Datasheet

11 10
Vout Vref
R R
Vref
-
=
( ) ( )
( )
( )
Tss ms Iss uA
Css nF
Vref V
×
=
max 0.25Iout
Vin
Cin fsw
×
D =
×
( )
min
min min
Vin Vout
Vout
Icirms Iout
Vin Vin
-
= × ×
TPS65251
www.ti.com SLVSAA4D –JUNE 2010–REVISED DECEMBER 2012
Input Capacitor
A minimum 10-µF X7R/X5R ceramic input capacitor is recommended to be added between VIN and GND. These
capacitors should be connected as close as physically possible to the input pins of the converters as they handle
the RMS ripple current shown in Equation 14. For this example, I
OUT
= 3 A, V
OUT
= 1.2 V, V
INmin
= 9.6 V, from
Equation 14, the input capacitors must support a ripple current of 0.99 A RMS.
(14)
The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be
calculated using Equation 15. Using the design example values, I
OUTmax
= 3 A, C
IN
= 10 µF, f
SW
= 500 kHz, yields
an input voltage ripple of 150 mV.
(15)
Soft Start Capacitor
The soft start capacitor determines the minimum amount of time it will take for the output voltage to reach its
nominal programmed value during power up. This is useful 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 soft start capacitor value can be calculated using Equation 16. In this example, the converter’s soft start time
is 0.8 ms. In TPS65251, Iss is 5 µA and Vref is 0.8 V. From Equation 16, the soft start capacitance is 5 nF. A
standard 4.7-nF ceramic capacitor is chosen in this design. In this example, C16 is 4.7nF
(16)
Bootstrap Capacitor Selection
A 0.047-µF ceramic capacitor must be connected between the BST to LX 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.
Adjustable Current Limiting Resistor Selection
The converter uses the voltage drop on the high-side MOSFET to measure the inductor current. The over current
protection threshold can be optimized by changing the trip resistor. Figure 24 governs the threshold of over
current protection for Buck 1. When selecting a resistor, do not exceed the graph limits. In this example, the over
current threshold is 3.2 A. In order to prevent a premature limit trip, the minimum line is used and the resistor is
100 kΩ.
When setting high-side current limit to large current values, ensure that the additional load immediately prior to
an overcurrent condition will not cause the switching node voltage to exceed 20 V. Additionally, ensure during
worst case operation, with all bucks loaded immediately prior to current limit, the maximum virtual junction
temperature of the device does not exceed 125°C.
Output Voltage and Feedback Resistors Selection
For the example design, 40.2 kΩ was selected for R10. Vout is 1.2 V, Vref = 0.8 V. Using Equation 17, R11 is
calculated as 80.4 kΩ. A standard 80.6-kΩ resistor is chose in this design.
(17)
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