LTC2923 Power Supply Tracking Controller U FEATURES DESCRIPTIO ■ The LTC®2923 provides a simple solution to power supply tracking and sequencing requirements. By selecting a few resistors, the supplies can be configured to ramp-up and ramp-down together or with voltage offsets, time delays or different ramp rates. ■ ■ ■ ■ ■ ■ ■ ■ Flexible Power Supply Tracking Tracks Both Up and Down Power Supply Sequencing Supply Stability is Not Affected Controls Two Supplies Without Series FETs Controls a Third Supply With a Series FET Adjustable Ramp Rates Electronic Circuit Breaker Available in 10-Lead MS and 12-Lead (4mm × 3mm) DFN Packages By introducing currents into the feedback nodes of two independent switching regulators, the LTC2923 causes their outputs to track without inserting any pass element losses. Because the currents are controlled in an openloop manner, the LTC2923 does not affect the transient response or stability of the supplies. Furthermore, it presents a high impedance when power-up is complete, effectively removing it from the DC/DC circuit. U APPLICATIO S ■ ■ ■ ■ VCORE and VI/O Supply Tracking Microprocessor, DSP and FPGA Supplies Servers Communication Systems For systems that require a third supply, one supply can be controlled with a series FET. This optional series FET can also control a supply that does not allow direct access to its feedback resistors (e.g., a power module) or a supply whose output cannot be forced to ground (e.g., a 3-terminal linear regulator). When the FET is used, an electronic circuit breaker provides protection from short-circuit conditions. , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patents Pending. U TYPICAL APPLICATIO Q1 VIN 3.3V 3.3V 2.5V 1.8V 3.3V 1V/DIV 10nF VIN 138k VCC GATE ON 100k RAMP FB1 IN DC/DC FB = 1.235V 35.7k 2923 TA02 16.5k STATUS VIN 16.5k TRACK1 SDO TRACK2 FB2 887k 412k 1ms/DIV 1.8V LTC2923 RAMPBUF 13k OUT IN DC/DC FB = 0.8V 3.3V 2.5V 1.8V OUT 1V/DIV 2.5V GND 887k 412k 2923 TA01 1ms/DIV 2923 F08b 2923fa 1 LTC2923 W W W AXI U U ABSOLUTE RATI GS (Note 1) Supply Voltage (VCC) ................................ – 0.3V to 10V Input Voltages ON ........................................................ – 0.3V to 10V TRACK1, TRACK2 ...................... – 0.3V to VCC + 0.3V RAMP ........................................... – 0.3V to VCC + 1V Output Voltages FB1, FB2, SDO, STATUS ........................– 0.3V to 10V RAMPBUF ................................. – 0.3V to VCC + 0.3V GATE (Note 2) ................................... – 0.3V to 11.5V Average Current TRACK1, TRACK2 .............................................. 5mA FB1, FB2 ............................................................ 5mA RAMPBUF ......................................................... 5mA Operating Temperature Range LTC2923C ............................................... 0°C to 70°C LTC2923I ............................................ – 40°C to 85°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec) MS Package .................................................... 300°C U U W PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW VCC ON TRACK1 TRACK2 RAMPBUF 1 2 3 4 5 10 9 8 7 6 RAMP GATE FB1 FB2 GND MS PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125°C, θJA = 120°C/W LTC2923CMS LTC2923IMS VCC 1 12 RAMP ON 2 11 GATE TRACK1 3 LTC2923CDE LTC2923IDE 10 STATUS 13 MS PART MARKING LTAED LTAEE ORDER PART NUMBER TOP VIEW TRACK2 4 9 SDO RAMPBUF 5 8 FB1 GND 6 7 FB2 DE PART MARKING DE12 PACKAGE 12-LEAD (4mm × 3mm) PLASTIC DFN 2923 TJMAX = 125°C, θJA = 43°C/W EXPOSED PAD (PIN 13) INTERNALLY CONNECTED TO GND (PCB CONNECTION OPTIONAL) Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● 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 3). SYMBOL PARAMETER VCC Input Supply Range ICC Input Supply Current CONDITIONS MIN TYP MAX UNITS 5.5 V mA mA ● 2.9 IFBx = 0, ITRACKx = 0 IFBx = –1mA, ITRACKx = –1mA, IRAMPBUF = –2mA ● ● 5 1.3 7 3 10 VCC Rising ● 2.2 2.5 2.7 VCC(UVL) Input Supply Undervoltage Lockout ∆VCC(UVLHYST) Input Supply Undervoltage Lockout Hysteresis ∆VGATE External N-Channel Gate Drive (VGATE – VCC) IGATE = –1µA ● 5 5.5 6 IGATE GATE Pin Current Gate On, VGATE = 0V, No Faults Gate Off, VGATE = 5V, No Faults Gate Off, VGATE = 5V, Short-Circuit Fault ● ● ● –7 7 5 –10 10 20 –13 13 50 VON(TH) ON Pin Threshold Voltage VON Rising ● 1.212 1.230 1.248 ∆VON(HYST) ON Pin Hysteresis ● 30 75 150 25 V mV V µA µA mA V mV 2923fa 2 LTC2923 ELECTRICAL CHARACTERISTICS The ● 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 3). SYMBOL PARAMETER CONDITIONS VON(FC) ON Pin Fault Clear Threshold Voltage ION ON Pin Input Current ∆VDS(TH) FET Drain-Source Overcurrent Voltage Threshold (VCC – VRAMP) IRAMP RAMP Pin Input Current 0V < RAMP < VCC, VCC = 5.5V ● 0 ±1 µA VRAMPBUF(OL) RAMPBUF Low Voltage IRAMPBUF = 2mA ● 90 150 mV VRAMPBUF(OH) RAMPBUF High Voltage (VCC – VRAMPBUF) IRAMPBUF = –2mA ● 100 200 mV VOS Ramp Buffer Offset (VRAMPBUF – VRAMP) VRAMPBUF = VCC/2, IRAMPBUF = 0A 0 30 mV IERROR(%) IFBx to ITRACKx Current Mismatch IERROR(%) = (IFBx – ITRACKx)/ITRACKx ITRACKx = –10µA ITRACKx = –1mA ● ● 0 0 ±5 ±5 % % VTRACKx TRACK Pin Voltage ITRACKx = –10µA ITRACKx = –1mA ● ● 0.8 0.8 0.824 0.824 V V IFB(LEAK) IFB Leakage Current VFB = 1.5V, VCC = 5.5V ● ±1 ±100 nA VFB(CLAMP) VFB Clamp Voltage 1µA < IFB < 1mA ● 1.7 2 V VSDO(OL) SDO Output Low Voltage ISDO = 3mA ● 0.2 0.4 V VSTATUS(OL) STATUS Output Low Voltage ISTATUS = 3mA ● 0.2 0.4 V tPSC Short-Circuit Propagation Delay VDS High to GATE Low VDS = VCC, VCC = 2.9V 10 20 µs ● TYP MAX 0.3 0.4 0.5 0 ±100 nA 160 200 240 mV ● VON = 1.2V, VCC = 5.5V ● Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The GATE pin is internally limited to a minimum of 11.5V. Driving this pin to voltages beyond the clamp may damage the part. MIN –30 0.776 0.776 1.5 UNITS V Note 3: 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. U W TYPICAL PERFOR A CE CHARACTERISTICS Specifications are at TA = 25°C unless otherwise noted. VGATE vs VCC VGATE vs IGATE 12 IGATE vs VCC Fast Pull-Down 15 30 VGATE = 5V 25 VCC = 5.5V 11 10 IGATE (mA) 20 VGATE (V) VGATE (V) 10 VCC = 2.9V 5 15 10 9 5 0 8 2 3 4 VCC (V) 5 6 0 5 10 0 15 IGATE (µA) 2923 G01 2923 G02 0 1 2 3 VCC (V) 4 5 6 2923 G03 2923fa 3 LTC2923 U W TYPICAL PERFOR A CE CHARACTERISTICS Specifications are at TA = 25°C unless otherwise noted. IGATE Fast Pull-Down vs Temperature ICC vs VCC ITRACKx = IFBx = –1mA IRAMPBUF = –2mA ITRACKx = IFBx = 0mA IRAMPBUF = 0mA 35 VCC = 5.5V ICC vs VCC 8.0 1.5 40 1.4 7.9 1.3 7.8 ICC (mA) IGATE (mA) 25 20 VCC = 2.9V 15 10 ICC (mA) 30 1.2 7.7 7.6 1.1 5 –25 0 50 25 TEMPERATURE (°C) 75 1.0 2.5 100 3 3.5 4.5 4 5 VTRACKx vs Temperature VTRACKx (V) VRAMPBUF(OL) (mV) VCC = 5.5V ITRACKx = 10µA 0.795 0.790 VCC = 2.9V ITRACKx = 10µA 0.785 –50 –25 VCC = 2.9V ITRACKx = 1mA 75 0 25 50 TEMPERATURE (°C) 130 110 120 100 VCC = 2.9V 90 VCC = 5.5V 80 110 VCC = 2.9V 100 90 VCC = 5.5V 80 70 60 –50 100 –25 0 25 50 TEMPERATURE (°C) 75 60 –50 100 VSDO(OL) vs VCC 16 Tracking Cell Error vs ITRACKx VTRACKx = 0V ERROR = I VTRACKx • FBx – 1 0.8V ITRACKx 4 14 13 ERROR (%) ITRACKx (mA) VSDO(OL) (V) 0.8 100 5 15 ISDO = 5mA 75 2923 G09 ITRACKx vs VCC 0.2 50 25 0 TEMPERATURE (°C) –25 2923 G08 2923 G07 1.0 5.5 VRAMPBUF(OH) vs Temperature 120 70 0.6 5 2923 G06 VRAMPBUF(OL) vs Temperature VCC = 5.5V ITRACKx = 1mA 4.5 4 VCC (V) 2923 G05 0.810 0.800 3.5 3 VCC (V) 2923 G04 0.805 7.5 2.5 5.5 VRAMPBUF(OH) (mV) 0 –50 12 11 3 2 EXACTLY 2% 10 1 0.2 9 0 ISDO = 10µA 0 1 2 3 VCC (V) 4 5 2923 G11 8 3 3.5 4 4.5 VCC (V) 5 5.5 2923 G10 0 0 1 3 2 ITRACKx (mA) 4 5 2923 G12 2923fa 4 LTC2923 U U U PI FU CTIO S MS/DE Packages VCC (Pin 1): Positive Supply Input Pin. The operating supply input range is 2.9V to 5.5V. An undervoltage lockout circuit resets the part when the supply is below 2.5V. VCC should be bypassed to GND with a 0.1µF capacitor. ON (Pin 2): On Control Input. The ON pin has a threshold of 1.23V with 75mV of hysteresis. An active high will cause 10µA to flow from the GATE pin, ramping up the supplies. An active low pulls 10µA from the GATE pin, ramping the supplies down. Pulling the ON pin below 0.4V resets the electronic circuit breaker in the LTC2923. If a resistive divider connected to VCC drives the ON pin, the supplies will automatically start up when VCC is fully powered. TRACK1, TRACK2 (Pins 3, 4): Tracking Control Input. A resistive voltage divider between RAMPBUF and TRACKx determines the tracking profile of a slave supply (see Applications Information). TRACKx pulls up to 0.8V and the current supplied at TRACKx is mirrored at FBx. TRACKx is capable of supplying at least 1mA when VCC = 2.9V. Because a TRACKx pin is capable of supplying up to 30mA under short-circuit conditions, avoid connecting TRACKx to GND for extended periods. Limit the capacitance at each TRACKx pin to less than 25pF. Float the TRACKx pins if unused. RAMPBUF (Pin 5): Ramp Buffer Output. Provides a low impedance buffered version of the signal on the RAMP pin. This buffered output drives the resistive dividers that connect to the TRACKx pins. Limit the capacitance at the RAMPBUF pin to less than 100pF. GND (Pins 6, 13): Circuit Ground. FB1, FB2 (Pins 8, 7): Feedback Control Output. FBx pulls up on the feedback node of slave supplies. Tracking is achieved by mirroring the current from TRACKx into FBx. If the appropriate resistive divider connects RAMPBUF and TRACKx, the FBx current will force OUTx to track RAMP. To prevent damage to the slave supply, the FBx pin will not force the slave’s feedback node above 1.7V. In addition, it will not actively sink current from this node even when the LTC2923 is unpowered. Float the FB pins if unused. GATE (Pin 9/Pin 11): Gate Drive for External N-Channel FET. When the ON pin is high, an internal 10µA current source charges the gate of the external N-channel MOSFET. A capacitor connected from GATE to GND sets the ramp rate. An internal charge pump guarantees that GATE will pull up to 5V above VCC ensuring that logic level N-channel FETs are fully enhanced. When the ON pin is pulled low, the GATE pin is pulled to GND with a 10µA current source. Under a short-circuit condition, the electronic circuit breaker in the LTC2923 pulls the GATE low immediately with 20mA. Tie GATE to GND if unused. It is a good practice to add a 10Ω resistor between this capacitor and the FET’s gate to prevent high frequency FET oscillations. RAMP (Pin 10/Pin 12): Ramp Buffer Input. When the RAMP pin is connected to the source of the external N-channel FET, the slave supplies track the FET’s source as it ramps up and down. If the GATE is fully enhanced (GATE > RAMP + 4.9V) and (VCC – RAMP > 200mV) indicates a shorted output, then the electronic circuit breaker trips and GATE quickly pulls low with 20mA. The GATE will not ramp up again until ON is pulled below 0.4V and then above 1.23V. Alternatively, when no external FET is used, the RAMP pin can be tied directly to the GATE pin. In this configuration, the supplies track the capacitor on the GATE pin as it is charged and discharged by the 10µA current source controlled by the ON pin. RAMP must not be driven above VCC (except by the GATE pin). SDO (Pin 9, DE Package Only): Slave Supply Shutdown Output. SDO is an open-drain output that holds the shutdown (RUN/SS) pins of the slave supplies low until the ON pin is pulled above 1.23V. If the slave supply is capable of operating with an input supply that is lower than the LTC2923’s minimum operating voltage of 2.9V, the SDO pin can be used to hold off the slave supplies. SDO will be pulled low again when RAMP < 100mV and ON < 1.23V. STATUS (Pin 10, DE Package Only): Power Good Status Indicator. The STATUS pin is an open-drain output that pulls low until GATE has been fully charged at which time all supplies will have reached their final operating voltage. 2923fa 5 LTC2923 W FU CTIO AL BLOCK DIAGRA U U 1 VCC CHARGE PUMP 10µA 2 ON 1.2V 0.4V 10 + – – + GATE 11 (9) SHORT-CIRCUIT FAULT LATCH R Q S Q ONSIG 10µA STATUS GATE > RAMP + 4.9V 4.9V GATE RAMP > VCC VCC RAMP 12 (10) + – VCC – RAMP > 200mV 0.2V VCC RAMP < 100mV + – 9 SDO ONSIG VCC < 2.6V – + 5 RAMPBUF 0.1V VCC 2.6V 1× VCC 3 TRACK1 + – 0.8V FB1 8 VCC 4 TRACK2 + – 6 0.8V FB2 GND 7 2923 FBD PIN NUMBERS IN PARENTHESES CORRESPOND TO THE 10-LEAD MSOP PACKAGE 2923fa 6 LTC2923 U W U U APPLICATIO S I FOR ATIO Power Supply Tracking and Sequencing The LTC2923 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 the 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 LTC2923 provides a simple solution to all of the power supply tracking and sequencing profiles shown in Figures 1 to 4. A single LTC2923 controls up to three supplies with two “slave” supplies that track a “master” signal. With just two resistors, a slave supply is configured to ramp up as a function of the master signal. This master signal can be a third supply that is ramped up through an external FET, whose ramp rate is set with a single capacitor, or it can be a signal generated by tying the GATE and RAMP pins to an external capacitor. MASTER SLAVE1 SLAVE2 1V/DIV 1ms/DIV MASTER SLAVE1 SLAVE2 1V/DIV 1ms/DIV 2923 F01 Figure 1. Coincident Tracking 2923 F02 Figure 2. Offset Tracking MASTER SLAVE1 SLAVE1 1V/DIV SLAVE2 1ms/DIV Figure 3. Ratiometric Tracking 2923 F03 1V/DIV SLAVE2 1ms/DIV 2923 F04 Figure 4. Supply Sequencing 2923fa 7 LTC2923 U W U U APPLICATIO S I FOR ATIO Tracking Cell In a properly designed system, when the master signal has reached its maximum voltage the current from the TRACKx pin is zero. In this case, there is no current from the FBx pin and the LTC2923 has no effect on the output voltage accuracy, transient response or stability of the slave supply. The LTC2923’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 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. When the ON pin falls below VON(TH) – ∆VON(HYST), typically 1.225V, the GATE pin pulls down with 10µA and the master signal and the 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. Controlling the Ramp-Up and Ramp-Down Behavior The operation of the LTC2923 is most easily understood by referring to the simplified functional diagram in Figure 6. When the ON pin is low, the GATE pin is pulled to ground causing the master signal to remain low. Since the currents through RTB1 and RTB2 are at their maximum when the master signal is low, the currents from FB1 and FB2 are also at their maximum. These currents drive the slaves’ outputs to their minimum voltages. Optional External FET The Coincident Tracking Example (Figures 8 and 9) illustrates how an optional external N-channel FET can ramp up a single supply that becomes the master signal. When used, the FET’s gate is charged by the GATE pin and its source is tied to the RAMP pin. Under normal operation, the GATE pin sources or sinks 10µA to ramp the FET’s gate up or down at a rate set by the external capacitor connected to the GATE pin. It is a good practice to add 10Ω between the FET’s gate and the external capacitor to prevent high frequency oscillations. When the ON pin rises above 1.23V, the master signal rises and the slave supplies track the master signal. The ramp rate is set by an external capacitor driven by a 10µA current source from an internal charge pump. If no external FET is used, the ramp rate is set by tying the RAMP and GATE pins together at one terminal of the external capacitor (see the Ratiometric Tracking Example). VCC + + – MASTER 0.8V – RTB TRACK DC/DC FB FB OUT SLAVE RTA RFA RFB 2923 F05 Figure 5. Simplified Tracking Cell 2923fa 8 LTC2923 U W U U APPLICATIO S I FOR ATIO VCC RONB ON 10µA + GATE RONA 1.2V – CGATE 10µA VCC +– Q1 + 200mV – RAMPBUF RAMP 1× MASTER VCC + 0.8V – RTB1 FB1 TRACK1 VCC RTA1 DC/DC RFA1 + SLAVE1 RFB1 0.8V – RTB2 FB2 TRACK2 DC/DC SLAVE2 RTA2 RFA2 RFB2 2923 F06 Figure 6. Simplified Functional Diagram The LTC2923 features an electronic circuit breaker function that protects the optional series FET against short circuits. When the FET is fully enhanced (GATE > RAMP + 4.9V), the electronic circuit breaker is enabled. Then, if the voltage across the FET (VDS) exceeds 200mV as measured from VCC to the RAMP pin for more than about 10µs the gate of the FET is pulled down with 20mA, turning it off. Because the slaved supplies track the RAMP pin, they are pulled low by the tracking circuit when a short-circuit fault occurs. Following a short-circuit fault, the FET is latched off until the fault is cleared by pulling the ON pin below 0.4V. Ramp Buffer The RAMPBUF pin provides a buffered version of the RAMP pin voltage that drives the resistive dividers on the TRACKx pins. When there is no external FET, it provides up to 2mA to drive the resistors even though the GATE pin only supplies 10µA. The RAMPBUF pin also proves useful in systems with an external FET. Since the track cell in the simplified functional diagram above drives 0.8V on the TRACKx pins, if RTBx is connected directly to the FET’s source, the TRACKx pin could potentially pull up the FET’s source towards 0.8V when the FET is off. RAMPBUF blocks this path. 2923fa 9 LTC2923 U W U U APPLICATIO S I FOR ATIO Shutdown Output In some applications it might be necessary to control the shutdown or RUN/SS pins of the slave supplies using the 12-lead LTC2923CDE or LTC2923IDE. The LTC2923 may not be able to supply the rated 1mA of current from the FB1 and FB2 pins when VCC is below 2.9V. If the slave power supplies are capable of operating at low input voltages, use the open-drain SDO output to drive the SHDN or RUN/SS pins of the slave supplies (see Figure 7). This will hold the slave supplies’ outputs low until the ON pin is above 1.23V, VCC is above the 2.6V undervoltage lockout condition and there are no short-circuit faults latched. It pulls low again when the ON pin is pulled below 1.23V and the RAMP pin is below about 100mV. When two supplies Q1 VIN 3.3V must have their RUN/SS or SHDN pins controlled independently, tie a Schottky diode between each pin and the SDO output (see Figure 8). Status Output The STATUS pin provides an indication that the supplies are finished ramping up. This pin is an open-drain output that pulls low until the GATE has been fully charged. Since the GATE pin drives the gate of the external FET, or the RAMP pin directly when no FET is used, the supplies are completely ramped up when the GATE pin is fully charged. It will go low again when the GATE pin is pulled low, either because of a short-circuit fault or because the ON pin has been pulled low. Q1 VIN 3.3V 3.3V 3.3V 0.1µF 0.1µF 10Ω CGATE 10nF 10Ω VIN CGATE 10nF VIN RSTATUS 10k RONB 138k VCC GATE RAMP RONA 100k FB1 RONB 138k VIN STATUS ON RSTATUS 10k RUN/SS IN DC/DC FB = 1.235V VCC OUT RTA1 13k 1.8V RFA1 35.7k FB1 RFB1 16.5k SDO FB2 TRACK2 RTA2 412k RUN/SS IN DC/DC FB = 1.235V RUN/SS IN DC/DC FB = 0.8V OUT 2.5V RTA1 13k OUT 1.8V OUT 2.5V RFA2 412k RTB2 887k TRACK1 SDO FB2 TRACK2 RUN/SS IN DC/DC FB = 0.8V GND RFB2 887k RFA2 412k 2923 F07 Figure 7 RFB1 16.5k VIN RTA2 412k GND RFA1 35.7k RAMPBUF RTB1 16.5k VIN RTB2 887k VIN LTC2923 RAMPBUF TRACK1 RAMP STATUS RONA 100k LTC2923 RTB1 16.5k GATE ON RFB2 887k 2923 F08 Figure 8 2923fa 10 LTC2923 U W U U APPLICATIO S I FOR ATIO 3-Step Design Procedure 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 three supply application circuit is shown in Figure 9. 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. 3. Choose RTA to obtain the desired delay. 1. Set the ramp rate of the master signal. 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. Solve for the value of CGATE, the capacitor on the GATE pin, based on the desired ramp rate (V/s) of the master supply, SM. CGATE = IGATE where IGATE ≈ 10µA SM (1) RTA ′′ = If the external FET has a gate capacitance comparable to CGATE, then the external capacitor’s value should be reduced to compensate for the FET’s gate capacitance. If no external FET is used, tie the GATE and RAMP pins together. 2. Solve for the pair of resistors that provide the desired ramp rate of the slave supply, assuming no delay. Choose a ramp rate for the slave supply, SS. If the slave supply ramps up coincident with the master supply 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 supply has reached its final supply value. If not, the slave supply will be held below the intended regulation value by the master supply. 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: S RTB = RFB • M SS (4) RTA = RTA′||RTA′′ (5) the parallel combination of RTA′ and RTA′′ 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. Q1 VIN MASTER 0.1µF 10Ω CGATE VIN RONB VCC GATE ON RONA IN DC/DC RAMP FB1 FB OUT SLAVE1 OUT SLAVE2 LTC2923 RFA1 RAMPBUF RFB1 VIN RTB1 TRACK1 RTA1 (2) VTRACK • RTB tD • SM IN DC/DC RTB2 TRACK2 RTA2 FB2 FB GND RFB2 RTA ′ = VFB RFB VTRACK V V + FB – TRACK RFA RTB RFA2 (3) 2923 F09 Figure 9. Three Supply Application where VTRACK ≈ 0.8V. 2923fa 11 LTC2923 U W U U APPLICATIO S I FOR ATIO Coincident Tracking Example MASTER SLAVE2 SLAVE1 1V/DIV 1ms/DIV 1V/DIV 1ms/DIV 2923 F10a 2923 F10b Figure 10. Coincident Tracking (from Figure 11) A typical three supply application is shown in Figure 11. The master signal is a 3.3V module. 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 supply that is controlled with an external FET. The ramp rate of the supplies is 1000V/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. 10µA = 10nF 1000 V/s 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: RTB = 16.5kΩ • 1000 V/s = 16.5kΩ 1000 V/s From Equation 3: RTA ′ = 0.8 V ≈ 13kΩ 1.235V 1.235V 0.8 V + – 16.5kΩ 35.7kΩ 16.5kΩ 3. Choose RTA to obtain the desired delay. Since no delay is desired, RTA = RTA′ 12 3.3V MASTER 0.1µF 10Ω CGATE 10nF 3.3V RONB 138k RONA 100k RTB1 16.5k RTA1 13k VCC GATE ON RAMP FB1 RTB2 887k IN DC/DC FB = 1.235V OUT 1.8V SLAVE1 LTC2923 RFA1 RFB1 16.5k 35.7k 3.3V RAMPBUF TRACK1 FB2 IN DC/DC FB = 0.8V OUT TRACK2 RTA2 412k From Equation 1: CGATE = Q1 3.3V GND RFA2 412k 2.5V SLAVE2 RFB2 887k 2923 F11 Figure 11. Coincident Tracking Example In this example, all supplies remain low while the ON pin is held below 1.23V. When the ON pin rises above 1.23V, 10µA pulls up CGATE and the gate of the FET at 1000V/s. As the gate of the FET rises, the source follows and pulls up the output to 3.3V at 1000V/s. This output serves as the master signal and is buffered from the RAMP pin to the RAMPBUF pin. As this output and the RAMPBUF pin rise, the current from the TRACK pins is reduced. Consequently, the voltage at the slave supply’s outputs increases, and the slave supplies track the master supply. When the ON pin is again pulled below 1.23V, 10µA will pull down CGATE and the gate of the FET at 1000V/s. If the loads on the outputs are sufficient, all outputs will track down coincidently at 1000V/s. 2923fa LTC2923 U W U U APPLICATIO S I FOR ATIO Ratiometric Tracking Example SLAVE2 SLAVE1 1V/DIV 1ms/DIV 1V/DIV 1ms/DIV 2923 F12a 2923 F12b Figure 12. Ratiometric Tracking (from Figure 13) This example converts the coincident tracking example to the ratiometric tracking profile shown in Figure 12, using two supplies without an external FET. 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 600V/s and the 2.5V slave 2 supply ramps up at 850V/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 rampup at 500V/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. 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: 1000 V/s RTB = 16.5kΩ • ≈ 27.4kΩ 600 V/s From Equation 3: RTA ′ = 0.8 V = 10kΩ 1.235V 1.235V 0.8 V + – 16.5kΩ 35.7kΩ 27.5kΩ Step 3 is unnecessary because there is no delay, so RTA = RTA′. 3.3V CGATE 10nF 0.1µF RONB 138k VCC GATE ON RONA 100k RAMP FB1 IN DC/DC FB = 1.235V OUT 1.8V SLAVE1 LTC2923 RFA1 RFB1 35.7k 16.5k 3.3V RAMPBUF RTB1 27.4k RTA1 10k 3.3V RTB2 1M TRACK1 FB2 TRACK2 RTA2 383k IN DC/DC FB = 0.8V OUT 2.5V SLAVE2 GND RFA2 412k RFB2 887k 2923 F13 Figure 13. Ratiometric Tracking Example 2923fa 13 LTC2923 U W U U APPLICATIO S I FOR ATIO Offset Tracking Example MASTER SLAVE2 SLAVE1 1V/DIV 1ms/DIV 1V/DIV 1ms/DIV 2923 F14a 2923 F14b Figure 14. Offset Tracking (from Figure 15) Converting the circuit in the coincident tracking example to the offset tracking shown in Figure 14 is relatively simple. Here the 1.8V slave 1 supply ramps up 1V below the master. The ramp rate remains the same (1000V/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 offsets will allow the slave supplies to ramp up completely. In this example, if the voltage offset were 2V, the slave supply would only ramp up to 3.3V – 2V = 1.3V. 3. Choose RTA to obtain the desired delay. First, convert the desired voltage offset, VOS, to a delay, tD, using the ramp rate: tD = VOS 1V = = 1ms SS 1000 V/s (6) Q1 3.3V MASTER 3.3V 0.1µF 10Ω CGATE 10nF 3.3V RONB 138k RONA 100k GATE RAMP FB1 RTB2 887k 1.8V SLAVE1 RFA1 RFB1 35.7k 16.5k 3.3V TRACK1 FB2 TRACK2 RTA2 316k IN DC/DC FB = 1.235V OUT LTC2923 RAMPBUF RTB1 16.5k RTA1 6.65k VCC ON IN DC/DC FB = 0.8V OUT GND RFA2 412k 2.5V SLAVE2 RFB2 887k 2923 F15 Figure 15. Offset Tracking Example From Equation 4: RTA ′′ = 0.8 V • 16.5kΩ = 13.2kΩ 1ms • 1000 V/s From Equation 5: RTA = 13.1kΩ||13.2kΩ ≈ 6.65kΩ 2923fa 14 LTC2923 U W U U APPLICATIO S I FOR ATIO Supply Sequencing Example MASTER SLAVE2 SLAVE1 1V/DIV 10ms/DIV 1V/DIV 10ms/DIV 2923 F16a 2923 F16b Figure 16. Supply Sequencing (from Figure 17) In Figure 16, the slave 1 supply and the slave 2 supply are sequenced instead of tracking. The 3.3V supply ramps up at 100V/s with an external FET and serves as the master signal. 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 1 supply yields: 1. Set the ramp rate of the master signal. From Equation 3: RTA ′ = 0.8 V = –2.13kΩ 1.235V 1.235V 0.8 V – + 16.5kΩ 35.7kΩ 1.65kΩ 3. Choose RTA to obtain the desired delay. From Equation 4: RTA ′′ = 0.8 V • 1.65kΩ = 1.32kΩ 10ms • 100 V/s From Equation 5: RTA = – 2.13kΩ||1.32kΩ = 3.48kΩ Q1 From Equation 1: 0.1µF 10Ω CGATE = 10µA = 100nF 100 V/s 100 V/s RTB = 16.5kΩ • = 1.65kΩ 1000 V/s CGATE 100nF 3.3V RONB 138k 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: 3.3V MASTER 3.3V RONA 100k GATE ON RAMP FB1 RTB2 88.7k IN DC/DC FB = 1.235V OUT RFA1 RFB1 16.5k 35.7k 3.3V TRACK1 FB2 IN DC/DC FB = 0.8V OUT TRACK2 RTA2 36.5k 1.8V SLAVE1 LTC2923 RAMPBUF RTB1 1.65k RTA1 3.48k VCC GND 2.5V SLAVE2 2923 F17 RFA2 412k RFB2 887k Figure 17. Supply Sequencing Example 2923fa 15 LTC2923 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 ⎟ < VCC, where VTRACK = 0.8 V ⎝ RTA ⎠ It is possible to choose resistor values that require the LTC2923 to supply more current than the Electrical Characteristics table guarantees. To avoid this condition, check that ITRACKx does not exceed 1mA and IRAMPBUF does not exceed ±2mA. To confirm that ITRACKx < 1mA, the TRACKx pin’s maximum guaranteed current, verify that: VTRACK < 1mA RTA RTB Finally, check that the RAMPBUF pin will not be forced to sink more then 2mA when it is at 0V or be forced to source more than 2mA when it is at VCC. VTRACK VTRACK + < 2mA and RTB1 RTB2 VCC VCC + < 2mA RTA1 + RTB1 RTA2 + RTB2 Caution with Boost and Linear Regulators Note that the LTC2923’s tracking cell is not able to control the outputs of all types of power supplies. If it is necessary to control one of these types of supplies, where the output is not controllable through its feedback node, the series FET can be used to control one supply’s output. For example, boost regulators commonly contain an inductor and diode between the input supply and the output supply providing a DC current path when the output voltage falls below the input voltage. Therefore, the LTC2923’s tracking cell will not effectively drive the supply’s output below the input. Special caution should be taken when considering the use of linear regulators. Three-terminal linear regulators have a reference voltage that is referred to the output supply rather than to ground. In this case, driving current into the regulator’s feedback node will cause its output to rise rather than fall. Even linear regulators that have their reference voltage referred to ground, including low dropout regulators (LDOs), may be problematic. Linear regulators commonly contain circuitry that prevents driving their outputs below their reference voltage. This may not be obvious from the data sheets, so lab testing is recommended whenever the LTC2923’s tracking cell is used to control linear regulators. Load Requirements When the supplies are 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 18 shows the case when the 2.5V supply does not track the 1.8V and 3.3V supplies near ground. Start-Up Delays Often power supplies do not start-up immediately when their input supplies are applied. If the LTC2923 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 19). 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 20. 2923fa 16 LTC2923 U W U U APPLICATIO S I FOR ATIO Layout Considerations MASTER SLAVE2 1V/DIV SLAVE1 1ms/DIV 2923 F18 Figure 18. Weak Resistive Load MASTER SLAVE1 1V/DIV SLAVE2 Be sure to place a 0.1µF bypass capacitor as near as possible to the supply pin of the LTC2923. A 10Ω resistor located near the FET and connected between the FET’s gate and the external CGATE capacitor is recommended. This will almost assuredly eliminate the troublesome high frequency oscillations that can occur due to the FET interacting with PCB parasitics. To minimize the noise on the slave supplies’ outputs, keep the traces connecting the FBx pins of the LTC2923 and the feedback nodes of the slave supplies as short as possible. In addition, do not route those traces 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 FBx pin of the LTC2923. This resistor must not exceed: 1.5V – VFB ⎛ 1.5V ⎞ – 1⎟ (RFA || RFB) =⎜ ⎝ VFB ⎠ IMAX 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. RSERIES = ON 1ms/DIV 2923 F19 Figure 19. Power Supply Start-Ups Delayed 10Ω MASTER FET VCC CGATE OUT SLAVE1 1V/DIV SLAVE2 LTC2923 RAMP VCC GATE RSERIES FB1 GND ON MINIMIZE TRACE LENGTH DC/DC FB OUT RFA RFB 0.1µF 2923 F21 1ms/DIV Figure 20. ON Pin Delayed 2923 F20 Figure 21. Layout Considerations 2923fa 17 LTC2923 U PACKAGE DESCRIPTIO MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.50 0.305 ± 0.038 (.0197) (.0120 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 10 9 8 7 6 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) DETAIL “A” 0.497 ± 0.076 (.0196 ± .003) REF 0° – 6° TYP GAUGE PLANE 1 2 3 4 5 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 1.10 (.043) MAX 0.86 (.034) REF 0.18 (.007) SEATING PLANE 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) NOTE: BSC 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 0.127 ± 0.076 (.005 ± .003) MSOP (MS) 0603 2923fa 18 LTC2923 U PACKAGE DESCRIPTIO DE Package 12-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1695) 0.65 ±0.05 3.50 ±0.05 1.70 ±0.05 2.20 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 3.30 ±0.05 (2 SIDES) 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ±0.10 (2 SIDES) 7 R = 0.115 TYP 0.38 ± 0.10 12 R = 0.20 TYP PIN 1 TOP MARK (NOTE 6) 3.00 ±0.10 (2 SIDES) 1.70 ± 0.10 (2 SIDES) PIN 1 NOTCH (UE12/DE12) DFN 0603 0.200 REF 0.75 ±0.05 0.00 – 0.05 6 0.25 ± 0.05 3.30 ±0.10 (2 SIDES) 1 0.50 BSC BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE A VARIATION OF VERSION (WGED) 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 2923fa 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. 19 LTC2923 U TYPICAL APPLICATIO S Daisy-Chained Application High Voltage Supply Application 3.3V 3.3V 0.1µF 0.1µF GATE VCC ON FB1 IN DC/DC FB = 1.235V OUT CGATE 10nF 3.3V SLAVE1 RONB 138k LTC2923 RFB1 RFA1 RAMPBUF RONA 100k VCC GATE ON RAMP FB1 TRACK1 RTB2 FB2 TRACK2 GND RTA2 IN DC/DC FB = 0.8V OUT RFA1 RAMPBUF RAMP TRACK1 RTA1 RTB2 FB2 TRACK2 RTA2 RFA2 IN DC/DC FB = 0.8V OUT RFA2 0.1µF RAMP ON RFB2 2923 TA04 CGATE 10nF RONA 100k 5V SLAVE2 GND 3.3V VCC GATE RFB1 RTB1 2.5V SLAVE2 RFB2 RONB 138k 12V SLAVE1 LTC2923 RTB1 RTA1 IN DC/DC FB = 1.235V OUT FB1 IN DC/DC FB = 1.235V OUT 1.8V SLAVE1 LTC2923 RFB RFA RAMPBUF RTB TRACK1 RTA RTB FB2 TRACK2 RTA IN DC/DC FB = 0.8V OUT 1.5V SLAVE2 GND RFB RFA 2923 TA03 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS TM LTC1645 Dual Hot Swap Controller Operates from 1.2V to 12V, Allows Supply Sequencing 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 LTC2925 Multiple Power Supply Tracking Controller Up to 4 Supplies, Status and Fault Pins, Slave Supply Shutdown, Remote Sense Switch LT®4220 Dual Supply Hot Swap Controller ±2.7V to ±16.5V, Supply Tracking Mode 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. 2923fa 20 Linear Technology Corporation LT/TP 1104 1K REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003