AN557 datasheet(14/52 Pages) STMICROELECTRONICS

14 / 52 page

AN557 APPLICATION NOTE

14/52

The PWM comparator block commutates when Vr = Vc. Therefore:

from which is obtained

The time t obtained from this equation is equal to the ON time of the power transistor. The corresponding

duty-cycle is given by:

10)

Consequently, after leaving the discharged capacitor of Soft Start, the output of the regulator will reach its

value when the voltage across the Css capacitor, charged with constant current, has reached the value

Vr - 0.5V.

The time necessary in order that the output rises from zero to the nominal value is given by:

11)

in which Css is the Soft Start capacitor and Iss the Soft Start current. Considering Soft Start time as tss,

the required time for the Soft Start capacitor to change itself approx from (2Vbe - 0.5V) = (1.2V - 0.5V) to

Vr - 0.5V, is:

12)

By taking Vr from (10):

13)

and substituting it in (12), we obtain:

14)

UNDERVOLTAGE LOCKOUT

The device contains the protection block of under-voltage lockout which keeps the power stage turned-off

as long as the supply voltage does not reach at least 12V. At this point the device starts up with Soft Start.

The function of undervoltage is also provided with an hysteresis of 1V to make it better immune to the rip-

ple present on the supply voltage.

ERROR AMPLIFIER

The error amplifier is a transconductance type and deliver an output current proportional to the voltage in-

balance of the two inputs. The simplified diagram is presented in fig 12.The principal characteristics of this

uncompensated operational amplifier are the following: Gm = 4mA/V, Ro = 2.5Mohm, Avo = 80dB, Isource-

sink = 200

µA, Input Bias Current = 0.3µA. The frequency response of the op. amp. is given in fig. 13.

Ignoring the high frequency response and hypothesizing that the second pole is below the 0 dB axis in the

all the conditions of loop compensation, it is possible to make a first approximation with the equivalent cir-

cuit of fig. 14

---------- t

⋅

–

⋅

------------------------- t

⋅

Vr T

–

()

⋅⋅

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

-------

Vr T

–

()

⋅⋅

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

–

()

⋅

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

------

start

–

0.5 V

–

()

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

⋅

1.2V

–

()

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

⋅

------

–

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

⋅

---------

------

–

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

1.2V

–

⋅

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