View LTC2927CDDB_4409390.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

FEATURES
n n n n n n n n n
LTC2927 Single Power Supply Tracking Controller DESCRIPTIO
The LTC(R)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. 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.
, 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.
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 ThinSOTTM and 8-Lead (3mm x 2mm) DFN Packages
APPLICATIO S
n n n n
VCORE and VI/O Supply Tracking Microprocessor, DSP and FPGA Supplies Multiple Supply Systems Point-of-Load Supplies
TYPICAL APPLICATIO
EARLY VIN 3.3V 138k ON 100k LTC2927 SDO RAMPBUF 16.5k TRACK 13k GND FB VCC RAMP
Track-Up and Track-Down Waveforms
0.1F
0.1F RUN/SS
VIN IN DC/DC
1V/DIV OUT 1.8V
FB = 1.235V 16.5k
35.7k
EARLY VIN 3.3V VCC ON LTC2927 SDO RAMPBUF 887k TRACK 412k GND
2927 TA01a
0.1F RAMP VIN RUN/SS IN DC/DC FB FB = 0.8V 887k 10ms/DIV
2927 TA01c
RAMP (3.3V) 2.5V 1.8V OUT 2.5V 1V/DIV
412k
U
U
U
RAMP (3.3V) 2.5V 1.8V
10ms/DIV
2927 TA01b
2927fb
1
LTC2927 ABSOLUTE AXI U RATI GS (Note 1)
Average Current TRACK .................................................................5mA FB ........................................................................5mA RAMPBUF ............................................................5mA Operating Temperature Range LTC2927C ................................................ 0C to 70C LTC2927I.............................................. -40C to 85C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 sec) .................. 300C 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 ,
PIN CONFIGURATION
TOP VIEW TOP VIEW ON 1 RAMP 2 RAMPBUF 3 TRACK 4 9 8 VCC 7 SDO 6 FB 5 GND VCC 1 SDO 2 FB 3 GND 4 8 ON 7 RAMP 6 RAMPBUF 5 TRACK
DDB PACKAGE 8-LEAD (3mm x 2mm) PLASTIC DFN EXPOSED PAD (PIN 9) PCB GND, CONNECTION OPTIONAL TJMAX = 125C, JA = 76C/W
ORDER INFORMATION
LEAD FREE FINISH LTC2927CDDB#PBF LTC2927IDDB#PBF LTC2927CTS8#PBF LTC2927ITS8#PBF LEAD BASED FINISH LTC2927CDDB LTC2927IDDB LTC2927CTS8 LTC2927ITS8 TAPE AND REEL LTC2927CDDB#TRPBF LTC2927IDDB#TRPBF LTC2927CTS8#TRPBF LTC2927ITS8#TRPBF TAPE AND REEL LTC2927CDDB#TR LTC2927IDDB#TR LTC2927CTS8#TR LTC2927ITS8#TR PART MARKING* LBQH LBQH LTBQJ LTBQJ PART MARKING* LBQH LBQH LTBQJ LTBQJ PACKAGE DESCRIPTION 8-Lead (3mm x 2mm) Plastic DFN 8-Lead (3mm x 2mm) Plastic DFN 8-Lead Plastic TSOT-23 8-Lead Plastic TSOT-23 PACKAGE DESCRIPTION 8-Lead (3mm x 2mm) Plastic DFN 8-Lead (3mm x 2mm) Plastic DFN 8-Lead Plastic TSOT-23 8-Lead Plastic TSOT-23 TEMPERATURE RANGE 0C to 70C -40C to 85C 0C to 70C -40C to 85C TEMPERATURE RANGE 0C to 70C -40C to 85C 0C to 70C -40C to 85C
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/
2
U
WW
W
TS8 PACKAGE 8-LEAD PLASTIC TSOT-23 TJMAX = 125C, JA = 250C/W
2927fb
LTC2927 ELECTRICAL CHARACTERISTICS
SYMBOL VCC ICC VCC(UVLO) VCC(UVHYST) VON(TH) VON(HYST) ION IRAMP VRAMPBUF(OL) VRAMPBUF(OH) VOS IERROR(%) VTRACK IFB(LEAK) VFB(CLAMP) VSDO(OL) PARAMETER Supply Voltage Supply Current Supply Undervoltage Lockout Supply Undervoltage Lockout Hysteresis ON Pin Threshold Voltage ON Pin Hysteresis ON Pin Input Current RAMP Pin Input Current RAMPBUF Output Low Voltage RAMPBUF Output High Voltage, VRAMPBUF(OH) = VCC - VRAMPBUF Ramp Buffer Offset, VOS = VRAMPBUF - VRAMP IFB to ITRACK Current Mismatch IERROR(%) = (IFB - ITRACK)/ITRACK TRACK Pin Voltage FB Pin Leakage Current FB Pin Clamp Voltage SDO Output Low Voltage VON = 1.2V, VCC = 5.5V 0V < VRAMP < VCC, Ramp On 0V < VRAMP < VCC, Ramp Off IRAMPBUF = 1mA IRAMPBUF = -1mA VRAMP = VCC/2, IRAMPBUF = 0mA ITRACK = -10A ITRACK = -1mA ITRACK = -10A ITRACK = -1mA VFB = 2V, VCC = 5.5V 1A < IFB < 1mA ISDO = 1mA, VCC = 2.3V
l l l l l l l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. 2.9V < VCC < 5.5V unless otherwise noted (Note 2).
CONDITIONS
l
MIN 2.9 0.25 3 2.2 1.210 30 -9 9
l l l l l l
TYP 0.56 3.6 2.5 25 1.230 75 0 -10 10 20 45
MAX 5.5 1.2 4.2 2.7 1.250 150 100 -11 11 100 150 30 5 5 0.82 0.82 100 2.3 0.4
UNITS V mA mA V mV V mV nA A A mV mV mV % % V V nA V V
IFB = 0mA, ITRACK = 0mA IFB = -1mA, ITRACK = -1mA, IRAMPBUF = -1mA VCC Rising VON Rising
-30
0 0 0
0.77 0.77 1.5
0.800 0.800 1 2 0.1
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.
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
ICC vs VCC
750 ITRACK = IFB = 0mA IRAMBUF = 0mA 4.70
ICC vs VCC
ITRACK = IFB = -1mA IRAMBUF = -2mA 820
VTRACK vs Temperature
700
4.65 VTRACK (mV)
810
600
ICC (mA)
ICC (A)
650
4.60
800
4.55
790
550
4.50
780
500
2.5
3.0
3.5
4.0 4.5 VCC (V)
5.0
5.5
6.0
2927 G01
4.45
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 TYPICAL PERFOR A CE CHARACTERISTICS
VON(TH) vs Temperature
1.240 1.235 24 1.230 VON(TH) (V) 1.225 1.220 1.215 12 1.210 -50 -25 0 25 50 TEMPERATURE (C) 75 100
2927 G04
VRAMPBUF(OH) (mV)
VRAMPBUF(OL) (mV)
MAX ITRACK vs VCC
60 VTRACK = 0V 5
50 ITRACK (mA)
40
3
VSDO(OL) (V)
ERROR (%)
30
20
10
2.5
3.0
3.5
4.0 4.5 VCC (V)
5.0
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 0.1F 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
UW
5.5
VRAMPBUF(OL) vs Temperature
28 26 65 22 20 18 16 14 45 10 -50 60 55 50 70
VRAMPBUF(OH) vs Temperature
-25
0 25 50 TEMPERATURE (C)
75
100
2927 G05
40 -50
-25
0 25 50 TEMPERATURE (C)
75
100
2927 G06
Tracking Cell Error vs ITRACK
ERROR = 4 VTRACK I * FB -1 0.8V ITRACK 1.0
VSDO(OL) vs VCC
0.8
0.6 ISDO = 5mA 0.4
2
1
0.2 ISDO = 10A 0 1 2 3 VCC (V) 4 5
2927 G0
6.0
2927 G07
0
1
2
3 ITRACK (mA)
4
5
2927 G0
0.0
LTC2927 PI FU CTIO S TSOT/DFN Packages
GND (Pin 4/Pin 5): Device Ground. 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 . 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. 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 10A pull-up or pull-down currents. RAMP can also be connected to an external ramping signal for tracking. Ground RAMP if unused. 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 10A pull-up current to flow at the RAMP pin, which charges an external capacitor. An active low at the ON pin causes a 10A pull-down current at the RAMP pin to discharge the external capacitor relative to GND. Exposed Pad (NA/Pin 9): Exposed pad may be left open or connected to device ground.
FU CTIO AL BLOCK DIAGRA
5 RAMPBUF
5
ON
+ -
1.23V
2.5V
+
UVLO
VCC
-
VCC
+ -
5 TRACK GND 5
0.8V
-
+
W
U
U
U
U
U
5 VCC 1x VCC 10A
RAMP
5
10A
0.2V
SDO
5
FB
5
2927 BD
2927fb
5
LTC2927 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
1V/DIV
10ms/DIV
2927 F01
Figure 1. Coincident Tracking
1V/DIV
10ms/DIV
2927 F03
Figure 3. Ratiometric Tracking
6
U
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 SLAVE1 SLAVE2 1V/DIV MASTER SLAVE1 SLAVE2 10ms/DIV
2927 F02
W
U
U
Figure 2. Offset Tracking
MASTER SLAVE1 SLAVE2 1V/DIV
MASTER SLAVE1 SLAVE2
10ms/DIV
2927 F04
Figure 4. Supply Sequencing
2927fb
LTC2927 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 10A 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) - VON(HYST), typically 1.225V, the RAMP pin pulls down with 10A and the master signal and slave supplies will fall at the same rate as they rose previously.
VCC
+
MASTER
+ - -
TRACK
0.8V
RTB 5 RTA
DC/DC FB 5 FB OUT RFB SLAVE
2927 F05
RFA
Figure 5. Simplified Tracking Cell
U
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 RONA 1.2V ON 10A
W
U
U
+ -
10A 1x VCC RAMP 5 CRAMP MASTER
5
RAMPBUF
+ -
RTB 5 RTA 5 TRACK
0.8V
FB
5
DC/DC
SLAVE
2927 F06
RFA
RFB
Figure 6. Simplified Functional Diagram
2927fb
7
LTC2927 APPLICATIO S I FOR ATIO
EARLY VIN 3.3V RONB 138k ON RONA 100k LTC2927 SDO RAMPBUF RTB 16.5k TRACK RTA 13k GND
2927 F07
0.1F
VCC RAMP CRAMP 10pF RUN/SS MASTER VIN IN DC/DC FB FB = 1.235V OUT 1.8V
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. 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 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 10A SM (1)
2. Solve for the pair of resistors that provide the desired ramp rate of the slave supply, assuming no delay.
EARLY VIN 0.1F
RONB ON RONA
VCC RAMP CRAMP LTC2927 MASTER VIN IN RAMPBUF DC/DC FB TRACK FB OUT SLAVE
RTB
RTA
GND
2927 F08
RFA
RFB
Figure 8. Single Supply Application
8
U
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: RTB = RFB * RTA = SM SS VTRACK V V + FB - TRACK RFA RTB (2) (3) VFB RFB where VTRACK 0.8V. 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. 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. RTA = VTRACK * RTB tD * SM (4) (5) RTA = RTA || RTA 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.
2927fb
W
U
U
LTC2927 APPLICATIO S I FOR ATIO
Coincident Tracking Example
1V/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 10A = = 0.1F 100V / s
2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: RTB = 16.5k * 100V / s = 16.5k 100V / s
EARLY 3.3V
From Equation 3: RTA = 0.8V 13k 1.235V 1.235V 0.8V + - 16.5k 35.7k 16.5k
3. Choose RTA to obtain desired delay. Since no delay is desired, RTA = RTA'
U
MASTER SLAVE2 SLAVE1 1V/DIV 10ms/DIV
2927 F09
W
U
U
In this example, the supply remains low while the ON pin is held below 1.23V. When the ON pin rises above 1.23V, 10A 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, 10A 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 RONB 138k ON RONA 100k LTC2927 SDO RAMPBUF RTB1 16.5k TRACK RTA1 13k GND RFA1 35.7k RFB1 16.5k FB 0.1F VCC RAMP 0.1F RUN/SS 3.3V IN DC/DC FB = 1.235V OUT SLAVE1 1.8V MASTER 3.3V
0.1F VCC ON LTC2927 SDO RAMPBUF RTB2 887k TRACK RTA2 412k GND
2927 F10
RAMP 3.3V RUN/SS IN DC/DC FB FB = 0.8V RFB2 887k OUT SLAVE2 2.5V
RFA2 412k
Figure 10. Coincident Tracking Example
2927fb
9
LTC2927 APPLICATIO S I FOR ATIO
Ratiometric Tracking Example
1V/DIV
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. 2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. From Equation 2: RTB 100V / s = 16.5k * = 27.4k 60V / s
From Equation 3: RTA = 0.8V 10k 1.235V 1.235V 0.8V + - 16.5k 35.7k 27.4k
Step 3 is unnecessary because there is no delay, so RTA = RTA'
10
U
MASTER SLAVE2 SLAVE1 1V/DIV 10ms/DIV
2927 F11
W
U
U
EARLY VIN 3.3V
RONB 138k ON RONA 100k
0.1F VCC RAMP LTC2927 SDO RAMPBUF 0.1F RUN/SS 3.3V IN DC/DC FB TRACK FB = 1.235V RFB1 16.5k OUT SLAVE1 1.8V MASTER 3.3V
RTB1 27.4k RTA1 10k GND
RFA1 35.7k
EARLY 3.3V VCC ON LTC2927 SDO RAMPBUF RTB2 1M TRACK RTA2 383k GND
2927 F12
0.1F RAMP 3.3V RUN/SS IN DC/DC FB FB = 0.8V RFB2 887k OUT SLAVE2 2.5V
RFA2 412k
Figure 12. Ratiometric Tracking Example
2927fb
LTC2927 APPLICATIO S I FOR ATIO
Offset Tracking Example
1V/DIV
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. 3. Choose RTA to obtain desired delay. 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 From Equation 4: RTA = 0.8V * 16.5k = 13.2k 10ms * 100V / s (6)
From Equation 5: RTA = 13.1k 13.2k 6.65k
U
MASTER SLAVE2 SLAVE1 1V/DIV 10ms/DIV
2927 F13
W
U
U
EARLY VIN 3.3V
RONB 138k ON RONA 100k
0.1F VCC RAMP LTC2927 SDO RAMPBUF 0.1F RUN/SS 3.3V IN DC/DC FB TRACK FB = 1.235V RFB1 16.5k OUT SLAVE1 1.8V MASTER 3.3V
RTB1 16.5k RTA1 6.65k GND
RFA1 35.7k
EARLY 3.3V VCC ON LTC2927 SDO RAMPBUF RTB2 887k TRACK RTA2 316k GND
2927 F14
0.1F RAMP 3.3V RUN/SS IN DC/DC FB FB = 0.8V RFB2 887k OUT SLAVE2 2.5V
RFA2 412k
Figure 14. Offset Tracking Example
2927fb
11
LTC2927 APPLICATIO S I FOR ATIO
Supply Sequencing Example
1V/DIV
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: 1. Set the ramp rate of the master signal. From Equation 1: C RAMP = 10A = 0.1F 100V / s
2. Solve for the pair of resistors that provide the desired slave supply behavior, assuming no delay. RTB 100V / s = 16.5k * = 1.65k 1000V / s
From Equation 3: RTA = 0.8V = -2.13k 1.235V 1.235V 0.8V + - 16.5k 35.7k 1.65k
12
U
MASTER SLAVE2 SLAVE1 1V/DIV 10ms/DIV
2927 F15
W
U
U
3. Choose RTA to obtain desired delay. From Equation 4: RTA = 0.8V * 1.65k = 1.32k 10ms * 100V / s
From Equation 5: RTA = -2.13k 1.32k = 3.48k
EARLY VIN 3.3V RONB 138k ON RONA 100k LTC2927 SDO RAMPBUF RTB1 1.65k TRACK RTA1 3.48k GND RFA1 35.7k RFB1 16.5k FB 0.1F VCC RAMP 0.1F RUN/SS 3.3V IN DC/DC FB = 1.235V OUT SLAVE1 1.8V MASTER 3.3V
EARLY 3.3V VCC ON LTC2927 SDO RAMPBUF RTB2 88.7k TRACK RTA2 36.5k GND
2927 F16
0.1F RAMP 3.3V RUN/SS IN DC/DC FB FB = 0.8V RFB2 887k OUT SLAVE2 2.5V
RFA2 412k
Figure 16. Supply Sequencing Example
2927fb
LTC2927 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 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
U
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.
10ms/DIV
2927 F17
W
U
U
Figure 17. Weak Resistive Load
2927fb
13
LTC2927 APPLICATIO S I FOR ATIO
Start-Up Delays 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.
MASTER
1V/DIV
20ms/DIV
2927 F18
Figure 18. Power Supply Start-Up Delayed
VCC LTC2927 MASTER FB GND MINIMIZE TRACE LENGTH RFA SLAVE 1V/DIV 0.1F
2927 F20
5ms/DIV
2927 F19
Figure 19. ON Pin Delayed
14
U
Layout Considerations Be sure to place a 0.1F 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: RSERIES =
SLAVE
W
U
U
1.5V - VFB 1.5V = - 1 RFA RFB IMAX VFB
(
)
ON
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
DC/DC FB OUT
RFB
Figure 20. Layout Considerations
ON
2927fb
LTC2927 PACKAGE DESCRIPTIO U
DDB Package 8-Lead Plastic DFN (3mm x 2mm)
(Reference LTC DWG # 05-08-1702)
0.61 0.05 (2 SIDES) 0.675 0.05 2.50 0.05 1.15 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.20 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 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 PIN 1 BAR TOP MARK (SEE NOTE 6) 2.00 0.10 (2 SIDES) 3.00 0.10 (2 SIDES) R = 0.115 TYP 5 0.56 0.05 (2 SIDES) 0.38 0.10 8 0.200 REF 0.75 0.05 4 0.25 0.05 2.15 0.05 (2 SIDES) BOTTOM VIEW--EXPOSED PAD 1 PIN 1 CHAMFER OF EXPOSED PAD
(DDB8) DFN 1103
0.50 BSC
0 - 0.05
TS8 Package 8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637)
0.52 MAX 0.65 REF 2.90 BSC (NOTE 4)
1.22 REF
3.85 MAX 2.62 REF
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4) PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.65 BSC
0.22 - 0.36 8 PLCS (NOTE 3)
0.80 - 0.90 0.20 BSC 1.00 MAX DATUM `A' 0.01 - 0.10
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 TYPICAL APPLICATIO
EARLY VIN 3.3V RONB 138k ON RONA 100k LTC2927 SDO RAMPBUF RTB 16.5k TRACK RTA 13k GND
2927 TA02a
Single Supply Application
EARLY VIN 3.3V RONB 138k RAMP CRAMP 0.1F RUN/SS MASTER 3.3V IN DC/DC FB FB = 1.235V OUT 1.8V RTB 78.7k RONA 100k
0.1F
VCC
RFA 35.7k
RELATED PARTS
PART NUMBER LTC2920 LTC2921/LTC2922 LTC2923 LTC2924 LTC2925 LTC2926 LTC2928 LT4220 LTC4221 LTC4230 LTC4253 DESCRIPTION Power Supply Margining Controller Power Supply Tracker with Input Monitors Power Supply Tracking Controller Quad Power Supply Sequencer Multiple Power Supply Tracking Controller with Power Good Timeout MOSFET-Controlled Power Supply Tracker Multichannel Power Supply Sequencer and Supervisor Dual Supply Hot SwapTM Controller Dual Hot Swap Controller Triple Hot Swap Controller with Multifunction Current Control -48V Hot Swap Controller and Supply Sequencer COMMENTS Single or Dual Versions, Symmetric as Symmetric High and Low Margining Includes 3 (LTC2921) or 5 (LTC2922) Remote Sense Switches Controls 3 Supplies for Tracking or Sequencing Controls 4 Supplies for Sequencing Controls 4 Supplies for Tracking or Sequencing Active Tracking Control with Series MOSFETs Programmable with External Components; No Software Required 2.7V to 16.5V, Supply Tracking Mode Operates from 1V to 13.5V, Allows Supply Sequencing, Active Current Limiting 1.7V to 16.5V, Active Inrush Limiting, Fast Comparator Floating Supply from -15V, Active Current Limiting, Enables Three DC/DC Converters
Hot Swap is a trademark of Linear Technology Corporation.
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
U
High Voltage Supply Application
0.1F VCC ON LTC2927 SDO RAMPBUF FB TRACK RFB 16.5k RTA 28k GND
2927 TA02b
RAMP CRAMP 0.4F RUN/SS
MASTER 3.3V IN DC/DC FB = 1.235V OUT SLAVE 12V
RFA 35.7k
RFB 311k
2927fb LT 0308 REV B * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006

▲Up To Search▲

Price & Availability of LTC2927CDDB

	To Download LTC2927CDDB Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .