+

-

V

OUT

R2

50k

SW

FB

GND

SHDN

Current Sensing

Circuitry

Under Voltage

Lockout

400ns

One Shot

CL Adjust

CL

Comp

R1

50k

Q1

Q2

10x

R3

30k

R4

140k

Driver

2

4

5

3

1

Logic

R

F2

R

F1

Enable

Comp

V

OUT

C

OUT

L

D

V

IN

C

IN

V

IN

Enable

LM2705

www.ti.com

SNVS191E – NOVEMBER 2002 – REVISED MAY 2013

OPERATION

Figure 15. LM2705 Block Diagram

2) Inductor Current, 100mA/div, DC

T = 10µs/div

Figure 16. Typical Switching Waveform

The LM2705 features a constant off-time control scheme. Operation can be best understood by referring to

Figure 15 and Figure 16. Transistors Q1 and Q2 and resistors R3 and R4 of Figure 15 form a bandgap reference

used to control the output voltage. When the voltage at the FB pin is less than 1.237V, the Enable Comp in

Figure 15 enables the device and the NMOS switch is turned on pulling the SW pin to ground. When the NMOS

switch is on, current begins to flow through inductor L while the load current is supplied by the output capacitor

Copyright © 2002–2013, Texas Instruments Incorporated

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