Datasheet

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E 1 = 5 / div

SS2 = 500 mV / div

Vo1 = 500 mV / div

Vo2 = 500 mV / div

Time = 1 msec / div

SS/TR1

TPS54478

EN1

PWRGD1

SS/TR2

TPS54478

EN2

PWRGD2

VOUT1

VOUT 2

1.135

90066

RT (k ) =

Fs(kHz)

0.8813

23439

Fs(kHz)

RT(k )

TPS54478

www.ti.com

SLVSAS2 –JUNE 2011

Figure 27. Simultaneous Startup Sequence Figure 28. Simultaneous Start-Up using Coupled

SS/TR Pins

CONSTANT SWITCHING FREQUENCY and TIMING RESISTOR (RT/CLK Pin)

The switching frequency of the TPS54478 is adjustable over a wide range from 200 kHz to 2000 kHz by placing

a maximum of 150 kΩ and minimum of 16 kΩ, respectively, on the RT/CLK pin. An internal amplifier holds this

pin at a fixed voltage when using an external resistor to ground to set the switching frequency. The RT/CLK is

typically 0.5 V. To determine the timing resistance for a given switching frequency, use the curve in Figure 5 or

Equation 7.

(7)

vertical spacer

(8)

To reduce the solution size one would typically set the switching frequency as high as possible, but tradeoffs of

the efficiency, maximum input voltage and minimum controllable on time should be considered.

The minimum controllable on time is typically 100 ns at full current load and 120 ns at no load, and limits the

maximum operating input voltage or output voltage.

OVERCURRENT PROTECTION

The TPS54478 implements current mode control which uses the COMP pin voltage to turn off the high side

MOSFET and turn on the low side MOSFET on a cycle by cycle basis. Each cycle the switch current and the

COMP pin voltage are compared, when the peak switch current intersects the COMP voltage the high side

switch is turned off. During overcurrent conditions that pull the output voltage low, the error amplifier will respond

by driving the COMP pin high, increasing the switch current. The error amplifier output is clamped internally.

This clamp functions as a switch current limit. When the OCP reaches 512 cycles, the converter enters hiccup

mode in which no switching action happens for about 16000 cycles. This helps the reduction of the power

consumption during the over current event.