L6911E datasheet(17/34 Pages) STMICROELECTRONICS

17 / 34 page

L6911E

Device description

17/34

Equation 7

Equation 8

The worst condition depends on the input voltage available and the output voltage selected.

Anyway the worst case is the response time after removal of the load with the minimum

output voltage programmed and the maximum input voltage available.

5.7

Output capacitor

Since the microprocessors require a current variation beyond 10A doing load transients,

with a slope in the range of tenth A/

µsec, the output capacitor is a basic component for the

fast response of the power supply. In fact for first few microseconds they supply the current

to the load. The controller recognizes immediately the load transient and sets the duty cycle

at 100%, but the current slope is limited by the inductor value.

The output voltage has a first drop due to the current variation inside the capacitor

(neglecting the effect of the ESL):

Equation 9

A minimum capacitor value is required to sustain the current during the load transient

without discharge it. The voltage drop due to the output capacitor discharge is given by the

following equation:

Equation 10

Where DMAX is the maximum duty cycle value that is 100%. The lower is the ESR, the lower

is the output drop during load transient and the lower is the output voltage static ripple.

application

∆I

⋅

OUT

–

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

removal

∆I

⋅

OUT

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

∆V

OUT = ∆IOUT · ESR

∆V

OUT

∆I

OUT

INMIN

MAX

OUT

–

⋅

()

⋅⋅

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

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