-

Vref

R7 =

R6

Vo

Vref

(

)

(

)

(

)

(

)

(

)

(

)

Voutmin

Ontimemin Fsmax

Vinmax

Ioutmin

2 RDS

Ioutmin

RL RDS

=

´

´

-

´

´

-

´

+

TPS54519

www.ti.com

SLVSAT3

– SEPTEMBER 2011

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

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 k

Ω 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 k

Ω was selected for R6. Using Equation 32, R7 is calculated as 80 kΩ. The nearest

standard 1% resistor is 80.5 k

Ω.

(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 m

Ω)

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

Copyright

© 2011, Texas Instruments Incorporated

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