Electronic Components Datasheet Search |
|
LTC3900 Datasheet(PDF) 9 Page - Linear Technology |
|
LTC3900 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC3900 9 3900i APPLICATIO S I FOR ATIO Current Sense The differential input current sense comparator is used for sensing the voltage across the drain-to-source terminals of Q4 through the CS+ and CS– pins. If the inductor current reverses into the Q4 causing CS+ to rise above CS– by more than 10.5mV, the LTC3900 pulls CG low. This comparator is used to prevent inductor reverse current buildup during power down or Burst Mode operation, which may cause damage to the MOSFET. The 10.5mV input threshold has a positive temperature coefficient, which closely matches the TC of the external MOSFET RDS(ON). The current sense comparator is only active 250ns after CG goes high; this is to avoid any ringing immediately after Q4 is switched on. Under light load conditions, if the inductor average current is less than half of its peak-to-peak ripple current, the inductor current will reverse into Q4 during a portion of the switching cycle, forcing CS+ to rise above CS–. The current sense comparator input threshold is set at 10.5mV to prevent tripping under light load conditions. If the product of the inductor negative peak current and MOSFET RDS(ON) is higher than 10.5mV, the LTC3900 will operate in discon- tinuous current mode. Figure 6 shows the LTC3900 oper- ating in discontinuous current mode; the CG output goes low before the next negative SYNC pulse, as soon as the inductor current becomes negative. Discontinuous cur- rent mode is sometimes undesirable. To disable discon- tinuous current mode operation, add a resistor divider, RCS1 and RCS2 at the CS+ pin to increase the 10.5mV threshold so that the LTC3900 operates in continuous mode at no load. The LTC3900 CS+ pin has an internal current sinking clamp circuit (ZCS in the Block Diagram) that clamps the pin to 11V. The clamp circuit is to be used together with the external series resistor, RCS1 to protect the CS+ pin from high Q4 drain voltage in the power transfer cycle. During the power transfer cycle, Q4 is off, the drain voltage of Q4 is determined by the primary input voltage and the trans- former turns ratio. This voltage can be high and may damage the LTC3900 if CS+ is connected directly to the drain of Q4. The current sinking capability of the clamp circuit is 5mA minimum. The value of the resistors, RCS1, RCS2 and RCS3 should be calculated using the following formulas to meet both the threshold and clamp voltage requirements: k = {48 • IRIPPLE • RDS(ON)} –1 RCS2 = {200 • VIN(MAX) • NS/NP –2200 • (1 + k)} /k RCS1 = k • RCS2 RCS3 = {RCS1 • RCS2} / {RCS1 + RCS2} If k = 0 or less than zero, RCS2 is not needed and RCS1 = RCS3 = {VIN(MAX) • (NS/NP) – 11V} / 5mA where: IRIPPLE = Inductor peak-to-peak ripple current RDS(ON) = On-resistance of Q4 at IRIPPLE/2 VIN(MAX) = Primary side main supply maximum input voltage NS/NP = Power transformer T1, turn ratio SG SYNC FG CG 3900 F06a INDUCTOR CURRENT 0A CURRENT SENSE COMPARATOR TRIP SYNC FG CG 3900 F06b INDUCTOR CURRENT 0A ADJUSTED CURRENT SENSE THRESHOLD Figure 6a. Discontinuous Current Mode Operation at No Load Figure 6b. Continuous Current Mode Operation with Adjusted Current Sense Threshold |
Similar Part No. - LTC3900 |
|
Similar Description - LTC3900 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | 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 |