LTM4677 datasheet(42/116 Pages) AD

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LTC3883/LTC3883-1

Rev. F

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APPLICATIONS INFORMATION

To ensure that the application will deliver full load current

over the full operating temperature range, be sure to pick

the optimum ILIMIT value accounting for errors in the DCR

versus the MFR_IOUT_CAL_GAIN parameter entered.

Next, determine the DCR of the inductor. Where provided,

use the manufacturer’s maximum value, usually given

at 20°C. Increase this value to account for errors in the

temperature sensing element of 3°C to 5°C and any

additional errors associated with the proximity of the

temperature sensor element to the inductor.

C1 is usually selected to be in the range of 0.047µF to

4.7µF. This forces (R1 + R3)||R2 to be approximately 2k.

Adding optional elements R3 and C2 shown in Figure 18a

will minimize offset errors associated with the ISNS leak-

age currents. Set R3 equal to the value of R1. Set C2 to a

value of 1µF or greater to ensure adequate noise filtering.

The equivalent resistance (R1 + R3)||R2 is scaled to the

room temperature inductance and maximum DCR:

R1+ R3

(

)||R2 =

2 • L

DCR at 20°C

(

) • C1

The sense resistor values are:



R1= R3; R1=

R1 R2

; R2 =

R1• RD

1– RD

The maximum power loss in R1 is related to the duty

cycle, and will occur in continuous mode at the maximum

input voltage:

PLOSSR1=

VIN(MAX) – VOUT

(

) • VOUT

Ensure that R1 has a power rating higher than this value.

If high efficiency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing or

sense resistors. Light load power loss can be modestly

higher with a DCR network than with a sense resistor

due to the extra switching losses incurred through R1.

However, DCR sensing eliminates a sense resistor, reduc-

ing conduction losses and provides higher efficiency at

heavy loads. Peak efficiency is about the same with either

method. Selecting Burst Mode operation or discontinuous

mode will improve the converter efficiency at light loads

regardless of the current sensing method.

To maintain a good signal-to-noise ratio for the current

sense signal, use a minimum ∆VSENSE of 10mV to 15mV.

For a DCR sensing application, the actual ripple voltage

will be determined by the equation:

∆VSENSE =

VIN – VOUT

R1• C1

•

VOUT

VIN • fOSC

SLOPE COMPENSATION AND INDUCTOR PEAK

CURRENT

Slope compensation provides stability in constant

frequency current mode architectures by preventing

sub-harmonic oscillations at high duty cycles. This is

accomplished internally by adding a compensation ramp

to the inductor current signal at duty cycles in excess of

35%.TheLTC3883usesapatentedcurrentlimittechnique

that counteracts the compensating ramp. This allows the

maximum inductor peak current to remain unaffected

throughout all duty cycles.

INDUCTOR VALUE CALCULATION

Given the desired input and output voltages, the inductor

value and operating frequency, fOSC, directly determine

the inductor peak-to-peak ripple current:

IRIPPLE =

VOUT VIN – VOUT

(

)

VIN • fOSC • L

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors, and output voltage

ripple. Thus, highest efficiency operation is obtained at the

lowest frequency with a small ripple current. Achieving

this, however, requires a large inductor.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). Note that the largest ripple

current occurs at the highest input voltage. To guarantee

that the ripple current does not exceed a specified maxi-

mum, the inductor should be chosen according to:

L ≥

VOUT VIN – VOUT

(

)

VIN • fOSC •IRIPPLE

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