LT3045 datasheet(24/36 Pages) LINER

24 / 36 page

LT3045-1

30451fa

For more information www.linear.com/LT3045-1

APPLICATIONS INFORMATION

plication needs (PSRR vs. power dissipation) using the

following equation:

VLDOIN – VLDOOUT = VVIOC = VFBSWITCHER •

R1+R2

Furthermore, in the event the LT3045-1 SET pin opens up,

the LT3045-1 input voltage can rise up to the switcher’s

input voltage, and thus potentially violate the LT3045-1’s

absolute maximum rating. To prevent this, the maximum

LT3045-1 input voltage can be set using a resistor (R3)

between the VIOC and IN pins of the regulator such that:

V(MAX)LDOIN = VFBSWITCHER •

R1+R2+R3

+ISINK •R3

Moreover, the VIOC pin is capable of sourcing 200µA

and sinking 15µA of current. To mitigate the effect of the

sink current on the maximum LDO input voltage (shown

above), choose R1 such that the resistor divider typically

runs at least 100µA.

For VOUT > 1.5V, VIN = VOUT + VVIOC. The VIOC pin volt-

age (and hence the input-to-output differential) can be

programmed anywhere between a minimum of 1V and

a maximum of 4V or VOUT – 0.5V (for VOUT > 1.5V),

whichever is lower. For applications where the feedback

pin of the switching regulator is below 1V, use resistors

R1 and R2 to make sure the VIOC pin is within the afore-

mentioned range. Note that the VIOC pin voltage cannot

beprogrammedbelowtheupstreamswitchingconverter’s

feedback pin. For VOUT ≤ 1.5V, the VIOC programming

range is 1V ±5%. If VIOC is set to be outside this range,

then the LT3045-1 input voltage will rise to the maximum

value set using R3. If VIOC functionality is not used, float

the VIOC pin.

Given the maximum VIOC programming voltage is de-

pendent on VOUT, care must be taken in setting the VIOC

voltage. For instance, if VIOC is set to 1V, the LDO’s IN-

to-OUT differential will be regulated to 1V for VOUT > 1.5V.

Similarly, if VIOC is set to 2V, the regulator’s IN-to-OUT

differential will be regulated to 2V for VOUT > 2.5V (i.e.

VVIOC + 0.5V). However, if the output voltage is below

2.5V, for this example, then the LDO will not be able to

drive its VIOC pin to the right level of 2V. As a result, the

upstream pre-regulator’s output will rise, thereby caus-

ing the LT3045-1 input voltage to rise to the maximum

voltage set using R3. Hence, for protection under various

fault conditions, the use of R3 to set the maximum VIN

voltage (below 20V) is required.

Typical VIOC Application

Figure 11 shows a typical VIOC application used to post-

regulatetheoutputoftheLT8608buckconverter.TheVIOC

voltage is set at 1V with the maximum LDO input voltage

set to 16.5V. Figure 12 shows the LDO’s input and output

voltage when pulsing the LT3045-1’s EN/UV pin, and as

Figure 11. Typical LT3045-1 Post-Regulating Application

100µA

EN/UV

PGFB

VIOC

GND

OUT

LT3045-1

ILIM

10µF

47µF

VIN

20V

4.7µF

VOUT: VARIABLE

IOUT(MAX): 500mA

30451 F11

249

140k

SET

OUTS

LT8608

BST

EN/UV

TR/SS

MODE

INTVCC

7.68k

40.2k

2.2µH

2.21k

GND

10nF

1µF

0.22µF

fSW = 1MHz

L: XFL4020-222MEC

VLDOIN – VLDOUT = 1V

VMAXLDOIN = 16.5V

LT3045 датащи(PDF) 24 Page - Linear Technology

LT3045 датащи(HTML) 24 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

датащи скачать

ссылки URL