L7980ATR datasheet(11/44 Pages) STMICROELECTRONICS

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L7980

Functional description

Doc ID 15181 Rev 4

11/44

5.2

Soft-start

The soft-start is essential to assure correct and safe start up of the step-down converter. It

avoids inrush current surge and makes the output voltage increases monothonically.

The soft -start is performed by a staircase ramp on the non-inverting input (VREF) of the

error amplifier. So the output voltage slew rate is:

Equation 1

where SRVREF is the slew rate of the non-inverting input, while R1and R2 is the resistor

divider to regulate the output voltage (see Figure 7). The soft-start stair case consists of 64

steps of 9.5 mV each one, from 0 V to 0.6 V. The time base of one step is of 32 clock cycles.

So the soft start time and then the output voltage slew rate depend on the switching

frequency.

Figure 7.

Soft start scheme

Soft start time results:

Equation 2

For example with a switching frequency of 250 kHz the SSTIME is 8 ms.

5.3

Error amplifier and compensation

The error amplifier (E/A) provides the error signal to be compared with the sawtooth to

perform the pulse width modulation. Its non-inverting input is internally connected to a 0.6 V

voltage reference, while its inverting input (FB) and output (COMP) are externally available

for feedback and frequency compensation. In this device the error amplifier is a voltage

mode operational amplifier so with high DC gain and low output impedance.

The uncompensated error amplifier characteristics are the following:

OUT

VREF

--------

⎝⎠

⎛⎞

⋅

TIME

32 64

⋅

Fsw

-----------------

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