L6911ETR datasheet(16/34 Pages) STMICROELECTRONICS

16 / 34 page

Device description

L6911E

16/34

Figure 7.

Hiccup mode and Inductor ripple current vs. VOUT

5.6

Inductor design

The inductance value is defined by a compromise between the transient response time, the

efficiency, the cost and the size. The inductor has to be calculated to sustain the output and

the input voltage variation to maintain the ripple current

∆I

L between 20% and 30% of the

maximum output current. The inductance value can be calculated with this relationship:

Equation 6

Where fSW is the switching frequency, VIN is the input voltage and VOUT is the output

voltage. Figure 7 b shows the ripple current vs. the output voltage for different values of the

inductor, with vin = 5V and Vin = 12V.

Increasing the value of the inductance reduces the ripple current but, at the same time,

reduces the converter response time to a load transient. If the compensation network is well

designed, the device is able to open or close the duty cycle up to 100% or down to 0%. The

response time is now the time required by the inductor to change its current from initial to

final value. Since the inductor has not finished its charging time, the output current is

supplied by the output capacitors. Minimizing the response time can minimize the output

capacitance required.

The response time to a load transient is different for the application or the removal of the

load: if during the application of the load the inductor is charged by a voltage equal to the

difference between the input and the output voltage, during the removal it is discharged only

by the output voltage. The following expressions give approximate response time for

∆I load

transient in case of enough fast compensation network response:

0.5

1.5

2.5

3.5

Output Voltage [V]

L=3

µH,

Vin=12V

L=2

µH,

Vin=12V

L=1.5

µH, Vin=12V

L=2

µH,

Vin=5V

L=1.5

µH,

Vin=5V

L=3

µH, Vin=5V

OUT

–

∆

⋅

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

OUT

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

⋅

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