LTC3718_15 Datasheet, PDF(12/20 Page) Linear Technology – Low Input Voltage DC/DC Controller for DDR/QDR Memory Termination

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LTC3718_15 Datasheet, PDF (12/20 Pages) Linear Technology – Low Input Voltage DC/DC Controller for DDR/QDR Memory Termination

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LTC3718

APPLICATIO S I FOR ATIO

VOUT

RVON1

30k

RVON2

100k

CVON

0.01ÂµF

VON

LTC3718

ITH

VOUT

INTVCC

RVON1

RVON2

10k 10k

2N5087

CVON

0.01ÂµF

VON

LTC3718

ITH

3718 F03

(3a)

(3b)

Figure 3. Adjusting Frequency Shift with Load Current Changes

filter out the ITH variations at the switching frequency. The

resistor load on ITH reduces the DC gain of the error amp

and degrades load regulation, which can be avoided by

using the PNP emitter follower of Figure 3b.

Inductor L1 Selection

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple

current:

âIL

ï£« VOUT

ï£ï£¬ fL

ï£¶ï£«

ï£¸ï£· ï£ï£¬1â

VOUT

VIN

ï£¶

ï£¸ï£·

Lower ripple current reduces cores losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a tradeoff between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). The largest ripple current

occurs at the highest VIN. To guarantee that ripple current

does not exceed a specified maximum, the inductance

should be chosen according to:

ï£«

ï£ï£¬

VOUT

âIL(MAX)

ï£¶

ï£¸ï£·

ï£«

ï£ï£¬1â

VOUT

VIN(MAX)

ï£¶

ï£¸ï£·

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron

cores, forcing the use of more expensive ferrite,

molypermalloy or Kool MÂµÂ® cores. A variety of inductors

designed for high current, low voltage applications are

Kool MÂµ is a registered trademark of Magnetics, Inc.

available from manufacturers such as Sumida, Pana-

sonic, Coiltronics, Coilcraft and Toko.

Schottky Diode D1, D2 Selection

The Schottky diodes, D1 and D2, shown in Figure 1

conduct during the dead time between the conduction of

the power MOSFET switches. It is intended to prevent the

body diodes of the top and bottom MOSFETs from turning

on and storing charge during the dead time, which can

cause a modest (about 1%) efficiency loss. The diodes can

be rated for about one half to one fifth of the full load current

since they are on for only a fraction of the duty cycle. In

order for the diode to be effective, the inductance between

it and the bottom MOSFET must be as small as possible,

mandating that these components be placed adjacently.

The diodes can be omitted if the efficiency loss is tolerable.

CIN and COUT Selection

The input capacitance CIN is required to filter the square

wave current at the drain of the top MOSFET. Use a low

ESR capacitor sized to handle the maximum RMS current.

IRMS

IOUT(MAX)

VOUT

VIN

VIN â 1

VOUT

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT(MAX)/ 2. This simple worst-case condition is

commonly used for design because even significant

deviations do not offer much relief. Note that ripple

current ratings from capacitor manufacturers are often

based on only 2000 hours of life which makes it advisable

to derate the capacitor.

The selection of COUT is primarily determined by the ESR

required to minimize voltage ripple and load step

3718fa

▷