1.008

101756

sw (kHz) =

RT (k )

f

W

0.991

92417

RT (k ) =

sw (kHz)

f

W

f

=

SS

SW

1024

t

(ms)

(kHz)

=

-

+

ENA

UVLO2

START

ENA

1

UVLO1

V

R

V

V

I

R

-

=

START

STOP

UVLO1

HYS

V

V

R

I

TPS54560

i

VIN

RUVLO1

RUVLO2

EN

V

EN

ihys

1

VIN

R

UVLO1

R

UVLO2

EN

Node

5.8 V

10 kW

TPS54560

www.ti.com

SLVSBN0 – MARCH 2013

DETAILED DESCRIPTION (continued)

Figure 24. Adjustable Undervoltage Lockout

Figure 25.

(UVLO)

(2)

(3)

Internal Soft-Start

The TPS54560 has an internal digital soft-start that ramps the reference voltage from zero volts to its final value

in 1024 switching cycles. The internal soft-start time (10% to 90%) is calculated using Equation 4

(4)

If the EN pin is pulled below the stop threshold of 1.2 V, switching stops and the internal soft-start resets. The

soft-start also resets in thermal shutdown.

Constant Switching Frequency and Timing Resistor (RT/CLK) Pin)

The switching frequency of the TPS54560 is adjustable over a wide range from 100 kHz to 2500 kHz by placing

a resistor between the RT/CLK pin and GND pin. The RT/CLK pin voltage is typically 0.5 V and must have a

resistor to ground to set the switching frequency. To determine the timing resistance for a given switching

frequency, use Equation 5 or Equation 6 or the curves in Figure 5 and Figure 6. To reduce the solution size one

would typically set the switching frequency as high as possible, but tradeoffs of the conversion efficiency,

maximum input voltage and minimum controllable on time should be considered. The minimum controllable on

time is typically 135 ns which limits the maximum operating frequency in applications with high input to output

step down ratios. The maximum switching frequency is also limited by the frequency foldback circuit. A more

detailed discussion of the maximum switching frequency is provided in the next section.

(5)

(6)

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