LTC3858IUH-2#TRPBF Datasheet LTC3858EUH-2#PBF, LTC3858EUH-2#TRPBF, LTC3858IUH-2#TRPBF | P16-P18

LTC3858-2

38582f

APPLICATIONS INFORMATION

Figure 3. Sense Lines Placement with Inductor or Sense Resistor

The Typical Application on the first page is a basic

LTC3858-2 application circuit. LTC3858-2 can be config-

ured to use either DCR (inductor resistance) sensing or low

value resistor sensing. The choice between the two current

sensing schemes is largely a design trade-off between

cost, power consumption and accuracy. DCR sensing

is becoming popular because it saves expensive current

sensing resistors and is more power efficient, especially

in high current applications. However, current sensing

resistors provide the most accurate current limits for the

controller. Other external component selection is driven

by the load requirement, and begins with the selection of

SENSE

(if R

SENSE

is used) and inductor value. Next, the

power MOSFETs and Schottky diodes are selected. Finally,

input and output capacitors are selected.

Current Limit Programming

The I

LIM

pin is a tri-level logic input which sets the maximum

current limit of the converter. When I

LIM

is grounded, the

maximum current limit threshold voltage of the current

comparator is programmed to be 30mV. When I

LIM

floated, the maximum current limit threshold is 50mV.

When I

LIM

is tied to INTV

, the maximum current limit

threshold is set to 75mV.

SENSE

and SENSE

–

Pins

The SENSE

and SENSE

–

pins are the inputs to the current

comparators. The common mode voltage range on these

pins is 0V to 28V (Abs Max), enabling the LTC3858-2 to

regulate output voltages up to a nominal 24V (allowing

plenty of margin for tolerances and transients).

The SENSE

pin is high impedance over the full common

mode range, drawing at most ±1µA. This high impedance

allows the current comparators to be used in inductor

DCR sensing.

The impedance of the SENSE

–

pin changes depending on

the common mode voltage. When SENSE

–

is less than

INTV

– 0.5V, a small current of less than 1µA flows out

of the pin. When SENSE

–

is above INTV

+ 0.5V, a higher

current (~550µA) flows into the pin. Between INTV

–

0.5V and INTV

+ 0.5V, the current transitions from the

smaller current to the higher current.

Filter components mutual to the sense lines should be

placed close to the LTC3858-2, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 3). Sensing cur-

rent elsewhere can effectively add parasitic inductance

and capacitance to the current sense element, degrading

the information at the sense terminals and making the

programmed current limit unpredictable. If inductor DCR

sensing is used (Figure 4b), resistor R1 should be placed

close to the switching node, to prevent noise from coupling

into sensitive small-signal nodes.

OUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR R

SENSE

3858 F03

Low Value Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 4a. R

SENSE

is chosen based on the required

output current.

The current comparator has a maximum threshold

SENSE(MAX)

determined by the I

LIM

setting. The current

comparator threshold voltage sets the peak of the induc-

tor current, yielding a maximum average output current,

MAX

, equal to the peak value less half the peak-to-peak

ripple current, ΔI

. To calculate the sense resistor value,

use the equation:

SENSE

SENSE(MAX)

MAX

ΔI

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due to

the internal compensation required to meet stability cri-

terion for buck regulators operating at greater than 50%

duty factor. A curve is provided in the Typical Performance

Characteristics section to estimate this reduction in peak

output current depending upon the operating duty factor.

LTC3858-2

38582f

(4a) Using a Resistor to Sense Current (4b) Using the Inductor DCR to Sense Current

Figure 4. Current Sensing Methods

APPLICATIONS INFORMATION

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC3850 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 4b. The DCR of the inductor represents the small

amount of DC resistance of the copper wire, which can be

less than 1m for today’s low value, high current inductors.

In a high current application requiring such an inductor,

power loss through a sense resistor would cost several

points of efficiency compared to inductor DCR sensing.

If the external R1||R2 • C1 time constant is chosen to be

exactly equal to the L/DCR time constant, the voltage drop

across the external capacitor is equal to the drop across

the inductor DCR multiplied by R2/(R1 + R2). R2 scales the

voltage across the sense terminals for applications where

the DCR is greater than the target sense resistor value.

To properly dimension the external filter components, the

DCR of the inductor must be known. It can be measured

using a good RLC meter, but the DCR tolerance is not

always the same and varies with temperature; consult

the manufacturers’ data sheets for detailed information.

Using the inductor ripple current value from the Inductor

Value Calculation section, the target sense resistor value is:

SENSE(EQUIV)

SENSE(MAX)

MAX

ΔI

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimum value for the Maximum Current Sense Thresh-

old Voltage (V

SENSE(MAX)

) in the Electrical Characteristics

table (30mV, 50mV or 75mV depending on the state of

the I

LIM

pin).

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

use the manufacturer’s maximum value, usually given at

20°C. Increase this value to account for the temperature

coefficient of copper, which is approximately 0.4%/°C. A

conservative value for T

L(MAX)

is 100°C.

To scale the maximum inductor DCR to the desired sense

resistor (R

) value, use the divider ratio:

SENSE(EQUIV)

DCR

MAX

atT

L(MAX)

INTV

BOOST

PLACE CAPACITOR NEAR

SENSE PINS

SENSE

–

SGND

LTC3858-2

OUT

SENSE

38582 F04a

INTV

BOOST

*PLACE C1 NEAR

SENSE PINS

INDUCTOR

DCRL

SENSE

–

SGND

LTC3858-2

OUT

38582 F04b

R2C1*

(R1

R2) t C1 =

DCR

SENSE(EQ)

= DCR

R1 + R2

LTC3858-2

38582f

APPLICATIONS INFORMATION

C1 is usually selected to be in the range of 0.1µF to 0.47µF.

This forces R1||R2 to around 2k, reducing error that might

have been caused by the SENSE

pin’s ±1µA current.

The equivalent resistance R1||R2 is scaled to the room

temperature inductance and maximum DCR:

R1|| R2 =

DCR at 20°C

()

•C1

The sense resistor values are:

R1=

R1|| R2

; R2 =

R1• R

1–R

The maximum power loss in R1 is related to duty cycle,

and will occur in continuous mode at the maximum input

voltage:

LOSS

R1=

IN(MAX)

–V

OUT

()

•V

OUT

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 inductor

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, reduces conduction losses and provides higher

efficiency at heavy loads. Peak efficiency is about the same

with either method.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because

of MOSFET gate charge losses. In addition to this basic

trade-off, the effect of inductor value on ripple current and

low current operation must also be considered.

The inductor value has a direct effect on ripple current. The

inductor ripple current ΔI

decreases with higher induc-

tance or higher frequency and increases with higher V

ΔI

()

OUT

1–

OUT

⎛

⎝

⎜

⎞

⎠

⎟

Accepting larger values of ΔI

allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is ΔI

= 0.3(I

MAX

). The maximum

ΔI

occurs at the maximum input voltage.

The inductor value also has secondary effects. The tran-

sition to Burst Mode operation begins when the average

inductor current required results in a peak current below

30% of the current limit determined by R

SENSE

. Lower

inductor values (higher ΔI

) will cause this to occur at

lower load currents, which can cause a dip in efficiency in

the upper range of low current operation. In Burst Mode

operation, lower inductance values will cause the burst

frequency to decrease.

Inductor Core Selection

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 or molypermalloy

cores. Actual core loss is independent of core size for a

fixed inductor value, but it is very dependent on inductance

value selected. As inductance increases, core losses go

down. Unfortunately, increased inductance requires more

turns of wire and therefore copper losses will increase.

Ferrite designs have very low core loss and are preferred

for high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance 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!

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LTC3858IUH-2#TRPBF

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Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators Low IQ, Dual, 2-Phase Synchronous Step-Down Controller without OVP

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