LT8631 datasheet(13/24 Pages) LINER

13 / 24 page

LT8631

8631f

For more information www.linear.com/LT8631

applicaTions inForMaTion

Choosing the Output Voltage

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

= R2

VOUT

0.808

– 1









Reference designators refer to the Block Diagram in

Figure 1.

Iflowinputquiescentcurrentandgoodlight-loadefficiency

are desired, use large resistor values for the FB resistor

divider. The current flowing in the divider acts as a load

current, and will increase the no-load input current to the

converter, which is approximately:

IQ = IQVIN+ IQVOUT +

VOUT

+R2

















•

VOUT

VIN









•









where IQVIN is the quiescent current of the LT8631 and the

secondtermisthequiescentcurrentdrawnfromtheoutput

(Iqvout) plus current in the feedback divider reflected to

the input of the buck operating at its light load efficiency

n. For a 5V application with R1 = 1MΩ and R2 = 191kΩ,

the feedback divider draws 4.2µA. With VIN = 12V IQVIN

= 3.6µA, IQVOUT = 10µA and n = 50%, the no-load quies-

cent current is approximately 16µA. For applications with

output voltages less than 2.8V, IQVOUT = 0µA and IQVIN is

typically 16µA. Graphs of IQVIN and IQVOUT vs VOUT are in

the Typical Performance Characteristics section.

WhenusingFBresistorsgreaterthan200k,a4.7pFto10pF

phaseleadcapacitorshouldbeconnectedfromVOUTtoFB.

Choosing the Switching Frequency

TheLT8631switchingfrequencycanbeprogrammedover

a 100kHz to 1MHz range by using a resistor tied from RT

to ground. A table showing the necessary RT value for a

desired switching frequency is shown in Table 1.

Theswitchingfrequencyselecteddeterminestheefficiency,

solution size, and input voltage range for the desired

frequency. High frequency operation permits the use of

smaller inductor and capacitor values which reduces the

overall solution size. However, as the switching frequency

increases. efficiency decreasesaswellastheinputvoltage

range for constant frequency operation.

Table 1. SW Frequency vs RT Value

FREQUENCY (kHz)

RRT (kΩ)

100

187

200

60.4

300

35.7

400

25.5

500

19.6

600

15.8

700

13.3

800

11.5

900

1000

8.66

Switching Frequency and Input Voltage Range

Once the switching frequency has been determined, the

input voltage range for fixed frequency operation of the

regulator can be determined.

The minimum input voltage for fixed frequency operation

is determined by either the VIN undervoltage lockout, or

the following equation:

VIN(MIN) =

VOUT + VSW(BOT)

1– fSW • tOFF(MIN)

– VSW(BOT)+ VSW(TOP)

where VOUT is the output voltage, VSW(TOP) and VSW(BOT)

are the internal switch drops (~0.775V, ~0.550V, respec-

tively at maximum load), FSW is the switching frequency

(set by RT), and tOFF(MIN) is the minimum switch off-time

(see the Electrical Characteristics).

IftheinputvoltagefallsbelowVIN(MIN)(dropoutmode), the

LT8631 will automatically reduce the switching frequency

from the programmed value to obtain the highest possible

output voltage. The lower limit on the switching frequency

in dropout mode is determined by the boost threshold.

When the voltage between the BST and SW pins is less

than the boost threshold, a minimum off-time pulse is

generated to recharge the boost capacitor.

LT8631 bảng dữ liệu(PDF) 13 Page - Linear Technology

LT8631 bảng dữ liệu(HTML) 13 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

bảng dữ liệu tải về

Link URL