Texas-instruments SLVU013 User Manual download pdf (Page 24)

TPS56xx Functions

2-4

2.1.2 Inhibit

The inhibit circuit is a comparator with a 2.1-V start voltage and a 100-mV

hysteresis. When inhibit is low, the output drivers are low and the slowstart

capacitor is discharged. When inhibit is above the start threshold, the short

across the slowstart capacitor is released and normal operation begins.

When the system logic supply is connected to the inhibit pin, the inhibit pin also

controls power sequencing by locking out controller operation until the system

logic supply exceeds the input threshold voltage of the inhibit circuit; thus, the

12-V supply and the system logic supply must be above UVLO thresholds

before the controller is allowed to start up.

2.1.3 Slowstart Design

Slowstart or soft-start is added to reduce power-up transients. Without

slowstart, when input power is first applied, the TPS56xx attempts to raise the

output voltage (initially zero) to its normal operating level by turning on the top

MOSFET until the voltage is approximately Vref. This can cause high transient

currents to flow in the output inductor and output capacitor. Although this form

of startup usually does not cause component failures, it does apply stresses

much greater than those typically encountered in normal operation. It is good

design practice to include slowstart circuitry to avoid these unnecessary

stresses.

The slowstart circuit in the TPS56xx controls the rate at which the output

voltage powers up. A capacitor, C21, connected between SLOWST (pin 8) and

ANAGND (pin 7), is charged by an internal current source. This current source

is proportional to the reference voltage and is adjustable by an external

resistance selected by the user. The output voltage follows the voltage on the

slowstart capacitor during startup. Since the charging current is proportional

to the reference voltage, the slowstart time is independent of the reference

voltage for a given resistor value.

Choices of the slowstart time and the slowstart capacitor value are largely

arbitrary as long as system start-up time requirements are met. For this

example design, a slowstart time of 10 ms is chosen, and the slowstart

capacitor is chosen to be 0.1 µF. Therefore, to charge 0.1 µF from zero volts

to 3.3 V in 10 ms, the following equation holds:

SLOWSTART



SLOWSTART



 0.1 



3.3

 33 

The slowstart charging current is determined by the following equation:

SLOWSTART



(

VREFB

)

where I(VREFB) = the current out of VREFB (pin 5).

For an I

SLOWSTART

current equal to 33 µA, I(VREFB) should be set to:

(

VREFB

)

 5 

SLOWSTART

 5  33 

 165 

The voltage on VREFB (pin 5) is a buffered reference voltage. The resistance

from the VREFB pin to ANAGND can be calculated as:

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Texas-instruments SLVU013 User Manual Page 24

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