Datenblatt-Suchmaschine für elektronische Bauteile |
|
LM34910 Datenblatt(PDF) 8 Page - National Semiconductor (TI) |
|
|
LM34910 Datenblatt(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page ON-Time Timer, and Shutdown (Continued) Current Limit Current limit detection occurs during the off-time by monitor- ing the recirculating current through the free-wheeling diode (D1). Referring to Figure 1, when the buck switch is turned off the inductor current flows through the load, into S GND, through the sense resistor, out of I SEN and through D1. If that current exceeds 1.25A the current limit comparator output switches to delay the start of the next on-time period if the voltage at FB is below 2.5V. The next on-time starts when the current out of I SEN is below 1.25A and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 1.25A during each on-time, that is detected at the beginning of each off-time. The operating frequency may be lower due to longer-than-normal off-times. Figure 7 illustrates the inductor current waveform. During normal operation the load current is Io, the average of the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches 1.25A. During the Current Limited portion of Figure 7, the current ramps down to 1.25A during each off-time, initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an amount equal to: ∆I=(V IN -VOUT)xtON /L1 During this time the LM34910 is in a constant current mode, with an average load current (I OCL) equal to 1.25A + ∆I/2. The current limit threshold can be increased by connecting an external resistor between S GND and ISEN. The external resistor will typically be less than 1 Ω. The peak current out of SW and I SEN must not exceed 3.5A. The average current out of SW must be less than 3A, and the average current out of I SEN must be less than 2A. Therefore IPK in Figure 7 must not exceed 3.5A, and I OCL must not exceed 2A. N - Channel Buck Switch and Driver The LM34910 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak cur- rent allowed through the buck switch is 3.5A, and the maxi- mum allowed average current is 3A. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022 µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately -1V, and C4 charges from V CC through the internal diode. The minimum off-time of 280 ns ensures a minimum time each cycle to recharge the bootstrap capaci- tor. Softstart The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, after V CC reaches the under-voltage threshold, an internal 11.5 µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and the non- inverting input of the regulation comparator) ramps up the output voltage in a controlled manner. An internal switch grounds the SS pin if V CC is below the under-voltage lockout threshold, if a thermal shutdown oc- curs, or if the R ON/SD pin is grounded. 20110913 FIGURE 6. Shutdown Implementation 20110914 FIGURE 7. Inductor Current - Current Limit Operation www.national.com 8 |
Ähnliche Teilenummer - LM34910 |
|
Ähnliche Beschreibung - LM34910 |
|
|
Link URL |
Privatsphäre und Datenschutz |
ALLDATASHEETDE.COM |
War ALLDATASHEET hilfreich? [ DONATE ] |
Über Alldatasheet | Werbung | Kontakt | Privatsphäre und Datenschutz | Linktausch | Hersteller 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 |