10

Application information

10.1

Output inductor

The inductor value is defined by a compromise among the dynamic response, ripple, efficiency,cost and size.

Usually, the inductance is calculated to maintain the inductor ripple current (ΔIL) between 20% and 30% of the

maximum output current. Given the switching frequency (FSW), the input voltage (VIN), the output voltage (VOUT)

and the desired ripple current (ΔIL), the inductance can be calculated as follows:

L=VIN−VOUT

FSW⋅ΔIL ⋅VOUTVIN

(8)

Figure 12 shows the ripple current vs. the output voltage for different inductance, with VIN = 5 V and VIN = 12 V.

Increasing the value of the inductance, the inductor ripple current (and output voltage ripple accordingly) reduces

but, at the same time, the converter response time to load transients increases. Higher inductance means that

the inductor needs more time to change its current from initial to final value. Until the inductor has finished its

charging, the additional output current is supplied by the output capacitors. Minimizing the response time, the

required output capacitance can be minimized. If the compensation network is designed with high bandwidth,

during a load transient the device can saturate duty cycle (0% or 80%). When this condition is reached, the

response time is limited only by the time required to charge the inductor.

Figure 12. Inductor current ripple vs. output voltage

0

2

4

6

8

10

0

1

2

3

4

5

Output voltage [V]

Vin=12 V, L=1 µH

Vin=12 V, L=2 µH

Vin=5 V, L=500 nH

Vin=5 V, L=1.5 µH

AM16776v1

A6727B

Application information

DS14159 - Rev 2

page 16/26