DEMO CIRCUIT 817A QUICK START GUIDE LTC2927 Single Power Supply Tracking Controller DESCRIPTION Demonstration circuit DC817A is intended for evaluating the performance of the LTC2927 Single Power Supply Tracking Controller. The board contains two LTC2927s tied to a common ramping signal, providing tracking and sequencing for two power supplies. As assembled, DC817A operates directly with two DC222 buck outputs. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT and PowerPath are trademarks of Linear Technology Corporation. PERFORMANCE SUMMARY Specification s are at TA = 25°C SYMBOL VCC VCC(UVLO) •VCC(UVLO) VON(TH) •VON(HYST) PARAMETER Input Supply Voltage Range Supply Undervoltage Lockout Supply Undervoltage Lockout Hysteresis ON Pin Threshold Voltage ON Pin Hysteresis VTRACK TRACK Pin Voltage VFB(CLAMP) FB Pin Clamp Voltage CONDITIONS VCC RISING MIN 2.6 2.2 TYP VON RISING 1.210 30 2.5 25 1.230 75 VTRACK= -10µA VTRACK= -1mA 1µA•IFB•1mA 0.77 0.77 1.5 0.8 0.8 2.0 MAX 5.5 2.7 1.250 150 UNITS V V mV V mV 0.82 0.82 2.3 V V V OPERATING PRINCIPLES The LTC2927 controls the output voltage of a power supply by introducing a small current into the supply’s feedback node. When this current is at its maximum, the supply’s output is forced to zero; decreasing this current to zero allows the supply’s output to rise to its normal, regulated output. The RAMP pin is driven by an internally generated constant current source, which when loaded with capacitor, produces a linear voltage ramp. By varying the injected current under the control of this “master” ramping signal, the slave supply output is made to follow the master ramp in a predictable fashion. Any slave power supply, which has an accessible inverting feedback node of less than 1.5V, and a feedback divider current in the range of 10µA to 1mA, may be controlled with the LTC2927. Tracking behavior is configured using two resistors, RTA and RTB. A valid Vcc supply for the LTC2927 must be present before the tracked supply is enabled or powered up. Boost converters and certain linear regulators are generally incompatible with the LTC2927 control technique. DC817A includes two LTC2927 controllers driven by a common ramp signal. The first controller 1 generates the ramp signal and monitors Vcc with its ON pin. The second controller’s RAMP pin is driven by REMPBUF of the first; its ON pin is defeated by being tied to VCC. To reconfigure DC817A for use with other supplies, five components may be changed: C3 to control the master ramp rate, and RTA1, RTB1, RTA2, and RTB2 to control the relative behavior of the two supplies. Hold-off time, ramp rate, and offset are all defined by the resistors. For details on calculating these component values, consult the LTC2927 data sheet. QUICK START PROCEDURE As a design example, DC817A is populated for operation with two DC222 slave supplies, using the LT1735 Buck Regulator, which are configured for 3.3V and 2.5V output. The first LTC2927 controls the 3.3V supply, while the second controls the 2.5V supply. Connect the slave supplies as shown in Figure 1, attaching both the RUN/SS and VOSENSE lines to the DC817A. Next apply 5V to all boards; the outputs will begin tracking up once the ON pin of U1 detects a valid 5V input. The tracking waveforms are shown in Figure 2 and Figure 3. Also shown in Figure 2 and Figure 3 is an optional “enable “ waveform, which is simply a switch closure to ground on U1’s ON pin (see Figure 1 for location of this pad). This may be driven by an open collector, such as an enable signal from a microcontroller or separate supply monitor. . Figure 1. Test Setup 2 Figure 2. Startup Waveforms Figure 3. Shutdown Waveforms 3 Figure 4. Reference A 4 Figure 5. Reference B 5