(

)

-

=

×

×

Vinmin

Vout

Vout

Icirms

Iout

Vinmin

Vinmin

0.25

f

×

D

=

×

Ioutmax

Vin

Cin

sw

(

)

(

)

( )

Tss ms Iss

A

C7(nF)

Vref V

×

=

m

Vout

Vref

R8

R9

Vref

-

=

TPS54521

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SLVS981 – JUNE 2010

Input Capacitor Selection

The TPS54521 requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of roughly 4.7 µF on

each input voltage rail (VIN and PVIN). In some applications, additional bulk capacitance may also be required

for the PVIN input. The voltage rating of the input capacitor must be greater than the maximum input voltage.

The capacitor must also have a ripple current rating greater than the maximum input current ripple of the

TPS54521. The input ripple current for this design, using Equation 30, is 2.46 A.

(30)

The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the

capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that

is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors

because they have a high capacitance to volume ratio and are fairly stable over temperature. The capacitance

value of a capacitor decreases as the DC bias across a capacitor increases. For this example design, a ceramic

capacitor with at least a 25 V voltage rating is required to support the maximum input voltage. For this example,

one 10 mF and one 4.7 µF 25 V capacitors in parallel have been selected as the VIN and PVIN inputs are tied

together so the TPS54521 may operate from a single supply. The input capacitance value determines the input

ripple voltage of the regulator. The input voltage ripple can be calculated using Equation 31. Using the design

example values, Ioutmax=5 A, Cin=14.7 mF, Fsw=480 kHz, Equation 31 yields an input voltage ripple of 177 mV.

(31)

Slow Start Capacitor Selection

The slow start capacitor determines the minimum amount of time it takes for the output voltage to reach its

nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This

is also used if the output capacitance is very large and would require large amounts of current to quickly charge

the capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the

TPS54521 reach the current limit or excessive current draw from the input power supply may cause the input

voltage rail to sag. Limiting the output voltage slew rate solves both of these problems. The soft start capacitor

value can be calculated using Equation 32. The example circuit has the soft start time set to an arbitrary value of

3.5 ms which requires a 10 nF capacitor. In the TPS54521, Iss is 2.3 uA and Vref is 0.8 V.

(32)

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 Lockout Set Point

The Under Voltage Lock Out (UVLO) can be adjusted using the external voltage divider network of R1 and R2.

R1 is connected between VIN and the EN pin of the TPS54521 and R2 is connected between EN and GND. The

UVLO has two thresholds, one for power up when the input voltage is rising and one for power down or

brownouts when the input voltage is falling. For the example design, the supply should turn on and start

switching once the input voltage increases above 6.806V (UVLO start or enable). After the regulator starts

switching, it should continue to do so until the input voltage falls below 4.824 V (UVLO stop or disable).

Equation 2 and Equation 3 can be used to calculate the values for the upper and lower resistor values. For the

stop voltages specified, the nearest standard resistor value for R1 is 511 k

Ω and for R2 is 100 kΩ.

Output Voltage Feedback Resistor Selection

The resistor divider network, R8 and R9, is used to set the output voltage. For this example design, 10 k

Ω was

selected for R9. Using Equation 33, R8 is calculated as 31.25 k

Ω. The nearest standard 1% resistor is 31.6 kΩ.

(33)

Copyright © 2010, Texas Instruments Incorporated

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