Datasheet

0.4

0.8

1.2

1.6

Input Voltage (V)

Time (ms)

2.0

Calibration

Time(1.9 ms)

0.7 V

1.3 V

SS_INT

EN/SS

TPS56221

SLUSAH5B –MARCH 2011–REVISED SEPTEMBER 2012

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APPLICATION INFORMATION

Introduction

The TPS56221 is a 25-A high performance synchronous buck converter with two integrated N-channel

NexFET™ power MOSFETs. The device implements a voltage-mode control with voltage feed-forward

compensation that responds instantly to input voltage change. Pre-bias capability eliminates concerns about

damaging sensitive loads.

Voltage Reference

The 600-mV bandgap cell is internally connected to the non-inverting input of the error amplifier. The reference

voltage is trimmed with the error amplifier in a unity gain configuration to remove amplifier offset from the final

regulation voltage. The 1% tolerance on the reference voltage allows the user to design a very accurate power

supply.

Figure 19. Startup Sequence and Timing

Enable Functionality, Startup Sequence and Timing

After input power is applied, an internal 40-μA current source begins to charge the soft-start capacitor connected

from EN/SS to GND. When the voltage across that capacitor increases to 0.7 V, it enables the internal BP

regulator followed by a calibration. Total calibration time is approximately 1.9 ms. See Figure 19. During the

calibration, the device performs the following two functions.

COMP Pin Impedance Sensing

The device samples the impedance at the COMP pin and determines the appropriate operating switching

frequency. If there is no resistor connected from the COMP pin to GND, the switching frequency is set to the

default value of 500 kHz. If a resistor of 40.2 kΩ ± 10% is connected from the COMP pin to GND, the switching

frequency is set to 300 kHz. Alternatively, if a resistor of 13.3 K ± 10% is connected from the COMP pin to GND,

the switching frequency is set to 1 MHz.

After a 1.1-ms time period, the COMP pin is then brought low for 0.8 ms. This ensures that the feedback loop is

preconditioned at startup and no sudden output rise occurs at the output of the converter when it is allowed to

start switching.

Overcurrent Protection (OCP) setting

The device sources 10 μA (typical) to the resistor connected from the ILIM pin to GND. The voltage developed

across that resistor multiplied by a factor of 2 is then sampled and latched off internally as the OCP trip level for

the low-side FET until one cycles the input or toggles the EN/SS.

Product Folder Links :TPS56221