Texas-instruments SLVU013 User Manual download pdf (Page 25)

TPS56xx Functions

2-5

Design Procedure

VREFB

3.3

165 m

+ 20

This value is used to determine the values of R10 and R14 that set the

hysteresis level.

The equations above can be used to derive a simplified relationship for the

slowstart time as shown:

SLOWSTART

+ 5

SLOWSTART

VREFB

start-up waveforms for different reference voltage settings are given in

Figures 3–7, 3–8, 3–9, 3–16, 3–17, 3–18, 3–25, 3–26, 3–27, 3–34, 3–35, and

3–36 in the test results section, showing that slowstart time is independent of

the reference voltage.

2.1.4 Hysteresis Setting

The next step in this design is choosing the desired output voltage ripple. As

a first approximation, the output voltage ripple is simply the difference between

the two levels (V

and V

) shown in Figure 1–2. The hysteresis setting of

the hysteresis comparator of the TPS56xx sets these two levels. The

hysteresis is set by two external resistors and is centered around VREF (pin

5). The hysteretic comparator is designed with low input offset voltage

(±2.5 mV max) low propagation delays (250ns max to gate driver outputs with

10mV overdrive) and accurate hysteresis setting (±3.5 mV max). The

hysteresis is proportional to the reference voltage; setting Vref to a new value

automatically adjusts the hysteresis to be the same percentage of Vref.

The total output ripple is greater than the ripple set by the hysteresis compara-

tor. To accurately choose the output voltage ripple, all of the propagation de-

lays must be considered. The first step is to calculate the amount of ripple ex-

pected due to the delays, in addition to the ripple set by the hysteresis compa-

rator. Using the terminology of Section 2.2.4.1, this additional ripple is equal

to V

p-p

– Hyst and will be referred to V

del

for ease of reference. Or:

p-p

Hyst

)

del

Where:

p-p

=the total output ripple,

Hyst = the hysteresis setting of the hysteretic comparator, and

del

= the output ripple exceeding Hyst and due to all propagation

delays.

Assuming the output ripple during the delay time is primarily caused by the

inductor ripple current flowing through the output capacitors’ ESR, the

additional ripple, V

del

, can be estimated as:

del

–V

del

ESR

)

del

ESR

del

Where L is the value of the output inductor.

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Texas-instruments SLVU013 User Manual Page 25

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