 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
LT3420 Datasheet(PDF) 8 Page - Linear Technology |
|
||||||||||||||||||||||||||||||
LT3420 Datasheet(HTML) 8 Page - Linear Technology |
|
|
8 / 20 page  LT3420/LT3420-1 8 3420fa Overview The following text focuses on the operation of the LT3420. The operation of the LT3420-1 is nearly identical with the differences discussed at the end of this section. The LT3420 uses an adaptive on-time/off-time control scheme to provide excellent efficiency and precise control of switching currents. Please refer to Figure 3 for the fol- lowing overview of the part’s operation. At any given instant, the master latch determines which mode the LT3420 is in: “charging” or “refresh”. In charging mode, the circuitry enclosed by the smaller dashed box is enabled, providing power to charge photoflash capacitor C1. The output volt- age is monitored via the flyback pulse on the primary of the transformer. When the target output voltage is reached, the charging mode is terminated and the part enters the refresh mode. In refresh mode, the power delivery block is disabled, reducing quiescent current, while the refresh timer is en- abled. The refresh timer simply generates a user program- mable delay, after which the part reenters the charging mode. Once in the charging mode, the LT3420 will again provide power to the output until the target voltage is reached. Figure 5 is an oscillograph photo showing both the initial charging of the photoflash capacitor and the subse- quent refresh action. The upper waveform is the output voltage. The middle waveform is the voltage on the CT pin. The lower waveform shows the input current. The mode of the part is indicated below the photo. The user can defeat the refresh timer and force the part into charging mode by toggling the CHARGE pin OPERATIO (high →low→high). The low to high transition on the CHARGE pin fires a one-shot that sets the master latch, putting the part in charging mode. Bringing CHARGE low puts the part in shutdown. The refresh timer can be programmed to wait indefinitely by simply grounding the CT pin. In this configuration, the LT3420 will only reenter the charging mode by toggling the CHARGE pin. Power Delivery Block The power delivery block consists of all circuitry enclosed by the smaller dashed box in Figure 3. This circuit block contains all elements needed for charging and output voltage detection. To better understand the circuit opera- tion, follow the subsequent description of one cycle of operation and refer to Figure 6. Assume that initially there is no current in the primary or secondary of the trans- former, so the output of comparator A1 is low, while that of A2 is high (note the small offset voltages at the inputs of A1 and A2). The SR latch is thus set and the power NPN switch, Q1, is turned on. Current increases linearly in the primary of the transformer at a rate determined by the VBAT voltage and the primary inductance of the transformer. As the current builds up, the voltage across the 14m Ω resistor increases. When this voltage exceeds the 20mV offset voltage of A1, the output of A1 goes high, resetting the SR latch and turning off Q1. The current needed to reset the latch is approximately 1.4A (~20mV/14m Ω). When Q1 turns off, the secondary side current quickly jumps from zero current to the primary side current di- vided by N (the turns ratio of transformer T1). In this ex- ample, the peak secondary current is 116mA (1.4A/12). Diode D1 now conducts, providing power to the output. Since a positive voltage exists across the secondary wind- ing of the transformer, the secondary current decreases linearly at a rate determined by the secondary inductance and the output voltage (neglecting the diode voltage drop). When the secondary side current drops below 40mA (10mV/0.25 Ω), the output of A2 goes high, setting the SR latch and turning on Q1. The initial primary current is sim- ply the minimum secondary current times N, in this case 0.48A (40mA • 12) . Q1 will now remain on until the pri- mary current again reaches 1.4A. This cycle of operation repeats itself, automatically adjusting the On and Off times Figure 5. Demonstrating 3 Operating Modes of LT3420: Shutdown, Charging and Refresh of Photoflash Capacitor VOUT 100V/DIV MODE IIN 1A/DIV VCT 1V/DIV 1s/DIV 3420 F05 SHUTDOWN CHARGING REFRESH |
Similar Part No. - LT3420 |
|
Similar Description - LT3420 |
|
|
|
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 |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
