TPS54541 Datasheet, PDF (15/48 Pages) Texas Instruments – 4.5-V to 42-V Input, 5-A Step-Down DC-DC Converter With Soft-Start and Eco-mode

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VIN
TPS54541
R_UVLO1
EN
R_UVLO2
i1 ihys
V_EN
TPS54541
SLVSC57 – OCTOBER 2013
VIN
R_UVLO1
EN
R_UVLO2
TPS54541
10 kW
Node
5.8 V
Figure 26. Adjustable Undervoltage Lockout
(UVLO)
Figure 27. Internal EN Pin Clamp
Soft-Start/Tracking Pin (SS/TR)
The TPS54541 device effectively uses the lower voltage of the internal voltage reference or the SS/TR pin
voltage as the power-supply's reference voltage and regulates the output accordingly. A capacitor on the SS/TR
pin to ground implements a soft start time. The TPS54541 device has an internal pullup current source of 1.7 μA
that charges the external soft-start capacitor. The calculations for the soft start time (10% to 90%) are shown in
Equation 5. The voltage reference (V_REF) is 0.8 V and the soft-start current (I_SS) is 1.7μA. The soft-start capacitor
should remain lower than 0.47 μF and greater than 0.47 nF.
C_SS
(nF) =
T_SS (ms) ´ I_SS (µA)
V_REF (V) ´ 0.8
(5)
At power up, the TPS54541 device does not begin switching until the soft start pin is discharged to less than 54
mV to ensure a proper power-up, see Figure 28.
Also, during normal operation, the TPS54541 device stops switching, the SS/TR must discharge to 54 mV, and,
when the VIN UVLO is exceeded, the EN pin must pull below 1.2 V, otherwise a thermal shutdown event occurs.
The FB voltage follows the SS/TR pin voltage with a 42-mV offset up to 85% of the internal voltage reference.
When the SS/TR voltage is greater than 85% on the internal reference voltage the offset increases as the
effective system reference transitions from the SS/TR voltage to the internal voltage reference (see Figure 23).
The SS/TR voltage ramps linearly until clamped at 2.7 V typically as shown in Figure 28.
Figure 28. Operation of SS/TR Pin when Starting
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