LTC3880 datasheet(16/50 Pages) LINER

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LTC3839

3839fa

APPLICATIONS INFORMATION

Once the required output voltage and operating frequency

have been determined, external component selection is

driven by load requirement, and begins with the selec-

tion of inductors and current sense method (either sense

resistors RSENSE or inductor DCR sensing). Next, power

MOSFETs are selected. Finally, input and output capaci-

tors are selected.

Output Voltage Programming

As shown in Figure 1, an external resistor divider is

used from the regulated output to its ground references

to program the output voltage. The resistive divider is

tapped by the VOUTSENSE+ pin and the ground reference

is remotely sensed by the VOUTSENSE– pin. By regulating

the differential feedback voltages to the internal reference

0.6V, the resulting output voltage is:

VOUT – VOUTSENSE– = 0.6V • (1 + RFB2/RFB1)

For example, if VOUT is programmed to 5V and the out-

put ground reference is sitting at –0.5V with respect to

SGND, then the absolute value of the output will be 4.5V

with respect to SGND. The minimum differential output

voltage is limited to the internal reference 0.6V, and the

maximum is 5.5V.

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.

The variations may be 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 LTC3839’s differential

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

in the output’s power and ground lines.

The LTC3839’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 resistor 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 LTC3839 allows for seamless differential output sens-

ing by sensing the resistively divided feedback voltage

differentially. This allows for differential sensing in the full

output range from 0.6V to 5.5V. The difference amplifier

(DIFFAMP) has a bandwidth of 8MHz, high enough so that

it will not affect main loop compensation and transient

behavior.

To avoid noise coupling into the feedback voltage, the

resistor dividers should be placed close to the VOUTSENSE+

and VOUTSENSE– pins. Remote output and ground traces

should be routed together as a differential pair to the

remote output. For best accuracy, these traces to the

remote output and ground should be connected as close

as possible to the desired regulation point.

Switching Frequency Programming

The choice of operating frequency is a trade-off between

efficiency and component size. Lowering the operating fre-

quency improves efficiency by reducing MOSFET switching

losses but requires larger inductance and/or capacitance

Figure 1. Setting Output Voltage

RFB2

VOUTSENSE

LTC3839

VOUTSENSE

–

COUT

3839 F01

VOUT

RFB1

The VOUTSENSE+ pin is a high impedance pin with no in-

put bias current other than leakage in the nA range. The

VOUTSENSE– pin has about 30μA of current flowing out

of the pin.

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