L3751 datasheet(40/41 Pages) STMICROELECTRONICS

40 / 41 page

List of figures

Figure 1.

Simplified block diagram ............................................................. 2

Figure 2.

QFN pin connection (top view) ......................................................... 3

Figure 3.

Typical application.................................................................. 5

Figure 4.

EN / UVLO resistor divider ........................................................... 11

Figure 5.

RRT resistor value for fSW programming .................................................. 12

Figure 6.

Internal sawtooth ramp feedforward - voltage feed forward ..................................... 13

Figure 7.

Internal sawtooth ramp feedforward - Fsw adjusted by external resistor ............................ 13

Figure 8.

Soft-start block diagram ............................................................. 14

Figure 9.

Soft-start phase .................................................................. 14

Figure 10.

Schematic to implement input voltage tracking ............................................. 15

Figure 11.

Input voltage tracking .............................................................. 15

Figure 12.

SS/TRK clampler operation .......................................................... 16

Figure 13.

DEM vs. FPWM management in light load condition ......................................... 17

Figure 14.

DEM to FPWM smooth transition in 1000 clks at powerup ..................................... 18

Figure 15.

Efficiency typ. @ 240 KHz, VOUT = 5 V .................................................. 18

Figure 16.

Efficiency typ. @ 240 KHz, VOUT = 12 V ................................................. 18

Figure 17.

Efficiency typ. @ 240 KHz DEM vs. FPWM ............................................... 19

Figure 18.

DEM output voltage ripple at zero load .................................................. 19

Figure 19.

FPWM output voltage ripple at zero load ................................................. 20

Figure 20.

RILIM resistor connection to define lossless and precise OCP setting .............................. 20

Figure 21.

Constant current in persistent short-circuit ................................................ 21

Figure 22.

Hiccup in persistent short-circuit ....................................................... 21

Figure 23.

Voltage mode control loop ........................................................... 23

Figure 24.

Type II compensation network ........................................................ 24

Figure 25.

Type II compensation - Bode plot ...................................................... 25

Figure 26.

Type III compensation network ........................................................ 25

Figure 27.

Type III compensation - Bode plot ...................................................... 26

Figure 28.

L3751 application schematic ......................................................... 27

Figure 29.

Pulsed current paths in buck topology ................................................... 28

Figure 30.

High frequency gate-driving loops ...................................................... 29

Figure 31.

Board layout example .............................................................. 30

Figure 32.

QFN20 (3.5 x 4.5 mm) package outline .................................................. 32

Figure 33.

QFN20 (3.5 x 4.5 mm) recommended footprint ............................................. 34

Figure 34.

QFN20 (3.5 x 4.5 mm) tape drawing .................................................... 35

Figure 35.

QFN20 (3.5 x 4.5 mm) tape oriented .................................................... 35

Figure 36.

QFN20 (3.5 x 4.5 mm) reel dimensions .................................................. 36

L3751

List of figures

DS13841 - Rev 1

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