LM2743MTCX Datasheet, PDF (12/37 Pages) Texas Instruments – LM2743 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller

LM2743
SNVS276G – APRIL 2004 – REVISED MARCH 2013
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For the example case of V_OUT1 = 5V and V_OUT2 = 1.8V, with R_T1 set to 150Ω as before, R_T2 is calculated from the
above equation to be 265Ω. A timing diagram for the case of equal slew rates is shown in Figure 25.
5V
VOUT1
1.8V
VOUT2
1.8V
Figure 25. Tracking with Equal Slew Rates
SEQUENCING
The start up/soft-start of the LM2743 can be delayed for the purpose of sequencing by connecting a resistor
divider from the output of a master power supply to the SD pin, as shown in Figure 26.
Master Power
Supply
VOUT1
RS2
SD
LM2743
RS1
FB
VOUT2
RFB2
VFB
RFB1
Figure 26. Sequencing Circuit
A desired delay time t_DELAY between the startup of the master supply output voltage and the LM2743 output
voltage can be set based on the SD pin low-to-high threshold V_SD-IH and the slew rate of the voltage at the SD
pin, SR_SD:
t_DELAY = V_SD-IH / SR_SD
Note again, that in Figure 26, the LM2743’s output voltage has been represented symbolically as V_OUT2, i.e.
without explicitly showing the power components.
V_SD-IH is typically 1.08V and SR_SD is the slew rate of the SD pin voltage. The values of the sequencing divider
resistors R_S1 and R_S2 set the SR_SD based on the master supply output voltage slew rate, SR_OUT1, using the
following equation:
R_S1
SR_SD = SR_{OUT1 RS1 + RS2}
12
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