LTC3415 datasheet(13/36 Pages) LINER

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LTC3613

3613fa

The Typical Application on the first page of this data sheet

is a basic LTC3613 application circuit. The LTC3613 can

be configured to sense the inductor current either through

a series sense resistor, RSENSE, or through an RC filter

across the inductor (DCR). 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, cur-

rent sensing resistors provide the most accurate current

limits for the controller. Once the required output voltage

and operating frequency have been determined, external

component selection is driven by load requirements, and

begins with the selection of inductor and current sensing

components. Next, the proper current sense threshold is

programmed using the VRNG pin. Finally, input and output

capacitors are selected.

Output Voltage Programming and

Differential Output Sensing

The LTC3613 integrates differential output sensing with

output voltage programming, allowing for simple and

seamless design. As shown in Figure 1, the output voltage

is programmed by an external resistor divider from the

regulated output point to its ground reference. The resis-

tive divider is tapped by the VOSNS+ pin, and the ground

reference is sensed by VOSNS–. An optional feed-forward

capacitor, CFF, can be used to improve the transient per-

formance of the regulator system as discussed under

OPTI-LOOP® Compensation. The resulting output voltage

is given according to the following equation:

APPLICATIONS INFORMATION

VOUT =0.6V• 1+

RFB2

RFB1

⎛

⎝⎜

⎞

⎠⎟

More precisely, the VOUTvalueprogrammedintheprevious

equation is with respect to the output’s ground reference,

and thus is a differential quantity. For example, if VOUT

is programmed to 5V and the output ground reference

is at –0.5V, then the output will be 4.5V with respect to

signal ground. The minimum differential output voltage is

limited to the internal reference, 0.6V, and the maximum

differential output voltage is 5.5V.

Figure 1. Setting Output Voltage

RFB2

VOSNS

LTC3613

VOSNS

–

COUT

CFF

(OPT)

3613 F01

VOUT

RFB1

The VOSNS+ pin is high impedance with no input bias cur-

rent. The VOSNS– pin has about 35μA of current flowing

out of the pin.

Differential output sensing allows for more accurate output

regulation in high power distributed systems having large

line losses. Figure 2 illustrates the potential variations in

the power and ground lines due to parasitic elements.

These variations are exacerbated in multi-application

systems with shared ground planes. Without differential

output sensing, these variations directly reflect as an error

in the regulated output voltage. The LTC3613’s differential

output sensing can correct for up to ±500mV of variation

in the output’s power and ground lines.

The LTC3613’s differential output sensing scheme is

distinct from conventional schemes where the regulated

output and its ground reference are directly sensed with

a difference amplifier whose output is then divided down

with an external resistive divider and fed into the error

amplifier input. This conventional scheme is limited by

the common mode input range of the difference amplifier

and typically limits differential sensing to the lower range

of output voltages.

The LTC3613 allows for seamless differential output

sensing by sensing the resistively divided feedback volt-

age differentially. This allows for differential sensing in

the full output range from 0.6V to 5.5V. The difference

amplifier of the LTC3613 has a –3dB bandwidth of 8MHz,

high enough to not affect main loop compensation and

transient behavior.

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