Datasheet

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Voutmin Ontimemin Fsmax Vinmax Ioutmin 2 RDS Ioutmin RL RDS= ´ ´ - ´ ´ - ´ +
-
Vref
R7 = R6
Vo Vref
TPS54519
SLVSAT3A –SEPTEMBER 2011–REVISED DECEMBER 2013 www.ti.com
BOOTSTRAP CAPACITOR SELECTION
A 0.1 μF ceramic capacitor must be connected between the BOOT to PH pin for proper operation. It is
recommended to use a ceramic capacitor with X5R or better grade dielectric. The capacitor should have 10 V or
higher voltage rating.
UNDER VOLTAGE LOCK OUT SET POINT
The Under Voltage Lock Out (UVLO) can be adjusted using an external voltage divider on the EN pin of the
TPS54519. The UVLO has two thresholds, one for power up when the input voltage is rising and one for power
down or brown outs when the input voltage is falling. For the example design, the supply should turn on and start
switching once the input voltage increases above 2.794 V (V
START
). After the regulator starts switching, it should
continue to do so until the input voltage falls below 2.595 V (V
STOP
).
The programmable UVLO and enable voltages are set using a resistor divider between Vin and ground to the EN
pin. Equation 2 and Equation 3 can be used to calculate the resistance values necessary. From Equation 2 and
Equation 3, a 14.3 kΩ between Vin and EN and a 11.5 kΩ between EN and ground are required to produce the
2.794 and 2.595 volt start and stop voltages.
OUTPUT VOLTAGE AND FEEDBACK RESISTORS SELECTION
For the example design, 100 kΩ was selected for R6. Using Equation 32, R7 is calculated as 80 kΩ. The nearest
standard 1% resistor is 80.5 kΩ.
(32)
Due to the internal design of the TPS54519, there is a minimum output voltage limit for any given input voltage.
The output voltage can never be lower than the internal voltage reference of 0.6 V. Above 0.6 V, the output
voltage may be limited by the minimum controllable on time. The minimum output voltage in this case is given by
Equation 33
Where:
Voutmin = minimum achievable output voltage
Ontimemin = minimum contollable on-time (60 ns typical. 110 nsec no load)
Fsmax = maximum switching frequency including tolerance
Vinmax = maximum input voltage
Ioutmin = minimum load current
RDS = minimum high side MOSFET on resistance (30 - 44 mΩ)
RL = series resistance of output inductor (33)
There is also a maximum achievable output voltage which is limited by the minimum off time. The maximum
output voltage is given by Equation 34
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