nx2124 Microsemi Corporation, nx2124 Datasheet - Page 7

• Manufacturer Part Number: nx2124
• Description: 300khz Synchronous Pwm Controller
• Manufacturer: Microsemi Corporation

pacitors, both terms in equation (3) need to be evalu-
ated to determine the overall ripple. Usually when this
type of capacitors are selected, the amount of capaci-
tance per single unit is not sufficient to meet the tran-
sient specification, which results in parallel configura-
tion of multiple capacitors .
with 2m
needs to be studied for transient requirement.
Based On Transient Requirement
is specified as:
transient is composed of two sections. One Section is
dependent on the ESR of capacitor, the other section is
a function of the inductor, output capacitance as well as
input, output voltage. For example, for the overshoot,
when load from high load to light load with a
transient load, if assuming the bandwidth of system is
high enough, the overshoot can be estimated as the fol-
lowing equation.
tance of each capacitor if multiple capacitors are used
in parallel.
put inductor is smaller than the critical inductance, the
voltage droop or overshoot is only dependent on the ESR
Rev.1.8
02/28/08
L
crit
For example, one 100uF, X5R ceramic capacitor
Although this meets DC ripple spec, however it
Typically, the output voltage droop during transient
During the transient, the voltage droop during the
where
where
where ESR
The above equation shows that if the selected out-
V
V
V
0
L
RIPPLE
DROOP
overshoot
ESR C
V
ESR is used
OUT
if L L
I
< V
step
is the a function of capacitor, etc.
2m
15mV
E
ESR
I
OUT
TRAN
step
and C
ESR C
crit
2.56A
V
@ step load DI
.
OUT
E
The amount of output ripple is
I
represents ESR and capaci-
step
OUT
ESR
8 300kHz 100uF
2 L C
if L L
V
E
OUT
2.56A
C
I
step
STEP
OUT
E
V
crit
OUT
2
...(8)
...(6)
...(7)
DI
STEP
of output capacitor. For low frequency capacitor such
as electrolytic capacitor, the product of ESR and ca-
pacitance is high and
transient spec is dependent on the ESR of capacitor.
capacitors in parallel. The number of capacitors can be
calculated by the following
sient is 100mV for 9A load step.
(1500uF, 13m ) is used, the critical inductance is given
as
than critical inductance. In that case, the output voltage
transient only dependent on the ESR.
and transient requirement. Overall, we can choose N=2.
N
In most cases, the output capacitors are multiple
where
For example, assume voltage droop during tran-
If the SANYO electrolytic capaictor 16ME1500WG
The selected inductor is 1.5uH which is smaller
number of capacitors is
The number of capacitors has to satisfied both ripple
N
2 1.5 H 220 F 100mV
0
L
ESR
1.2
13m
13m
L
V
crit
100mV
OUT
ESR
if L L
I
E
V
step
tran
ESR
E
V
9A
1500 F 1.8V
I
1.8V
tran
ESR
step
9A
E
I
step
crit
L L
E
2 L C
C
I
step
E
C
2 L C
NX2124/2124A
E
crit
V
V
OUT
is true. In that case, the
OUT
E
if L L
V
3.9 H
OUT
E
(0)
V
tran
2
V
tran
crit
2
2
...(10)
...(9)
7