a function of the master signal. This master signal can be a separate supply or it can be a ramp signal generated by tying the RAMP pin to an external capacitor. Tracking Cell The LTC2927’s operation is based on the tracking cell shown in Figure 5, which uses a proprietary wide-range current mirror. The tracking cell shown in Figure 5 servos the TRACK pin at 0.8V. The current supplied by the TRACK MASTER MASTER SLAVE1 SLAVE1 1V/DIV SLAVE2 10ms/DIV 1V/DIV 2927 F01 SLAVE2 10ms/DIV Figure 1. Coincident Tracking 2927 F02 Figure 2. Offset Tracking MASTER MASTER SLAVE1 SLAVE2 1V/DIV 10ms/DIV Figure 3. Ratiometric Tracking 2927 F03 SLAVE1 1V/DIV SLAVE2 10ms/DIV 2927 F04 Figure 4. Supply Sequencing 2927fb 6 LTC2927 U U W U APPLICATIO S I FOR ATIO pin is mirrored at the FB pin to establish a voltage at the output of the slave supply. The slave output voltage varies with the master signal, enabling the slave supply to be controlled as a function of the master signal with terms set by RTA and RTB. By selecting appropriate values of RTA and RTB, it is possible to generate any of the profiles in Figures 1 to 4. Controlling the Ramp-Up and Ramp-Down Behavior The operation of the LTC2927 is most easily understood by referring to the simplified functional diagram in Figure 6. When the ON pin is low, the master signal at the RAMP pin is pulled to ground. Since the current through RTB is at its maximum when the master signal is low, the current from FB is also at its maximum. This current drives the slave output to its minimum voltage. When the ON pin rises above 1.23V, the master signal rises and the slave supply tracks the master signal. The ramp rate is set by an external capacitor driven by a 10μA current source at the RAMP pin. Alternatively, the RAMP pin can be connected to a separate supply to be used as the master signal. In a properly designed system, when the master signal has reached its maximum voltage the current from the TRACK pin is zero. In this case, there is no current from the FB pin and the LTC2927 has no effect on the output voltage accuracy, transient response or stability of the slave supply. When the ON pin falls below VON(TH) – ΔVON(HYST), typically 1.225V, the RAMP pin pulls down with 10μA and the master signal and slave supplies will fall at the same rate as they rose previously. The ON pin can be controlled by a digital I/O pin or it can be used to monitor an input supply. By connecting a resistive divider from an input supply to the ON pin, the supplies will ramp up only after the monitored supply has reached a preset voltage. If a resistive divider is used to set the ON pin voltage, choose values that will keep this voltage above the maximum ON pin threshold voltage of 1.25V at the lowest operating supply level. The Ramp Buffer The RAMPBUF pin provides a buffered version of the RAMP pin voltage that drives the resistive divider on the TRACK pin. The buffered master signal provides up to 2mA to drive the resistors. Shutdown Output In some applications it might be necessary to control the shutdown or RUN/SS pins of the slave supplies. The LTC2927 may not be able to supply the rated 1mA of current from the FB pin when VCC is below 2.9V. If the slave power supply is capable of operating at low input voltages, use the open-drain SDO output to drive the SHDN or RUN/SS pin of the slave supply (see Figure 7). This will hold the slave supply output low until the ON pin is above 1.23V and VCC is above the 2.5V undervoltage lockout condition. VCC 5 RONB 5 10μA ON + RONA 1.2V – 10μA 5 RAMPBUF + + – MASTER 0.8V 0.8V – – RTB DC/DC TRACK MASTER VCC + 5 5 CRAMP VCC RTB RAMP 1x FB 5 FB OUT 5 SLAVE TRACK FB 5 DC/DC SLAVE RTA RTA 2927 F05 RFA Figure 5. Simplified Tracking Cell RFB 5 2927 F06 RFA RFB Figure 6. Simplified Functional Diagram 2927fb 7 LTC2927 U U W U APPLICATIO S I FOR ATIO EARLY VIN 3.3V Choose a ramp rate for the slave supply, SS. If the slave supply ramps up coincident with the master signal or with a fixed voltage offset, then the ramp rate equals the master supply’s ramp rate. Be sure to use a fast enough ramp rate for the slave supply so that it will finish ramping before the master signal has reached its final supply value. If not, the slave supply will be held below the intended regulation value by the master signal. Use the following formulas to determine the resistor values for the desired ramp rate, where RFB and RFA are the feedback resistors in the slave supply and VFB is the feedback reference voltage of the slave supply: 0.1μF RONB 138k VCC ON RAMP RONA 100k MASTER CRAMP 10pF LTC2927 SDO RUN/SS IN DC/DC RAMPBUF RTB 16.5k VIN FB FB = 1.235V OUT 1.8V TRACK RTA 13k GND 2927 F07 RFA 35.7k RFB 16.5k Figure 7. SDO Shutdown Application SDO pulls low again when the ON pin is pulled below 1.23V and the RAMP pin is below about 200mV. RTB = RFB • SM SS 3-Step Design Procedure RTA′ = VTRACK V V + FB − TRACK RFA RTB The following 3-step procedure allows one to complete a design for any of the tracking or sequencing profiles shown in Figures 1 to 4. A basic single supply application circuit is shown in Figure 8. 1. Set the ramp rate of the master signal. Solve for the value of CRAMP, the capacitor on the RAMP pin, based on the desired ramp rate (V/s) of the master supply, SM. C RAMP = IRAMP where IRAMP ≈ 10μA SM (1) 2. Solve for the pair of resistors that provide the desired ramp rate of the slave supply, assuming no delay. EARLY VIN RONB VCC ON RAMP LTC2927 VIN 3. Choose RTA to obtain the desired delay. If no delay is required, such as in coincident and ratiometric tracking, then simply set RTA = RTA’. If a delay is desired, as in offset tracking and supply sequencing, calculate RTA” to determine the value of RTA where tD is the desired delay in seconds. FB RTB FB OUT TRACK GND RFA VTRACK • RTB tD • SM (4) (5) the parallel combination of RTA’ and RTA”. IN DC/DC RAMPBUF RTA Note that large ratios of slave ramp rate to master ramp rate, SS/SM, may result in negative values for RTA’. If sufficiently large delay is used in step 3, RTA will be positive, otherwise SS/SM must be reduced. RTA = RTA′ || RTA ″ MASTER CRAMP RONA (3) where VTRACK ≈ 0.8V. RTA ″ = 0.1μF VFB RFB (2) RFB 2927 F08 Figure 8. Single Supply Application SLAVE As noted in step 2, small delays and large ratios of slave ramp rate to master ramp rate (usually only seen in sequencing) may result in solutions with negative values for RTA. In such cases, either the delay must be increased or the ratio of slave ramp rate to master ramp rate must be reduced. 2927fb 8 LTC2927 U W U U APPLICATIO S I FOR ATIO Coincident Tracking Example MASTER SLAVE2 1V/DIV 1V/DIV SLAVE1 2927 F09 10ms/DIV 10ms/DIV Figure 9. Coincident Tracking (from Figure 10) A typical application is shown in Figure 10. The master signal is a 3.3V ramp generated by the LTC2927. The slave 1 supply is a 1.8V switching power supply and the slave 2 supply is a 2.5V switching power supply. Both slave supplies track coincidently with the 3.3V ramping master signal. The ramp rate of the supplies is 100V/s. The 3-step design procedure detailed previously can be used to determine component values. Only the slave 1 supply is considered here as the procedure is the same for the slave 2 supply. 1. Set the ramp rate of the master signal. From Equation 1: C RAMP In this example, the supply remains low while the ON pin is held below 1.23V. When the ON pin rises above 1.23V, 10μA pulls up the master signal on CRAMP at 100V/s. The master signal is buffered from the RAMP pin to the RAMPBUF pin. As this output and the RAMPBUF pin rise, the current from the TRACK pin is reduced. Consequently, the voltage at the slave supply’s output is increased, and the slave supply tracks the master signal. When the ON pin is again pulled below 1.23V, 10μA will pull down CRAMP at 100V/s. If the loads on the outputs are sufficient, all outputs will track down coincidently at 100V/s. EARLY VIN 3.3V 10μA = = 0.1μF 100V / s ON 0.1μF LTC2927 SDO RTB1 16.5k RUN/SS 3. Choose RTA to obtain desired delay. Since no delay is desired, RTA = RTA’ IN DC/DC FB FB = 1.235V RTA1 13k RFA1 35.7k GND EARLY 3.3V OUT SLAVE1 1.8V OUT SLAVE2 2.5V RFB1 16.5k 0.1μF VCC ON RAMP 3.3V LTC2927 0.8V ≈ 13kΩ 1.235V 1.235V 0.8V + − 16.5kΩ 35.7kΩ 16.5kΩ 3.3V TRACK From Equation 3: RTA′ = MASTER 3.3V RAMP RAMPBUF From Equation 2: 100V / s = 16.5kΩ 100V / s VCC RONA 100k 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. RTB = 16.5kΩ • 0.1μF RONB 138k SDO RUN/SS RTB2 887k IN DC/DC RAMPBUF FB FB = 0.8V TRACK RTA2 412k GND 2927 F10 RFA2 412k RFB2 887k Figure 10. Coincident Tracking Example 2927fb 9 LTC2927 U W U U APPLICATIO S I FOR ATIO Ratiometric Tracking Example MASTER SLAVE2 SLAVE1 1V/DIV 1V/DIV 10ms/DIV 2927 F11 10ms/DIV Figure 11. Ratiometric Tracking (from Figure 12) This example converts the coincident tracking example to the ratiometric tracking profile shown in Figure 11. The ramp rate of the master signal remains unchanged (Step 1) and there is no delay in ratiometric tracking (Step 3), so only the result of step 2 in the 3-step design procedure needs to be considered. In this example, the ramp rate of the 1.8V slave 1 supply ramps up at 60V/s and the 2.5V slave 2 supply ramps up at 85V/s. Always verify that the chosen ramp rate will allow the supplies to ramp-up completely before RAMPBUF reaches VCC. If the 1.8V supply were to ramp-up at 50V/s it would only reach 1.65V because the RAMPBUF signal would reach its final value of VCC = 3.3V before the slave supply reached 1.8V. EARLY VIN 3.3V VCC ON RONA 100k MASTER 3.3V RAMP 0.1μF LTC2927 SDO RTB1 27.4k 3.3V RUN/SS IN DC/DC RAMPBUF FB FB = 1.235V OUT SLAVE1 1.8V OUT SLAVE2 2.5V TRACK RTA1 10k RFA1 35.7k GND EARLY 3.3V RFB1 16.5k 0.1μF VCC ON RAMP 3.3V LTC2927 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: 0.1μF RONB 138k SDO RUN/SS RTB2 1M IN DC/DC RAMPBUF FB FB = 0.8V TRACK RTB 100V / s = 16.5kΩ • = 27.4kΩ 60V / s From Equation 3: RTA′ = RTA2 383k GND 2927 F12 RFA2 412k RFB2 887k Figure 12. Ratiometric Tracking Example 0.8V ≈ 10kΩ 1.235V 1.235V 0.8V + − 16.5kΩ 35.7kΩ 27.4kΩ Step 3 is unnecessary because there is no delay, so RTA = RTA’ 2927fb 10 LTC2927 U W U U APPLICATIO S I FOR ATIO Offset Tracking Example MASTER SLAVE2 1V/DIV 1V/DIV SLAVE1 10ms/DIV 2927 F13 10ms/DIV Figure 13. Offset Tracking (from Figure 14) Converting the circuit in the coincident tracking example to the offset tracking shown in Figure 13 is relatively simple. Here the 1.8V slave 1 supply ramps up 1V below the master. The ramp rate remains the same (100V/s), so there are no changes necessary to steps 1 and 2 of the 3-step design procedure. Only step 3 must be considered. Be sure to verify that the chosen voltage offset will allow the slave supply to ramp up completely. In this example, if the voltage offset were 2V, the slave supply would only ramp to 3.3V – 2V = 1.3V. EARLY VIN 3.3V 0.1μF RONB 138k VCC ON RONA 100k RAMP 0.1μF LTC2927 SDO FB RTA1 6.65k RTA ″ = 0.8V • 16.5kΩ = 13.2kΩ 10ms • 100V / s FB = 1.235V RFA1 35.7k GND OUT SLAVE1 1.8V OUT SLAVE2 2.5V RFB1 16.5k 0.1μF VCC ON RAMP 3.3V LTC2927 SDO From Equation 4: IN TRACK First, convert the desired voltage offset, VOS, to a delay tD, using the ramp rate: 1V V t D = OS = = 10ms S S 100 V / s MASTER 3.3V DC/DC EARLY 3.3V 3. Choose RTA to obtain desired delay. RUN/SS RAMPBUF RTB1 16.5k 3.3V (6) RUN/SS RTB2 887k IN DC/DC RAMPBUF FB FB = 0.8V TRACK RTA2 316k GND 2927 F14 RFA2 412k RFB2 887k Figure 14. Offset Tracking Example From Equation 5: RTA = 13.1kΩ 13.2kΩ ≈ 6.65kΩ 2927fb 11 LTC2927 U W U U APPLICATIO S I FOR ATIO Supply Sequencing Example MASTER SLAVE2 1V/DIV 1V/DIV SLAVE1 10ms/DIV 2927 F15 10ms/DIV Figure 15. Supply Sequencing (from Figure 16) In Figure 15, the slave 1 supply and the slave 2 supply are sequenced instead of tracking. The 3.3V master ramps up at 100V/s. The 1.8V slave 1 supply ramps up at 1000V/s beginning 10ms after the master signal starts to ramp up. The 2.5V slave 2 supply ramps up at 1000V/s beginning 25ms after the master signal begins to ramp up. Note that not every combination of ramp rates and delays is possible. Small delays and large ratios of slave ramp rate to master ramp rate may result in solutions that require negative resistors. In such cases, either the delay must be increased or the ratio of slave ramp rate to master ramp rate must be reduced. In this example, solving for the slave supply yields: 3. Choose RTA to obtain desired delay. From Equation 4: RTA ″ = From Equation 5: RTA = −2.13kΩ 1.32kΩ = 3.48kΩ EARLY VIN 3.3V VCC ON RTA′ = 0.8V = −2.13kΩ 1.235V 1.235V 0.8V + − 16.5kΩ 35.7kΩ 1.65kΩ IN DC/DC FB FB = 1.235V OUT SLAVE1 1.8V OUT SLAVE2 2.5V TRACK RTA1 3.48k RFA1 35.7k GND EARLY 3.3V RFB1 16.5k 0.1μF VCC 100V / s = 16.5kΩ • = 1.65kΩ 1000V / s From Equation 3: 3.3V RUN/SS RAMPBUF ON RTB 0.1μF LTC2927 RTB1 1.65k 10μA = 0.1μF 100V / s MASTER 3.3V RAMP SDO From Equation 1: 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. 0.1μF RONB 138k RONA 100k 1. Set the ramp rate of the master signal. C RAMP = 0.8V • 1.65kΩ = 1.32kΩ 10ms • 100V / s RAMP 3.3V LTC2927 SDO RUN/SS RTB2 88.7k IN DC/DC RAMPBUF FB FB = 0.8V TRACK RTA2 36.5k GND 2927 F16 RFA2 412k RFB2 887k Figure 16. Supply Sequencing Example 2927fb 12 LTC2927 U W U U APPLICATIO S I FOR ATIO Final Sanity Checks The collection of equations below is useful for identifying unrealizable solutions. As stated in step 2, the slave supply must finish ramping before the master signal has reached its final voltage. This can be verified by the following equation: ⎛ R ⎞ VTRACK ⎜ 1+ TB ⎟ < VMASTER ⎝ RTA ⎠ Here, VTRACK = 0.8V. VMASTER is the final voltage of the master signal (VCC if RAMP pin). It is possible to choose resistor values that require the LTC2927 to supply more current than the Electrical Characteristics table guarantees. To avoid this condition, check that ITRACK does not exceed 1mA and IRAMPBUF does not exceed ±2mA. To confirm that ITRACK < 1mA, the TRACK pin’s maximum guaranteed current, verify that: VTRACK < 1mA RTA RTB Finally, check that the RAMPBUF pin will not be forced to sink more than 2mA when it is at 0V or be forced to source more than 2mA when it is at VMASTER. VTRACK V < 2mA and MASTER < 2mA RTB RTA + RTB RONB 2.9V < –1 RONA 1.21V For example, if the typical application shown on page 1 has a 3.3V ±10% VIN, the lowest possible operating supply voltage will be 2.97. RONB 2.97V > – 1 = 1.376 RONA 1.25V If RONA is 100k then RONB must be greater than 137.6k. Therefore, 138k is chosen. These values must be checked to ensure the supply reaches the LTC2927 minimum operating supply voltage of 2.9V before the ON pin is above the minimum threshold. 1.38 < 2.9V – 1= 1.389 1.21V Load Requirements When the supply is ramped down quickly, either the load or the supply itself must be capable of sinking enough current to support the ramp rate. For example, if there is a large output capacitance on the supply and a weak resistive load, supplies that do not sink current will have their falling ramp rate limited by the RC time constant of the load and the output capacitance. Figure 17 shows the case when the slave supply does not track the master near ground. ON Pin Resistive Divider MASTER Check that the ON pin voltage is above the 1.25V maximum threshold at the lowest possible supply voltage value. RONB VCC(MIN) < –1 RONA 1.25V Also check that the supply voltage is above the minimum LTC2927 operating supply voltage of 2.9V before the ON pin is above the 1.21V minimum threshold voltage. 1V/DIV SLAVE 10ms/DIV 2927 F17 Figure 17. Weak Resistive Load 2927fb 13 LTC2927 U W U U APPLICATIO S I FOR ATIO Start-Up Delays Layout Considerations Often power supplies do not start-up immediately when their input supplies are applied. If the LTC2927 tries to ramp-up these power supplies as soon as the input supply is present, the start-up of the outputs may be delayed, defeating the tracking circuit (Figure 18). Often this delay is intentionally configured by a soft-start capacitor. This can be remedied either by reducing the soft-start capacitor on the slave supply or by including a capacitor in the ON pin’s resistive divider to delay the ramp up. See Figure 19. Be sure to place a 0.1μF bypass capacitor as near as possible to the supply pin of the LTC2927. To minimize the noise on the slave supply’s output, keep the trace connecting the FB pin of the LTC2927 and the feedback node of the slave supply as short as possible. In addition, do not route this trace next to signals with fast transition times. In some circumstances it might be advantageous to add a resistor near the feedback node of the slave supply in series with the FB pin of the LTC2927. This resistor must not exceed: MASTER RSERIES = SLAVE 1V/DIV ON 20ms/DIV 2927 F18 1.5V − VFB ⎛ 1.5V ⎞ =⎜ − 1⎟ RFA RFB IMAX ⎝ VFB ⎠ ( ) This resistor is most effective if there is already a capacitor at the feedback node of the slave supply (often a compensation component). Increasing the capacitance on a slave supply’s feedback node will further improve the noise immunity, but could affect the stability and transient response of the supply. Figure 18. Power Supply Start-Up Delayed VCC FB FB MASTER DC/DC RSERIES LTC2927 MINIMIZE TRACE LENGTH GND RFA OUT RFB 2927 F20 SLAVE 0.1μF 1V/DIV Figure 20. Layout Considerations ON 5ms/DIV 2927 F19 Figure 19. ON Pin Delayed 2927fb 14 LTC2927 U PACKAGE DESCRIPTIO DDB Package 8-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1702) 0.61 ±0.05 (2 SIDES) R = 0.115 TYP 5 0.56 ± 0.05 (2 SIDES) 3.00 ±0.10 (2 SIDES) 0.675 ±0.05 2.50 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 2.00 ±0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.25 ± 0.05 4 0.25 ± 0.05 0.75 ±0.05 0.200 REF 0.50 BSC 2.20 ±0.05 (2 SIDES) 0.38 ± 0.10 8 (DDB8) DFN 1103 0.50 BSC 0 – 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 1 PIN 1 CHAMFER OF EXPOSED PAD 2.15 ±0.05 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE TS8 Package 8-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1637) 0.52 MAX 2.90 BSC (NOTE 4) 0.65 REF 1.22 REF 1.4 MIN 3.85 MAX 2.62 REF 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.22 – 0.36 8 PLCS (NOTE 3) 0.65 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193 1.95 BSC TS8 TSOT-23 0802 2927fb 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. 15 LTC2927 U TYPICAL APPLICATIO Single Supply Application EARLY VIN 3.3V High Voltage Supply Application EARLY VIN 3.3V 0.1μF RONB 138k RONB 138k VCC ON RAMP RONA 100k SDO RONA 100k 3.3V RUN/SS FB FB = 1.235V MASTER CRAMP 0.4μF LTC2927 SDO 1.8V RTB 78.7k 3.3V RUN/SS IN DC/DC RAMPBUF OUT FB FB = 1.235V OUT SLAVE 12V TRACK TRACK RTA 13k RAMP IN DC/DC RAMPBUF VCC ON MASTER CRAMP 0.1μF LTC2927 RTB 16.5k 0.1μF GND 2927 TA02a RFB 16.5k RFA 35.7k RTA 28k GND 2927 TA02b RFA 35.7k RFB 311k RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC2920 Power Supply Margining Controller Single or Dual Versions, Symmetric as Symmetric High and Low Margining LTC2921/LTC2922 Power Supply Tracker with Input Monitors Includes 3 (LTC2921) or 5 (LTC2922) Remote Sense Switches LTC2923 Power Supply Tracking Controller Controls 3 Supplies for Tracking or Sequencing LTC2924 Quad Power Supply Sequencer Controls 4 Supplies for Sequencing LTC2925 Multiple Power Supply Tracking Controller with Power Good Timeout Controls 4 Supplies for Tracking or Sequencing LTC2926 MOSFET-Controlled Power Supply Tracker Active Tracking Control with Series MOSFETs LTC2928 Multichannel Power Supply Sequencer and Supervisor Programmable with External Components; No Software Required LT4220 Dual Supply Hot Swap™ Controller ±2.7V to ±16.5V, Supply Tracking Mode LTC4221 Dual Hot Swap Controller Operates from 1V to 13.5V, Allows Supply Sequencing, Active Current Limiting LTC4230 Triple Hot Swap Controller with Multifunction Current Control 1.7V to 16.5V, Active Inrush Limiting, Fast Comparator LTC4253 –48V Hot Swap Controller and Supply Sequencer Floating Supply from –15V, Active Current Limiting, Enables Three DC/DC Converters Hot Swap is a trademark of Linear Technology Corporation. 2927fb 16 Linear Technology Corporation LT 0308 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006