 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
IS31LT3910-GRLS2-TR Datasheet(PDF) 6 Page - Integrated Silicon Solution, Inc |
|
||||||||||||||||||||||||||||||
IS31LT3910-GRLS2-TR Datasheet(HTML) 6 Page - Integrated Silicon Solution, Inc |
|
|
6 / 10 page  IS31LT3910 Integrated Silicon Solution, Inc. – www.issi.com 6 Rev. B, 05/12/2014 APPLICATION INFORMATION The IS31LT3910 is optimized to drive buck LED drivers using peak current mode control. This control method provides fairly accurate LED current control without the need for a high side current sensing or the design of any additional loop compensation. The IC uses very few external components and provides for either Linear or PWM control of the LED current. A capacitor connected to the TOFF pin programs the off-time of the oscillator inside. The oscillator produces pulses at regular intervals. These pulses set the SR flip-flop in the IS31LT3910 which causes the GATE driver to turn on. When the MOSFET turns on, the current through the inductor starts ramping up. This current flows through the external sense resistor RCS and produces a ramp voltage at the CS pin. The comparators are constantly comparing the CS pin voltage to both the voltage at the LD pin and the internal 240mV. Once the blanking time is complete, the output of these comparators is allowed to reset the flip flop. When the output of either one of the two comparators goes high, the flip flop is reset and the GATE output goes low. If neither of the comparator goes high, the GATE keeps high. Assuming a 30% ripple in the inductor, the current sense resistor RCS can be set by using: (A) I . ) V(orV . R LED LD CS 15 1 24 0 A constant off-time peak current control scheme can easily operate at duty cycles greater than 0.5 and also gives inherent input voltage rejection making the LED current almost insensitive to input voltage variations. INPUT VOLTAGE REGULATOR When a voltage is applied through a suitable input resistor to the VIN pin, the IS31LT3910 maintains a constant 7.1V (Typ.) at the VIN pin. This voltage is used to power the IC. The VIN pin must be bypassed by a low ESR capacitor to provide a low impedance path for the high frequency current of the output GATE driver. The input current drawn from the VIN pin is a sum of the 0.5mA (Typ) current drawn by the internal circuit and the current drawn by the GATE driver (which in turn depends on the switching frequency and the GATE charge of the external MOSFET). S G IN f Q mA I 5 . 0 In the above equation, fS is the switching frequency and QG is the GATE charge of the external MOSFET (which can be obtained from the datasheet of the MOSFET). CURRENT SENSE The current sense input of the IS31LT3910 goes to the non-inverting inputs of two comparators. The inverting terminal of oe comparator is tied to an internal 240mV reference whereas the inverting terminal of the other comparator is connected to the LD pin. The outputs of both these comparators are fed into an OR GATE and the output of the OR GATE is fed into the reset pin of the flip-flop. Thus, the comparator which has the lowest voltage at the inverting terminal determines when the GATE output is turned off. The outputs of the comparators also include a typical 480ns blanking time which prevents spurious turn- offs of the external MOSFET due to the turn-on spike normally present in peak current mode control. In rare cases, this internal blanking time might not be enough to filter out the turn-on spike. In these cases, an external RC filter needs to be added between the external sense resistor (RCS) and the CS pin. Please note that the comparators are relatively fast with a typical 80ns response time. A proper layout minimizing external inductances will prevent false triggering of these comparators. OSCILLATOR The oscillator in the IS31LT3910 is controlled by a single capacitor connected at the TOFF pin. The equation governing the tOFF time of oscillation period is given by: ) 10 1 ( 10 51 . 0 ) ( 6 _ pF C s t OFF TIME OFF LINEAR DIMMING The Linear Dimming pin is used to control the LED current. An external voltage ranging from 50mV to 240mV can be applied to the LD pin to adjust the LED current during operation. There are two cases when it may be necessary to use the Linear Dimming pin. In some cases, it may not be possible to find the exact RCS value required to obtain the LED current when the internal 240mV is used. In these cases, an external voltage divider from the VIN pin can be connected to the LD pin to obtain a voltage (less than 240mV) corresponding to the desired voltage across RCS. Linear dimming may be desired to adjust the current level to reduce the brightness of the LEDs. Connecting a resistor between the VREF pin and the LD pin, and also connecting an NTC thermistor between the LD pin and ground (refer to Application Circuit), the IS31LT3910 is able to realize the temperature compensation function (See more detail |
Similar Part No. - IS31LT3910-GRLS2-TR |
|
Similar Description - IS31LT3910-GRLS2-TR |
|
|
|
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
