LTC3720 Datasheet, PDF(12/24 Page) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

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LTC3720 Datasheet, PDF (12/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

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LTC3720

APPLICATIO S I FOR ATIO

VOUT

RVON1

30k

RVON2

100k

CVON

0.01ÂµF

VON

LTC3720

ITH

3720 F03a

VOUT

INTVCC

RVON1

RVON2

10k 10k

2N5087

CVON

0.01ÂµF

VON

LTC3720

ITH

3720 F03b

(3a)

(3b)

Figure 3. Correcting Frequency Shift with Load Current Changes

shown in Figure 3a. Place capacitance on the VON pin to

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 Selection

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple

current:

âIL

ï£«

ï£ï£¬

VOUT

ï£¶ï£«

ï£¸ï£· ï£ï£¬1â

VOUT

VIN

ï£¶

ï£¸ï£·

Lower ripple current reduces core 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 af-

ford 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 available from manu-

facturers such as Sumida, Panasonic, Coiltronics, Coilcraft

and Toko.

Schottky Diode D1 Selection

The Schottky diode D1 shown in Figure 1 conducts during

the dead time between the conduction of the power

MOSFET switches. It is intended to prevent the body diode

of the bottom MOSFET from turning on and storing charge

during the dead time, which can cause a modest (about

1%) efficiency loss. The diode can be rated for about one

half to one fifth of the full load current since it is 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 diode can be omit-

ted 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

Kool MÂµ is a registered trademark of Magnetics, Inc.

3720f

▷