lm5022mme National Semiconductor Corporation, lm5022mme Datasheet - Page 12

• Manufacturer Part Number: lm5022mme
• Description: 60v Low Side Controller For Boost And Sepic
• Manufacturer: National Semiconductor Corporation

www.national.com
Inductance for Maximum Input Voltage
Maximum average inductor current occurs at V
corresponding inductor ripple current is 0.92A
an inductance that exceeds the ripple current requirement at
V
vides a tradeoff that allows smaller magnetics at the cost of
higher ripple current at maximum input voltage. For this ex-
ample, a 33 µH inductor will satisfy these requirements.
The second criterion for selecting an inductor is the peak cur-
rent carrying capability. This is the level above which the
inductor will saturate. In saturation the inductance can drop
off severely, resulting in higher peak current that may over-
heat the inductor or push the converter into current limit. In a
boost converter, peak current, I
average inductor current plus one half of the ripple current.
First, the current ripple must be determined under the condi-
tions that give maximum average inductor current:
Maximum average inductor current occurs at V
the selected inductance of 33 µH yields the following:
The highest peak inductor current over all operating condi-
tions is therefore:
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% in-
ductor that can handle a peak current of 3.2A and an average
current of 3.4A. Finally, the inductor current ripple is recalcu-
lated at the maximum input voltage:
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
ical parameter for all voltage regulators. Output capacitors are
IN(MIN)
Δi
D
and the requirement to stay in CCM for V
L-VIN(MAX)
VIN(MAX)
I
Δi
PK
L
I
= I
L-VIN(MIAX)
= (9 x 0.78) / (0.5 x 33) = 425 mA
L
= (40 - 16 + 0.5) / (40 + 0.5) = 60%
+ 0.5 x Δi
Δi
= (16 x 0.6) / (0.5 x 33) = 0.58A
L
= 0.4 x 1.25A = 0.5A
= 0.5 / (1 – 0.6) = 1.25A
L
= 2.3 + 0.213 = 2.51A
PK
, is equal to the maximum
IN(MIN)
P-P
P-P
IN(MIN)
IN(MAX)
. Selecting
P-P
, and the
O
. Using
, a crit-
pro-
12
selected based on their capacitance, C
ries resistance (ESR) and their RMS or AC current rating.
The magnitude of ΔV
steady state the ripple voltage during the on-time is equal to
the ripple voltage during the off-time. For simplicity the anal-
ysis 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:
The second portion of the ripple voltage is the increase due
to the charging of C
can be approximated as:
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:
The total change in output voltage is then:
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
Capacitors with high ESR such as tantalum and aluminum
electrolytic create an output voltage ripple that is dominated
by ΔV
capacitors, in contrast, have very low ESR and lower capac-
itance. The shape of the output ripple voltage is dominated by
ΔV
O2
, with a shape shown in Figure 6.
O1
FIGURE 5. ΔV
and ΔV
O3
ΔV
ΔV
, with a shape shown in Figure 5. Ceramic
O
O2
O
ΔV
ΔV
= ΔV
O
O
through the output diode. This portion
= (I
Using High ESR Capacitors
O3
is comprised of three parts, and in
O1
O
O1
= Δi
= I
/ C
+ ΔV
PK
O
L
) x (D / f
x ESR
x ESR
O2
- ΔV
O
, their equivalent se-
SW
O3
)
20212226
O
.