Electronic Components Datasheet Search |
|
SC1185ACSW Datasheet(PDF) 10 Page - Semtech Corporation |
|
SC1185ACSW Datasheet(HTML) 10 Page - Semtech Corporation |
10 / 12 page PROGRAMMABLE SYNCHRONOUS DC/DC CONVERTER, DUAL LOW DROPOUT REGULATOR CONTROLLER © 1999 SEMTECH CORP. PRELIMINARY - January 6, 1999 652 MITCHELL ROAD NEWBURY PARK CA 91320 SC1185 SC1185A 10 COMPONENT SELECTION SWITCHING SECTION OUTPUT CAPACITORS - Selection begins with the most critical component. Because of fast transient load current requirements in modern microprocessor core supplies, the output capacitors must supply all transient load current requirements until the current in the output inductor ramps up to the new level. Output capacitor ESR is therefore one of the most important criteria. The maximum ESR can be simply calculated from: step current Transient I excursion voltage transient Maximum V Where I V R t t t t ESR = = ≤ Each Capacitor Total Technology C (µF) ESR (m Ω) Qty. Rqd. C (µF) ESR (m Ω) Low ESR Tantalum 330 60 6 2000 10 OS-CON 330 25 3 990 8.3 Low ESR Aluminum 1500 44 5 7500 8.8 () O IN t ESR V V I C R L − ≤ OSC IN L f L 4 V I RIPPLE ⋅ ⋅ = IN O ) on ( DS 2 O COND V V cycle duty = where R I P ≈ δ δ ⋅ ⋅ = 2 IN O SW 10 V I P − ⋅ ⋅ = 4 f ) t t ( V I P OSC f r IN O SW ⋅ + ⋅ ⋅ = OSC IN RR RR f V Q P ⋅ ⋅ = For example, to meet a 100mV transient limit with a 10A load step, the output capacitor ESR must be less than 10m Ω. To meet this kind of ESR level, there are three available capacitor technologies. The choice of which to use is simply a cost/perfor- mance issue, with Low ESR Aluminum being the cheapest, but taking up the most space. INDUCTOR - Having decided on a suitable type and value of output capacitor, the maximum allowable value of inductor can be calculated. Too large an in- ductor will produce a slow current ramp rate and will cause the output capacitor to supply more of the tran- sient load current for longer - leading to an output volt- age sag below the ESR excursion calculated above. The maximum inductor value may be calculated from: The calculated maximum inductor value assumes 100% duty cycle, so some allowance must be made. Choosing an inductor value of 50 to 75% of the calculated maxi- mum will guarantee that the inductor current will ramp fast enough to reduce the voltage dropped across the ESR at a faster rate than the capacitor sags, hence en- suring a good recovery from transient with no additional excursions. We must also be concerned with ripple current in the output inductor and a general rule of thumb has been to allow 10% of maximum output current as ripple current. Note that most of the output voltage ripple is produced by the inductor ripple current flowing in the output capac- itor ESR. Ripple current can be calculated from: Ripple current allowance will define the minimum permit- ted inductor value. POWER FETS - The FETs are chosen based on several criteria with probably the most important being power dissipation and power handling capability. TOP FET - The power dissipation in the top FET is a combination of conduction losses, switching losses and bottom FET body diode recovery losses. a) Conduction losses are simply calculated as: b) Switching losses can be estimated by assuming a switching time, if we assume 100ns then: or more generally, c) Body diode recovery losses are more difficult to esti- mate, but to a first approximation, it is reasonable to as- sume that the stored charge on the bottom FET body diode will be moved through the top FET as it starts to turn on. The resulting power dissipation in the top FET will be: To a first order approximation, it is convenient to only consider conduction losses to determine FET suitability. For a 5V in; 2.8V out at 14.2A requirement, typical FET losses would be: |
Similar Part No. - SC1185ACSW |
|
Similar Description - SC1185ACSW |
|
|
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 |