Electronic Components Datasheet Search |
|
LT3505IDD-PBF Datasheet(PDF) 11 Page - Linear Technology |
|
LT3505IDD-PBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LT3505 11 3505fc Finite transistor bandwidth limits the speed at which the power switch can be turned on and off, effectively setting theminimumon-timeoftheLT3505.Foragivenoutputvolt- age,theminimumon-timedeterminesthemaximuminput voltage to remain in continuous mode operation, VIN(PS). See the “Input Voltage Range” section of the datasheet for more information on determining VIN(PS). For switching frequencies below 750kHz, operation above VIN(PS) (up to 40V) is safe provided that the system will tolerate the pulse-skipping behavior outlined in the “Minimum On Time” section of the datasheet. At switching frequencies exceeding 750kHz, edge loss limits operation to input voltages below VIN(PS). Finite transition time results in a small amount of power dissipation each time the power switch turns on and off (edge loss). Edge loss increases with frequency, switch current, and input voltage. Input Voltage Frequency Foldback In constant frequency operation (below VIN(PS)) edge loss only reduces the application efficiency. However, at high switching frequencies exceeding 750kHz and input voltagesexceedingVIN(PS),thepartoperatesinpulse-skip- ping mode and the switch current can increase above the current limit of the part, 1.75A. This further increases the power dissipated during switch transitions and increases die temperature. To remedy the situation a single resis- tor (R5) and a zener diode (D3) can be added to a typical LT3505 circuit as shown in Figure 4. When the input voltage is below 16V, the zener diode path conducts no current and the current flowing out of the RT pin (and through R4) is nominally 0.5V/20k = 25µA, which programs a 2.2MHz switching frequency. As the input voltage is increased above 16V, the zener diode begins to conduct and gradually reduces the cur- rent flowing out of the RT pin. This mechanism reduces the switching frequency as the input voltage is increased above 16V (up to 36V) to ensure that the part constantly operates in continuous mode without skipping pulses, thereby preventing the excessive die temperature rise encountered in pulse-skipping mode. Although the circuit can be operated indefinitely above VZENER, this frequency foldback method is intended to protect circuits during temporary periods of high input voltage. For example, in many automotive systems, the normal operating input range might be 9V to 16V, and the LT3505 can be programmed to operate above the AM band (>1.8MHz). At the same time, the circuit must be able to withstand higher input voltages due to load dump or double-battery jump starts. During these brief periods, it is usually acceptable to switch at a frequency within the AM band. Iftheoutputisshortedwhiletheinputvoltageisgreaterthan VZENER, the switching frequency will be reduced to 30kHz and the part will not be able to recover from the short until theinputvoltageisreducedbelowVZENER(seethefollowing discussion). APPLICATIONS INFORMATION Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit VIN 6.7V TO 36V ON OFF C3 0.1 µF L1 6.8 µH D2 1N4148 R1 61.9k C1 10 µF 3505 F04 C5 22pF C4 22pF C2 1 µF R4 20.0k R5 806k R2 11.5k VOUT 5V R3 100k VIN BOOST GND FB VC SHDN SW LT3505 RT D3 BZT52C16T D1 MBRM140 16V Input Voltage [V] 0 0 0.50 0.75 1.00 2.25 1.50 10 20 25 0.25 1.75 2.00 2.50 1.25 5 15 30 35 40 LTC3505 • F04b Switching Frequency Maximum Load Current |
Similar Part No. - LT3505IDD-PBF |
|
Similar Description - LT3505IDD-PBF |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |