LM5145 datasheet(22/60 Pages) TI1

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capacitor is required to keep the device from entering current limit during a start-up event. Likewise, the system

designer must also be aware of how fast the input supply ramps if the tracking feature is enabled.

PGOOD

2V/DIV

VOUT 1V/DIV

SS/TRK

160mV/DIV

94% VOUT

92% VOUT

10 ms/DIV

Figure 8-6. Typical Output Voltage Tracking and PGOOD Waveforms

Figure 8-6 shows a triangular voltage signal directly driving SS/TRK and the corresponding output voltage

tracking response. Nominal output voltage here is 5 V, with oscilloscope channel scaling chosen such that the

waveforms overlap during tracking. As expected, the PGOOD flag transitions at thresholds of 94% (rising) and

92% (falling) of the nominal output voltage setpoint.

Two practical tracking configurations, ratiometric and coincident, are shown in Tracking Implementation With

Primary, Ratiometric Secondary, and Coincident Secondary Rails. The most common application is coincident

tracking, used in core versus I/O voltage tracking in DSP and FPGA implementations. Coincident tracking forces

the primary and secondary channels to have the same output voltage ramp rate until the secondary output

reaches its regulated setpoint. Conversely, ratiometric tracking sets the output voltage of the secondary to a

fraction of the output voltage of the primary during start-up.

LV5144

SS/TRK

RTRK1

RTRK2

26.7 k

10 k

VOUTSECONDARY= 1.8

RFB1

RFB2

12.5 k

10 k

0.8 V

LV5144

SS/TRK

RTRK3

RTRK4

20 k

RFB3

RFB4

10 k

20 k

0.8 V

VOUTSECONDARY2 =

1.2 V

VOUTPRIMARY = 3.3 V

Secondary Regulator

#1 Ra ometric

Tracking

Secondary Regulator

#2 Coincident Tracking

SYNCIN

SYNCOUT

from Primary

CSS1

2.2 nF

CSS2

2.2 nF

SYNCIN

10 k

Figure 8-7. Tracking Implementation With Primary, Ratiometric Secondary, and Coincident Secondary

Rails

For coincident tracking, connect the SS/TRK input of the secondary regulator to a resistor divider from the output

voltage of the primary that is the same as the divider used on the FB pin of the secondary. In other words,

simply select RTRK3 = RFB3 and RTRK4 = RFB4 as shown in Tracking Implementation With Primary, Ratiometric

Secondary, and Coincident Secondary Rails. As the primary voltage rises, the secondary voltage rises identically

LV5144

SNVSCJ1 – AUGUST 2023

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LM5145 Datasheet(PDF) 22 Page - Texas Instruments

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