SC1109 Просмотр технического описания (PDF) - Semtech Corporation

Номер в каталоге

Компоненты Описание

производитель

SC1109

SYNCHRONOUS PWM CONTROLLER WITH DUAL LOW DROPOUT REGULATOR CONTROLLERS

Semtech Corporation 

SYNCHRONOUS PWM CONTROLLER WITH

DUAL LOW DROPOUT REGULATOR

CONTROLLERS

SC1109

PRELIMINARY - October 16, 2000

COMPONENT SELECTION

fast enough to reduce the voltage dropped across the

ESR at a faster rate than the capacitor sags, hence en-

SWITCHING SECTION

suring a good recovery from transient with no additional

OUTPUT CAPACITORS - Selection begins with the most excursions.

critical component. Because of fast transient load current We must also be concerned with ripple current in the

requirements in modern microprocessor core supplies, output inductor and a general rule of thumb has been to

the output capacitors must supply all transient load cur- allow 10% of maximum output current as ripple current.

rent requirements until the current in the output inductor Note that most of the output voltage ripple is produced

ramps up to the new level. Output capacitor ESR is by the inductor ripple current flowing in the output capac-

therefore one of the most important criteria. The maxi- itor ESR. Ripple current can be calculated from:

mum ESR can be simply calculated from:

RESR

≤

Vt

It

Where

I = LRIPPLE

VIN

4⋅L⋅ fOSC

Ripple current allowance will define the minimum permit-

ted inductor value.

Vt = Maximumtransientvoltageexcursion POWER FETS - The FETs are chosen based on several

It = Transientcurrentstep

criteria with probably the most important being power

dissipation and power handling capability.

For example, to meet a 100mV transient limit with a

TOP FET - The power dissipation in the top FET is a

10A load step, the output capacitor ESR must be less combination of conduction losses, switching losses and

than 10mΩ. To meet this kind of ESR level, there are

three available capacitor technologies.

bottom FET body diode recovery losses.

a) Conduction losses are simply calculated as:

Each Capacitor

Total

Technology

Low ESR Tantalum

C ESR Qty. C ESR

(µF) (mΩ) Rqd. (µF) (mΩ)

330

60 6 2000 10

OS-CON

330

25 3 990 8.3

Low ESR Aluminum 1500

44 5 7500 8.8

PCOND = IO2 ⋅ RDS(on) ⋅ δ

where

δ = duty cycle ≈ VO

VIN

b) Switching losses can be estimated by assuming a

switching time, if we assume 100ns then:

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.

PSW = IO ⋅ VIN ⋅ 10 −2

or more generally,

INDUCTOR - Having decided on a suitable type and

value of output capacitor, the maximum allowable value

of inductor can be calculated. Too large an inductor will

produce a slow current ramp rate and will cause the

output capacitor to supply more of the transient load

current for longer - leading to an output voltage sag

below the ESR excursion calculated above.

The maximum inductor value may be calculated from:

( ) L ≤ RESR C

It

VIN − VO

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

PSW

=

IO ⋅ VIN ⋅ (tr + t f ) ⋅ fOSC

4

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:

PRR = QRR ⋅ VIN ⋅ fOSC

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:

© 2000 SEMTECH CORP.

15

TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com

Share Link: