LTC3602 datasheet(15/36 Pages) LINER

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LTC3613

3613fa

APPLICATIONS INFORMATION

The inductor value has a direct effect on ripple current.

The inductor ripple current, ΔIL, decreases with higher

inductance or frequency and increases with higher VIN:

ΔI

L =

VOUT

f•L

•1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of ΔIL allows the use of low induc-

tances, but results in higher output voltage ripple, higher

ESR losses in the output capacitor, and greater core losses.

A reasonable starting point for setting ripple current is

ΔIL = 0.4 • IOUT(MAX) where IOUT(MAX) is the maximum

output current for the application. The maximum ΔIL

occurs at the maximum input voltage. To guarantee that

ripple current does not exceed a specified maximum, the

inductance should be chosen according to:

VOUT

•ΔI

L(MAX)

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

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

be selected. High efficiency converters generally cannot

tolerate the core loss of low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy

or Kool Mμ cores. Ferrite core material saturates hard,

meaning that inductance collapses abruptly when the

peak design current is exceeded. This results in an abrupt

increase in inductor ripple current and consequent output

voltage ripple. Do not allow the core to saturate!

A variety of inductors designed for high current, low volt-

age applications are available from manufacturers such as

Sumida, Panasonic, Coiltronics, Coilcraft, Toko, Vishay,

Pulse and Würth.

Current Sense Pins and Current Limit Programming

Inductor current is sensed through the SENSE+ and

SENSE– pins and fed into the internal current compara-

tors. The common mode input voltage range of the cur-

rent comparators is –0.5V to 5.5V. Both SENSE pins are

high impedance inputs. When the common mode range

is between –0.5V to 1.1V, there is no input bias current,

and when between 1.4V and 5.5V, there is less than 1μA

of current flowing into the pins. Between 1.1V and 1.4V,

the input bias current will be zero if the common mode

voltage is ramped up from 1.1V and less than 1μA if the

common mode voltage is ramped down from 1.4V. The

high impedance inputs to the current comparator allow

accurate DCR sensing. However, care must be taken not

to float these pins during normal operation.

The maximum allowed sense voltage VSENSE(MAX) between

SENSE+ and SENSE– is set by the voltage applied to the

VRNG pin and is given by:

VSENSE(MAX) = 0.05 • VRNG

The current mode control loop does not allow the induc-

tor current valleys to exceed 0.05 • VRNG. The maximum

output current is given by:

IOUT(MAX) =

VSENSE(MAX)

RSENSE

ΔIL

The VSENSE(MAX) is shown in the figure “Maximum Current

Sense Voltage vs Temperature” in the Typical Performance

Characteristics. Note that ITH is close to 2.4V when in

current limit.

An external resistive divider from INTVCC can be used

to set the voltage on the VRNG pin between 0.6V and 2V,

resulting in maximum sense voltages between 30mV and

100mV. The wide voltage sense range allows for a variety of

applications. The VRNG pin can also be tied to either SGND

or INTVCC to force internal defaults. When VRNG is tied to

SGND, the device operates with a maximum sense voltage

of 30mV. When the VRNG pin is tied to INTVCC, the device

operates with a maximum sense voltage of 50mV. When

setting current limit, ensure that the junction temperature

does not exceed the rating of 125°C.

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