LM2743MTCSLASHNOPB datasheet(11/44 Pages) TI2

11 / 44 page

0.65 = VOUT1

RT1

RT1 + RT2

Master Power

Supply

VOUT1 = 5V

SS/TRACK

LM2743

RT2

1 k:

RT1

150:

RFB2

10 k:

RFB1

5 k:

VOUT2 = 1.8V

VSS = 0.65V

VFB

RFADJ = -5.93 + 3.06

fSW

+ 0.24

(fSW)

RFB1

VOUT =

RFB1

RFB2

VFB

(VFB = 0.6V)

LM2743

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SNVS276I – APRIL 2004 – REVISED FEBRUARY 2019

Product Folder Links: LM2743

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Feature Description (continued)

•

CSS is in µF and tSS is in ms

(1)

During soft start the Power Good flag is forced low and it is released when the FB pin voltage reaches 70% of

0.6V. At this point the chip enters normal operation mode, and the output overvoltage and undervoltage

monitoring starts.

7.3.2 Normal Operation

While in normal operation mode, the LM2743 regulates the output voltage by controlling the duty cycle of the

high side and low side MOSFETs (see Typical Application Diagram). The equation governing output voltage is:

(2)

The PWM frequency is adjustable between 50 kHz and 1 MHz and is set by an external resistor, RFADJ, between

the FREQ pin and ground. The resistance needed for a desired frequency is approximately:

(3)

Where fSW is in Hz and RFADJ is in kΩ.

7.3.3 Tracking a Voltage Level

The LM2743 can track the output of a master power supply during soft-start by connecting a resistor divider to

the SS/TRACK pin. In this way, the output voltage slew rate of the LM2743 will be controlled by the master

supply for loads that require precise sequencing. When the tracking function is used no soft-start capacitor

should be connected to the SS/TRACK pin. Otherwise, a CSS value of at least 1 nF between the soft-start pin

and ground should be used.

Figure 19. Tracking Circuit

One way to use the tracking feature is to design the tracking resistor divider so that the master supply’s output

voltage (VOUT1) and the LM2743’s output voltage (represented symbolically in Figure 19 as VOUT2, that is, without

explicitly showing the power components) both rise together and reach their target values at the same time. For

this case, the equation governing the values of the tracking divider resistors RT1 and RT2 is:

(4)

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