LT8602 datasheet(27/38 Pages) LINER

27 / 38 page

LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

limitations on the achievable duty cycle range and oper-

ating frequency. For buck regulators, the duty cycle is

given by:

D =

VOUT

PVIN

Further, the minimum duty cycle achievable at a given

operating frequency is given by:

DMIN = tON(MIN) • fSW

where fSW is the programmed operating frequency.

The maximum duty cycle achievable at a given operating

frequency is given by:

DMAX = 1 – (tOFF(MIN) • fSW )

Combining these equations, the minimum PVIN voltage

allowed while regulating at full frequency is:

PVINx(MIN) =

VOUTx

1 – (tOFF(MIN) • fSW )

and the maximum VIN voltage allowed while regulating at

full frequency is:

PVINx(MAX) =

VOUTx

tON(MIN) • fSW

If the PVIN(MAX) given above is exceeded during regula-

tion, the buck regulator will skip switch-on cycles and no

longer maintain the programmed operating frequency.

Buck: Inductor Selection

Inductor selection involves inductance, saturation current,

series resistance (DCR) and magnetic loss.

A good starting point for choosing inductor values is:

L =

1.05 • (VOUTx + VBOTx )

fSW

for Channels 1 and 3

and

L =

0.70 • (VOUTx + VBOTx )

fSW

for Channel 2

Figure 9. Feedback Resistor Divider

8603 F09

SWx

LT8603

FBx

COUT

CFF

(OPTIONAL)

The value of input capacitance is a function of the source

impedance. In general, the higher the source impedance,

the higher the required input capacitance. The Typical

Applications section provides reasonable starting values

for input capacitance but careful evaluation of each appli-

cation must be made to ensure adequate design margin.

BUCK REGULATORS

Setting the Output Voltages

The output voltages of the buck channels are set with a

resistor divider from the output to the related FBx pin as

shown in Figure 9.

The value of R2 is best selected first as this establishes

how much current is in the string based on I = VFB/R2

where VFB = 1.0V for the high voltage channels and 0.8V

for the low voltage channel. The current should be chosen

such that it is not influenced by anticipated leakage or

noise. R1 can then be calculated from:

R1= R2 •

VOUTx

FBREF

– 1

















CFF can optionally be used to improve the transient

response and stability of the internally compensated feed-

back loops. The values shown in the Typical Applications

section will provide a good starting point for selecting CFF

though careful evaluation of regulator stability should be

made to ensure adequate design margin.

Buck: Operating Frequency and Input Voltage Range

Each buck regulator’s respective minimum on-time,

tON(MIN), and minimum off-time, tOFF(MIN), impose

LT8602 датащи(PDF) 27 Page - Linear Technology

LT8602 датащи(HTML) 27 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

датащи скачать

ссылки URL