Electronic Components Datasheet Search |
|
LT1107MJ8 Datasheet(PDF) 11 Page - Linear Technology |
|
LT1107MJ8 Datasheet(HTML) 11 Page - Linear Technology |
11 / 12 page 11 LT1107 Figure 7 details current limit circuitry. Sense transistor A1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2’s collector current flows in Q1’s collector. This current is passed through internal 80 Ω resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN sets the current limit. When suffi- cient switch current flows to develop a VBE across R1 + RLIM, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 800ns. The current trip point becomes less accurate for switch ON times less than 3 µs. Resistor values programming switch ON time for 800ns or less will cause spurious response in the switch circuitry although the device will still maintain output regulation. Using the Gain Block The gain block (GB) on the LT1107 can be used as an error amplifier, low-battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 1.25V reference. The positive input comes out on the SET pin. Arrangement of the gain block as a low-battery detector is straightforward. Figure 8 shows hookup. R1 and R2 need only be low enough in value so that the bias current of the SET input does not cause large errors. 33k for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to “snap” when the trip point is reached. Values in the 1M to 10M range are optimal. The addition of R3 will change the trip point, however. Output ripple of the LT1107, normally 50mV at 5VOUT can be reduced significantly by placing the gain block in front of the FB input as shown in Figure 9. This effectively reduces the comparator hysteresis by the gain of the gain block. Output ripple can be reduced to just a few millivolts using this technique. Ripple reduction works with step- down or inverting modes as well. For this technique to be effective, output capacitor C1 must be large, so that each switching cycle increases VOUT by only a few millivolts. 1000 µF is a good starting value. C1 should be a low ESR type as well. S APPLICATI I FOR ATIO 1107 F07 SW2 SW1 DRIVER OSCILLATOR VIN ILIM R1 80 Ω (INTERNAL) RLIM (EXTERNAL) Q3 Q2 Q1 Figure 7. LT1107 Current Limit Circuitry L1 1107 F09 D1 R3 270k + VOUT R2 R1 C1 VBAT LT1107 GND SW2 SET SW1 ILIM VIN FB AO VOUT = + 1 1.25V R2 R1 ()( ) Figure 9. Output Ripple Reduction Using Gain Block Figure 8. Setting Low-Battery Detector Trip Point 1107 F08 VBAT R1 R2 1.25V REF SET GND VIN LT1107 47k 5V TO PROCESSOR AO R3 R1 = VLB = BATTERY TRIP POINT R2 = 33k R3 = 1.6M () VLB – 1.25V 35.1 µA + – Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. |
Similar Part No. - LT1107MJ8 |
|
Similar Description - LT1107MJ8 |
|
|
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 |