NX9511B
APPLICATION INFORMATION
V -VOUT VOUT
1
IN
DIRIPPLE
=
=
´
´
LOUT
V
F
S
Symbol Used In Application Information:
IN
...(2)
VIN
- Input voltage
- Output voltage
- Output current
12V-1.8V 1.8V
1
´
´
= 2.25A
0.68uH 12V 1000kHz
VOUT
IOUT
∆VRIPPLE - Output voltage ripple
- Working frequency
Output Capacitor Selection
FS
Output capacitor is basically decided by the
amount of the output voltage ripple allowed during steady
state(DC) load condition as well as specification for the
load transient. The optimum design may require a couple
of iterations to satisfy both condition.
∆IRIPPLE - Inductor current ripple
Design Example
The following is typical application for NX9511B:
The amount of voltage ripple during the DC load
condition is determined by equation(3).
VIN = 12V
VOUT=1.8V
DIRIPPLE
FS=1000kHz
DVRIPPLE = ESR´ DIRIPPLE
+
...(3)
8´ F ´ COUT
IOUT=9A
S
∆VRIPPLE <=20mV
∆VDROOP<=100mV @ 9A step
Where ESR is the output capacitors' equivalent
series resistance,COUT is the value of output capacitors.
Typically ceramic capacitors are selected as out-
put capacitors in NX9811B applications. DC ripple spec
is easy to be met, usually mutiple ceramic capacitors
are required at the output to meet transient requirement.
In this example, two 47uF,X5R are used.
Output Inductor Selection
The selection of inductor value is based on induc-
tor ripple current, power rating, working frequency and
efficiency. Larger inductor value normally means smaller
ripple current. However if the inductance is chosen too
large, it brings slow response and lower efficiency. Usu-
ally the ripple current ranges from 20% to 40% of the
output current. This is a design freedom which can be
decided by design engineer according to various appli-
cation requirements. The inductor value can be calcu-
lated by using the following equations:
Compensator Design
Due to the double pole generated by LC filter of the
power stage, the power system has 180o phase shift ,
and therefore, is unstable by itself. In order to achieve
accurate output voltage and fast transient
response,compensator is employed to provide highest
possible bandwidth and enough phase margin.Ideally,the
Bode plot of the closed loop system has crossover fre-
quency between1/10 and 1/5 of the switching frequency,
phase margin greater than 50o and the gain crossing
0dB with -20dB/decade. Power stage output capacitors
usually decide the compensator type. If electrolytic
capacitors are chosen as output capacitors, type II com-
pensator can be used to compensate the system, be-
cause the zero caused by output capacitor ESR is lower
than crossover frequency. Otherwise type III compensa-
tor should be chosen.
V -VOUT VOUT
1
IN
LOUT
=
´
´
DIRIPPLE
V
F
S
IN
...(1)
IRIPPLE =k ´ IOUTPUT
where k is between 0.2 to 0.4.
Select k=0.3, then
12V-1.8V 1.8V
1
LOUT
=
´
´
0.4´ 9A 12V 1000kHz
LOUT =0.42uH
Choose inductor from COILCRAFT DO3316H-
681MLD with L=0.68uH is a good choice.
Current Ripple is recalculated as
Rev.1.5
01/16/08
12
