LT8705 datasheet(19/44 Pages) LINER

19 / 44 page

LTC7812

7812fc

For more information www.linear.com/LTC7812

Figure 1. Sense Lines Placement with Inductor or Sense Resistor

APPLICATIONS INFORMATION

Cascaded Boost+Buck Regulator

The LTC7812 can be configured to regulate two separate,

completely independent outputs, one boost and one buck.

Or, it can be configured as a cascaded Boost+Buck single

output converter that regulates an output voltage from

an input voltage that can be above, below, or equal to the

output voltage. When cascaded, the input voltage feeds

the boost regulator, which generates an intermediate node

supply (VMID) that then serves as the input to the buck

regulator, which then regulates the output voltage.

When used as a cascaded Boost+Buck regulator, the

LTC7812 has distinct advantages compared to traditional

single inductor buck-boost regulators. Even though it

requires two inductors, these inductors are individually

smaller and provide inherent filtering at the input and

output, substantially reducing conducted EMI and volt-

age ripple, thereby requiring less input and output filter-

ing. Even though they are cascaded, the boost and buck

regulatorsareindependentlyoptimizedandcompensated.

The buck regulator on the output provides a very fast

transient response compared to a buck-boost regulator,

further reducing the amount of output capacitance that is

required. The LTC7812 also features a very low quiescent

current Burst Mode operation which dramatically reduces

power loss and increases efficiency at light loads. Thus,

for those applications that require low EMI, low ripple,

fasttransientresponse,lowquiescentcurrent,and/orhigh

light load efficiency, the LTC7812 cascaded Boost+Buck

regulator provides an excellent solution.

TheTypicalApplicationonthefirstpageisabasicLTC7812

application circuit. LTC7812 can be configured 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

RSENSE (if RSENSE is used) and inductor value. Next, the

power MOSFETs are selected. Finally, input and output

capacitors are selected.

SENSE+ and SENSE– Pins

The SENSE+ and SENSE– pins are the inputs to the cur-

rent comparators.

BuckController(SENSE1+/SENSE1–):Thecommonmode

voltage range on these pins is 0V to 28V (absolute maxi-

mum), enabling the LTC7812 to regulate a buck output

voltage up to a nominal 24V set point (allowing margin

for tolerances and transients). The SENSE1+ 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 SENSE1– pin changes depending on

the common mode voltage. When SENSE1– is less than

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

of the pin. When SENSE1– is above INTVCC+0.5V,ahigher

current(≈700µA)flowsintothepin.BetweenINTVCC–0.5V

andINTVCC+0.5V, thecurrenttransitionsfromthesmaller

current to the higher current.

Boost Controller (SENSE2+/SENSE2–): The common mode

input range for these pins is 2.5V to 38V, allowing the boost

converter to operate from inputs over this full range. The

SENSE2+ pin also provides power to the current compara-

tor and draws about 170µA during normal operation (when

not shut down or asleep in Burst Mode operation). There is

a small bias current of less than 1µA that flows out of the

SENSE2–pin.ThishighimpedanceontheSENSE2–pinallows

the current comparator to be used in inductor DCR sensing.

Filter components mutual to the sense lines should be

placed close to the LTC7812, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 1). Sensing cur-

7812 F01

TO SENSE FILTER

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

CURRENT FLOW

LT8705 Datasheet(PDF) 19 Page - Linear Technology

LT8705 Datasheet(HTML) 19 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

Datasheet Download

Link URL