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SC1186CSW

PROGRAMMABLE SYNCHRONOUS DC/DC CONVERTER, DUAL LOW DROPOUT REGULATOR CONTROLLER

Semtech Corporation 

PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER, DUAL LOW DROPOUT

REGULATOR CONTROLLER

SC1186

PRELIMINARY - December 2, 1999

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

excursions.

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

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

supplies, the output capacitors must supply all transient allow 10% of maximum output current as ripple current.

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

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

ESR is therefore one of the most important criteria. The pacitor ESR. Ripple current can be calculated from:

maximum ESR can be simply calculated from:

RESR

≤

Vt

It

Where

I = LRIPPLE

VIN

4⋅L⋅ fOSC

Ripple current allowance will define the minimum per-

mitted inductor value.

Vt = Maximum transient voltage excursion

POWER FETS - The FETs are chosen based on sev-

It = Transient current step

eral 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 bottom FET body diode recovery losses.

three available capacitor technologies.

a) Conduction losses are simply calculated as:

Each Capacitor

Total

PCOND = I2O ⋅ RDS(on) ⋅ δ

Technology

Low ESR Tantalum

OS-CON

Low ESR Aluminum

C ESR Qty. C ESR

(µF) (mΩ) Rqd. (µF) (mΩ)

330

60 6 2000 10

330

25 3 990 8.3

1500

44 5 7500 8.8

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 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:

( ) L ≤ RESR C

It

VIN − VO

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 = Q RR ⋅ VIN ⋅ fOSC

The calculated maximum inductor value assumes 100% To a first order approximation, it is convenient to only

duty cycle, so some allowance must be made. Choosing consider conduction losses to determine FET suitability.

an inductor value of 50 to 75% of the calculated maxi- For a 5V in; 2.8V out at 14.2A requirement, typical FET

mum will guarantee that the inductor current will ramp losses would be:

© 1999 SEMTECH CORP.

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