Datasheet

I
D
V
O
Âv
O
LM5022
SNVS480G –JANUARY 2007–REVISED DECEMBER 2013 www.ti.com
Hence an inductor must be selected that has a peak current rating greater than 2.5A and an average current
rating greater than 2.3A. One possibility is an off-the-shelf 33 µH ±20% inductor that can handle a peak current
of 3.2A and an average current of 3.4A. Finally, the inductor current ripple is recalculated at the maximum input
voltage:
Δi
L-VIN(MAX)
= (16 x 0.6) / (0.5 x 33) = 0.58A
P-P
(19)
OUTPUT CAPACITOR
The output capacitor in a boost regulator supplies current to the load during the MOSFET on-time and also filters
the AC portion of the load current during the off-time. This capacitor determines the steady state output voltage
ripple, ΔV
O
, a critical parameter for all voltage regulators. Output capacitors are selected based on their
capacitance, C
O
, their equivalent series resistance (ESR) and their RMS or AC current rating.
The magnitude of ΔV
O
is comprised of three parts, and in steady state the ripple voltage during the on-time is
equal to the ripple voltage during the off-time. For simplicity the analysis will be performed for the MOSFET
turning off (off-time) only. The first part of the ripple voltage is the surge created as the output diode D1 turns on.
At this point inductor/diode current is at the peak value, and the ripple voltage increase can be calculated as:
ΔV
O1
= I
PK
x ESR (20)
The second portion of the ripple voltage is the increase due to the charging of C
O
through the output diode. This
portion can be approximated as:
ΔV
O2
= (I
O
/ C
O
) x (D / f
SW
) (21)
The final portion of the ripple voltage is a decrease due to the flow of the diode/inductor current through the
output capacitor’s ESR. This decrease can be calculated as:
ΔV
O3
= Δi
L
x ESR (22)
The total change in output voltage is then:
ΔV
O
= ΔV
O1
+ ΔV
O2
- ΔV
O3
(23)
The combination of two positive terms and one negative term may yield an output voltage ripple with a net rise or
a net fall during the converter off-time. The ESR of the output capacitor(s) has a strong influence on the slope
and direction of ΔV
O
. Capacitors with high ESR such as tantalum and aluminum electrolytic create an output
voltage ripple that is dominated by ΔV
O1
and ΔV
O3
, with a shape shown in Figure 16. Ceramic capacitors, in
contrast, have very low ESR and lower capacitance. The shape of the output ripple voltage is dominated by
ΔV
O2
, with a shape shown in Figure 17.
Figure 16. ΔV
O
Using High ESR Capacitors
16 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM5022