TPS54361_16 Datasheet, PDF (16/51 Pages) Texas Instruments – 4.5-V to 60-V Input, 3.5-A, Step-Down DC-DC Converter

TPS54361
SLVSC39C – NOVEMBER 2013 – REVISED FEBRUARY 2016
Feature Description (continued)
VIN
TPS54361
R_UVLO1
EN
R_UVLO2
i1 ihys
V_EN
www.ti.com
VIN
R_UVLO1
EN
R_UVLO2
TPS54361
10 kW
Node
5.8 V
Figure 26. Adjustable Undervoltage Lockout
(UVLO)
Figure 27. Internal EN Pin Clamp
7.3.8 Soft-Start/Tracking Pin (SS/TR)
The TPS54361 device effectively uses the lower voltage of the internal voltage reference or the SS/TR pin
voltage as the reference voltage of the power-supply and regulates the output accordingly. A capacitor on the
SS/TR pin to ground implements a soft-start time. The TPS54361 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 1.7 μA. The soft-start
capacitor must 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 TPS54361 device does not start 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 TPS54361 device stops switching and the SS/TR must discharge to 54 mV
when one of the following occurs: the VIN UVLO is exceeded, the EN pin pulled below 1.2 V, or 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.
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