LTC1909-8_15 Datasheet, PDF(19/32 Page) Linear Technology – Wide Operating Range, No RSENSE Step-Down DC/DC Controller with SMBus Programming

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC1909-8_15 Datasheet, PDF (19/32 Pages) Linear Technology – Wide Operating Range, No RSENSE Step-Down DC/DC Controller with SMBus Programming

◁

LTC1909-8

APPLICATIO S I FOR ATIO

[ ] f =

VOUT

VVON RON(10pF)

To hold frequency constant during output voltage changes,

tie the VON pin to VOUT. The VON pin has internal clamps

that limit its input to the one-shot timer. If the pin is tied

below 0.7V, the input to the one-shot is clamped at 0.7V.

Similarly, if the pin is tied above 2.4V, the input is clamped

at 2.4V.

Because the voltage at the ION pin is about 0.7V, the

current into this pin is not exactly inversely proportional to

VIN, especially in applications with lower input voltages.

To correct for this error, an additional resistor RON2

connected from the ION pin to the 5V INTVCC supply will

further stabilize the frequency.

RON2

0.7V

RON

Changes in the load current magnitude will also cause

frequency shift. Parasitic resistance in the MOSFET

switches and inductor reduce the effective voltage across

the inductance, resulting in increased duty cycle as the

load current increases. By lengthening the on-time slightly

as current increases, constant frequency operation can be

maintained. This is accomplished with a resistive divider

from the ITH pin to the VON pin and VOUT. The values

required will depend on the parasitic resistances in the

specific application. A good starting point is to feed about

25% of the voltage change at the ITH pin to the VON pin as

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

f âIL(MAX)

ï£¶ï£«

ï£¸ï£· ï£ï£¬ 1â

VOUT ï£¶

VIN(MAX) 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, Coil-

craft and Toko.

Kool MÂµ is a registered trademark of Magnetics, Inc.

VOUT

RVON1

30k

RVON2

100k

CVON

0.01ÂµF

VON

LTC1909-8

ITH

VOUT

INTVCC

RVON1

RVON2 CVON

10k 10k 0.01ÂµF

2N5087

VON

LTC1909-8

ITH

19098 F03

(3a)

(3b)

Figure 3. Correcting Frequency Shift with Load Current Changes

19098f

▷