LT1070 Design Manual (Linear Technology) - 6
| 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 / 6 — New Switch Current Specification. High Supply Voltages. Discontinuous … |
| File Format / Size | PDF / 656 Kb |
| Document Language | English |
New Switch Current Specification. High Supply Voltages. Discontinuous “Oscillations” (Ringing)
Text Version of Document
Application Note 19
New Switch Current Specification
VIN-to-ground pin voltage exceeds 40V during this state because R The LT1070 was specified at 5A peak switch current, for Z is too large to allow harmful currents to flow. duty cycles of 50% or less. At higher duty cycles the peak Some attention needs to be paid to CZ. The LT1070 is very current was limited to 4A. This abrupt change in specifica- tolerant of noise and ripple on the VIN pin, but CZ may be tion at 50% duty cycle was bothersome because many necessary in some applications. The problem is that D1 designs operate near 50% duty cycle and require maxi- must charge CZ when power is applied. If power comes up mum possible output power. To solve this problem, very rapidly, D1 might exceed its one cycle surge rating. switch current limit on new data sheets will be specified as a linearly decreasing function, from 5A at 50% duty cycle V+ to 4A at 80% duty cycle. The LT1071 and LT1072 will also be specified this way. R D1 Z ≈ 1k 20V 0.5W
High Supply Voltages
+ CIN It has become apparent that many applications for the VIN VSW LT1070 have maximum input voltages which exceed 40V. LT1070 The straightforward approach is to use the “HV” devices CZ FB (OPTIONAL) that are specified at 60V, but in some cases the standard GND part can be used at lower cost simply by dropping supply voltage with a Zener diode as shown. The LT1070 supply AN19 F00 pin (VIN) requires only a few volts to operate, so in most cases the unregulated input voltage range is not compro- mised with this Zener. Zener dissipation can be calculated
Discontinuous “Oscillations” (Ringing)
from IZ ≈ 6mA + ISW(0.0015 + DC/40): Many customers have called about oscillations occurring I on the switch pin during a portion of the switch “off” time. SW = LT1070 average switch current during “on” time DC = duty cycle These are not oscillations. They are a damped ringing caused by the transition to a zero-current state in the For ISW = 4A, DC = 30%; IZ = 42mA inductor or transformer primary. At light loads, or with low A 20V Zener would dissipate (20)(42) = 0.84W. Note that inductance values, inductor current will drop to zero this power would be dissipated anyway in the LT1070, so during switch off time. This causes the inductor voltage to no loss in efficiency occurs. The resistor, RZ, is necessary collapse toward zero. In doing so, however, energy is for start-up. Without it, a latch-off condition exists where transferred back to the inductor from the parasitic capaci- the VIN pin sits more than 16V negative with respect to the tance of the switch, inductor, and catch diode. The induc- switch pin. If the LT1070 is not switching and the FB pin tor and capacitance form a parallel resonant tank which is below 0.5V, the LT1070 is in the “isolated flyback” mode “rings.” This ringing is not harmful as long as its peak where it is trying to regulate the VIN -to-VSW voltage. When amplitude does not result in a negative voltage on the this voltage exceeds 16V, the regulator thinks it should switch pin. It can be damped, if desired, by paralleling the reduce duty cycle to zero, resulting in a permanent “no- inductor/primary with a series R/C damper, typically 100Ω switching” state. RZ forces the VIN pin to rise enough to to 1kΩ, and 500pF to 5000pF. Typical undamped ringing initiate start-up. The user need not be concerned that the frequency is 100kHz to 1MHz. an19fc AN19-6
New Switch Current Specification
VIN-to-ground pin voltage exceeds 40V during this state because R The LT1070 was specified at 5A peak switch current, for Z is too large to allow harmful currents to flow. duty cycles of 50% or less. At higher duty cycles the peak Some attention needs to be paid to CZ. The LT1070 is very current was limited to 4A. This abrupt change in specifica- tolerant of noise and ripple on the VIN pin, but CZ may be tion at 50% duty cycle was bothersome because many necessary in some applications. The problem is that D1 designs operate near 50% duty cycle and require maxi- must charge CZ when power is applied. If power comes up mum possible output power. To solve this problem, very rapidly, D1 might exceed its one cycle surge rating. switch current limit on new data sheets will be specified as a linearly decreasing function, from 5A at 50% duty cycle V+ to 4A at 80% duty cycle. The LT1071 and LT1072 will also be specified this way. R D1 Z ≈ 1k 20V 0.5W
High Supply Voltages
+ CIN It has become apparent that many applications for the VIN VSW LT1070 have maximum input voltages which exceed 40V. LT1070 The straightforward approach is to use the “HV” devices CZ FB (OPTIONAL) that are specified at 60V, but in some cases the standard GND part can be used at lower cost simply by dropping supply voltage with a Zener diode as shown. The LT1070 supply AN19 F00 pin (VIN) requires only a few volts to operate, so in most cases the unregulated input voltage range is not compro- mised with this Zener. Zener dissipation can be calculated
Discontinuous “Oscillations” (Ringing)
from IZ ≈ 6mA + ISW(0.0015 + DC/40): Many customers have called about oscillations occurring I on the switch pin during a portion of the switch “off” time. SW = LT1070 average switch current during “on” time DC = duty cycle These are not oscillations. They are a damped ringing caused by the transition to a zero-current state in the For ISW = 4A, DC = 30%; IZ = 42mA inductor or transformer primary. At light loads, or with low A 20V Zener would dissipate (20)(42) = 0.84W. Note that inductance values, inductor current will drop to zero this power would be dissipated anyway in the LT1070, so during switch off time. This causes the inductor voltage to no loss in efficiency occurs. The resistor, RZ, is necessary collapse toward zero. In doing so, however, energy is for start-up. Without it, a latch-off condition exists where transferred back to the inductor from the parasitic capaci- the VIN pin sits more than 16V negative with respect to the tance of the switch, inductor, and catch diode. The induc- switch pin. If the LT1070 is not switching and the FB pin tor and capacitance form a parallel resonant tank which is below 0.5V, the LT1070 is in the “isolated flyback” mode “rings.” This ringing is not harmful as long as its peak where it is trying to regulate the VIN -to-VSW voltage. When amplitude does not result in a negative voltage on the this voltage exceeds 16V, the regulator thinks it should switch pin. It can be damped, if desired, by paralleling the reduce duty cycle to zero, resulting in a permanent “no- inductor/primary with a series R/C damper, typically 100Ω switching” state. RZ forces the VIN pin to rise enough to to 1kΩ, and 500pF to 5000pF. Typical undamped ringing initiate start-up. The user need not be concerned that the frequency is 100kHz to 1MHz. an19fc AN19-6
