LT1070 Design Manual (Linear Technology) - 8
| Authors | Carl Nelson |
| Manufacturer | Linear Technology |
| Description | Application Note 19. This design manual is an extensive discussion of all standard switching configurations for the LT1070; including buck, boost, flyback, forward, inverting and "Cuk." The manual includes comprehensive information on the LT1070, the external components used with it, and complete formulas for calculating component values. |
| Pages / Page | 80 / 8 — PIN FUNCTIONS. Ground Pin. Input Supply (VIN). Figure 1. External … |
| File Format / Size | PDF / 656 Kb |
| Document Language | English |
PIN FUNCTIONS. Ground Pin. Input Supply (VIN). Figure 1. External Undervoltage Lockout. Figure 2
Text Version of Document
Application Note 19
PIN FUNCTIONS Ground Pin Input Supply (VIN)
The ground pin (case) of the LT1070 is important because it acts as both the negative sense point for the internal error The LT1070 is designed to operate with input voltages amplifier and as the high current path for the 5A switch. from 3V to 40V (standard) or 60V (HV units). Supply This is not normally good design practice, but was neces- current is essentially flat over this range at about 6mA sary in a 5-pin package configuration. To avoid degrada- (with zero output current). With increasing switch current, tion of load regulation, Kelvin connections should be made the supply current (during switch on-time) increases at a to the ground pin. This is done on the TO-3 package by rate approximately 1/40 of switch current, corresponding tying one end of the package to power ground and the other to a forced hFE of 40 for the switch. end to the feedback divider resistor (analog ground). This Undervoltage lockout is incorporated on the LT1070 by is illustrated in Figure 2. sensing saturation of the lateral PNP pass transistor which For best load regulation, the resistance in the switch drives an internal 2.3V regulator. A remote collector on current path must be kept low. 0.01Ω of wire resistance this transistor conducts current and locks out the switch creates 50mV drop at 5A switch current. This is a 1% for input voltages below 2.5V. No hysteresis is used to change in a 5V output, and actually causes the output to maximize the useful range of input voltage. Operating the increase with increasing load current. regulator right at the 2.5V threshold may result in a “burping” action as the LT1070 turns on and off in With the TO-220 package, (Figure 3) connect the feedback response to wobbles in input voltage, but this will not harm resistor directly to the ground pin with a separate wire if no the device. External undervoltage lockout can be added if case connection is made. The case can be used as a second it is desirable to raise the threshold voltage. The circuit ground pin if desired. shown in Figure 1 is one example of how to implement Avoid long wire runs to the ground pin to minimize load this. regulation effects and inductive voltages created by the The threshold of this circuit is approximately VZ + 1.5V. high di/dt switch current. A ground plane will keep EMI to Below that voltage, D2 pulls the VC pin low to shut off the a minimum regulator. VIN LT1070 V FB Z GND VC PIN D2 SWITCH CURRENT FEEDBACK PATH—KEEP DIVIDER RESISTANCE TO A MINIMUM 1.5k LOAD CURRENT TO OUTPUT (+) PATH—KEEP RESISTANCE TO A MINIMUM AN19 F01 –VIN TO OUTPUT GND AN19 F02
Figure 1. External Undervoltage Lockout Figure 2
an19fc AN19-8
PIN FUNCTIONS Ground Pin Input Supply (VIN)
The ground pin (case) of the LT1070 is important because it acts as both the negative sense point for the internal error The LT1070 is designed to operate with input voltages amplifier and as the high current path for the 5A switch. from 3V to 40V (standard) or 60V (HV units). Supply This is not normally good design practice, but was neces- current is essentially flat over this range at about 6mA sary in a 5-pin package configuration. To avoid degrada- (with zero output current). With increasing switch current, tion of load regulation, Kelvin connections should be made the supply current (during switch on-time) increases at a to the ground pin. This is done on the TO-3 package by rate approximately 1/40 of switch current, corresponding tying one end of the package to power ground and the other to a forced hFE of 40 for the switch. end to the feedback divider resistor (analog ground). This Undervoltage lockout is incorporated on the LT1070 by is illustrated in Figure 2. sensing saturation of the lateral PNP pass transistor which For best load regulation, the resistance in the switch drives an internal 2.3V regulator. A remote collector on current path must be kept low. 0.01Ω of wire resistance this transistor conducts current and locks out the switch creates 50mV drop at 5A switch current. This is a 1% for input voltages below 2.5V. No hysteresis is used to change in a 5V output, and actually causes the output to maximize the useful range of input voltage. Operating the increase with increasing load current. regulator right at the 2.5V threshold may result in a “burping” action as the LT1070 turns on and off in With the TO-220 package, (Figure 3) connect the feedback response to wobbles in input voltage, but this will not harm resistor directly to the ground pin with a separate wire if no the device. External undervoltage lockout can be added if case connection is made. The case can be used as a second it is desirable to raise the threshold voltage. The circuit ground pin if desired. shown in Figure 1 is one example of how to implement Avoid long wire runs to the ground pin to minimize load this. regulation effects and inductive voltages created by the The threshold of this circuit is approximately VZ + 1.5V. high di/dt switch current. A ground plane will keep EMI to Below that voltage, D2 pulls the VC pin low to shut off the a minimum regulator. VIN LT1070 V FB Z GND VC PIN D2 SWITCH CURRENT FEEDBACK PATH—KEEP DIVIDER RESISTANCE TO A MINIMUM 1.5k LOAD CURRENT TO OUTPUT (+) PATH—KEEP RESISTANCE TO A MINIMUM AN19 F01 –VIN TO OUTPUT GND AN19 F02
Figure 1. External Undervoltage Lockout Figure 2
an19fc AN19-8
