LTC3829 datasheet(18/54 Pages) LINER

18 / 54 page

LTC3838

3838fa

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, external resistor dividers are used

from the regulated outputs to their respective ground refer-

ences to program the output voltages. On Channel 1, the

resistive divider is tapped by the VOUTSENSE1+ pin, and the

ground reference is remotely sensed by the VOUTSENSE1–

pin, this voltage is sensed differentially. On Channel 2, the

resistive divider is tapped by the VFB2 pin, with respect to

signal ground at the SGND pin. By regulating the tapped

(differential) feedback voltages to the internal reference

0.6V, the resulting output voltages are:

VOUT1 – VOUTSENSE1– = 0.6V • (1 + RFB2/RFB1)

and

VOUT2 = 0.6V • (1 + RFB2/RFB1)

For example, if VOUT1 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

voltages are limited to the internal reference 0.6V, and the

maximum are 5.5V.

The VOUTSENSE1+ pin is a high impedance pin with no

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

VOUTSENSE1– pin has about 30μA of current flowing out

of the pin. The VFB2 pin is quasi-high impedance pin with

minimum bias current 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.

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 LTC3838 Channel 1’s

differential output sensing can correct for up to ±500mV

of variation in the output’s power and ground lines.

The LTC3838 Channel 1’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 differ-

ence amplifier and typically limits differential sensing to

the lower range of output voltages.

The LTC3838’s Channel 1 allows for seamless differential

output sensing 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

RFB2

VOUTSENSE1+/VFB2

LTC3838

VOUTSENSE1–/SGND

COUT

3838 F01

VOUT

RFB1

Figure 1. Setting Output Voltage

LTC3829 Datasheet(PDF) 18 Page - Linear Technology

LTC3829 Datasheet(HTML) 18 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Datasheet Download

Link URL