0.8 V

Tss

Css

4.7 uA

×

æ

ö

=

× ç

÷

×

è

ø

FB

R1

40.2kΩ

R2

7.7kΩ

C1

4.7 nF

10

COMP

2

-

+

EA

0.8V

Reference

Vout=5V

2

1

OUT

0.8V

R

R (

)

V

0.8V

=

×

-

TPS65281, TPS65281-1

SLVSBH7B – JULY 2012 – REVISED DECEMBER 2012

www.ti.com

BUCK DC/DC CONVERTER DETAILED DESCRIPTION

Output Voltage

The TPS65281/TPS65281-1 regulates output voltage set by a feedback resistor divider to 0.8-V reference

voltage. This pin should be directly connected to middle of resistor divider. It is recommended to use 1%

tolerance or better divider resistors.Great care should be taken to route the FB line away from noise sources,

such as the inductor or the LX switching node line. Start with 40.2 k

Ω for the R1 resistor and use Equation 4 to

calculate R2.

(4)

Figure 29. Buck Feedback Resistor Divider

Switching Frequency Selection

The selection of switching frequency is a tradeoff between efficiency and component size. Low frequency

operation increases efficiency by reducing MOSFET switching losses, but requires larger inductance and

capacitance to maintain low output ripple voltage. The switching frequency of the TPS65281/TPS65281-1 buck

controller can be selected with the connection at ROSC pin. The ROSC pin can be connected to AGND, tied to

V7V, open or programmed through an external resistor. Tying ROSC pin to AGND selects 300 kHz, while tying

ROSC ping to V7V or floating ROSC pin selects 600 kHz. Placing a resistor between ROSC and AGND allows

the buck switching frequency to be programmed between 300 kHz to 1.4 MHz, as shown in Figure 14. The

programmed clock frequency by an external resistor can be calculated with the following equation:

(5)

Soft-Start Time

The start-up of buck output is controlled by the voltage on the SS pin. When the voltage on the SS pin is less

than the internal 0.8-V reference, the TPS65281/TPS65281-1 regulates the internal feedback voltage to the

voltage on the SS pin instead of 0.8 V. The SS pin can be used to program an external soft-start function or to

allow output of the buck to track another supply during start-up. The device has an internal pull-up current source

of 4.7 µA that charges an external soft-start capacitor to provide a linear ramping voltage at SS pin. The

rise smoothly from 0 V to its final regulated value. The total soft-start time will be approximately:

(6)

20

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Product Folder Links: TPS65281 TPS65281-1