0

0.4

0.8

1.2

1.6

Time (ms)

2.0

Calibration

Time(1.9 ms)

0.7 V

1.3 V

V

SS_INT

V

EN/SS

11

TPS56121

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SLUSAH4D – MARCH 2011 – REVISED FEBRUARY 2016

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Feature Description (continued)

Figure 19. Startup Sequence and Timing

7.3.2 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.

7.3.2.1 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 k

Ω ± 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.

7.3.2.2 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.

The voltage at EN/SS is internally clamped to 1.3 V before and/or during calibration to minimize the discharging

time once calibration is complete. The discharging current is from an internal current source of 140

μA and it

pulls the voltage down to 0.4 V. It then initiates the soft-start by charging up the capacitor using an internal

current source of 10

μA. The resulting voltage ramp on this pin is used as a second non-inverting input to the

error amplifier after an 800 mV (typical) downward level-shift; therefore, the actual soft-start does not take place

until the voltage at this pin reaches 800 mV.

If the EN/SS pin is left floating, the controller starts automatically. EN/SS must be pulled down to less than 270

mV to ensure that the chip is in shutdown mode.

7.3.3 Soft-Start Time

The soft-start time of the TPS56121 is user programmable by selecting a single capacitor. The EN/SS pin

sources 10

μA to charge this capacitor. The actual output ramp-up time is the amount of time that it takes for the

10

μA to charge the capacitor through a 600 mV range. There is some initial lag due to calibration and an offset

(800 mV) from the actual EN/SS pin voltage to the voltage applied to the error amplifier.