NX7102
®
3A High Voltage Synchroous Buck Converter
TM
PRODUCTION DATA SHEET
APPLICATION INFORMATION
Where ESR is the output capacitor’s equivalent series
resistance, COUT is the value of output capacitor.
SYMBOL USED IN APPLICATION INFORMATION:
VIN
VOUT
IOUT
- Input voltage
- Output voltage
- Output current
Typically when large value capacitors are selected such as
Aluminum Electrolytic, POSCAP and OSCON types are
used, the amount of the output voltage ripple is dominated by
the first term in equation(2) and the second term can be
neglected.
If ceramic capacitors are chosen as output capacitors, both
terms in equation (2) need to be evaluated to determine the
overall ripple. Usually when this type of capacitor is selected,
the amount of capacitance per single unit is not sufficient to
meet the transient specification, which results in parallel
configuration of multiple capacitors.
ΔVRIPPLE - Output voltage ripple
FS
- Working frequency
ΔIRIPPLE
- Inductor current ripple
DESIGN EXAMPLE
The following is typical application for NX7102, the
schematic is figure 1.
V
V
IN = 12V
OUT =3.3V
In this design two 22uF 6.3V X5R ceramic capacitors are
chosen as output capacitors.
I
OUT =3A
INPUT CAPACITOR SELECTION
OUTPUT INDUCTOR SELECTION
Input capacitors are usually a mix of high frequency ceramic
capacitors and bulk capacitors. Ceramic capacitors bypass the
high frequency noise, and bulk capacitors supply current to
the MOSFETs. Usually 1uF ceramic capacitor is chosen to
decouple the high frequency noise. The bulk input capacitors
are determined by voltage rating and RMS current rating. The
RMS current in the input capacitors can be calculated as:
The selection of inductor value is based on inductor ripple
current, power rating, working frequency and efficiency. A
larger inductor value normally means smaller ripple current.
However if the inductance is chosen too large, it results in
slow response and lower efficiency. Usually the ripple
current ranges from 20% to 40% of the output current. This
is a design freedom which can be determined by the design
engineer according to various application requirements. The
inductor value can be calculated by using the following
equations:
IRMS = IOUT × D × 1-D
... (3)
VOUT
D =
V
IN
In this design two 10uF 25V X5R ceramic capacitors are
chosen.
V - VOUT VOUT
1
IN
LOUT
=
×
×
ΔIRIPPLE
V
F
S
IN
OUTPUT VOLTAGE CALCULATION
IRIPPLE = k ×IOUTPUT
... (1)
Output voltage is set by reference voltage and external
voltage divider. The reference voltage is fixed at 0.925V. The
divider consists of two ratioed resistors so that the output
voltage applied at the FB pin is 0.925V when the output
voltage is at the desired value. The following equation and
picture show the relationship between and voltage divider.
where k is between 0.2 to 0.4.
In this design, k is set at 0.23 and 10uH inductor value is
chosen. In order to avoid output oscillation at light load, a
minimum 8.2uH inductor is required for all NX7102
application.
OUTPUT CAPACITOR SELECTION
Vout
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.
NX7102
R1
FB
COMP
The amount of voltage ripple during the DC load condition is
determined by equation (2).
R2
Vref
ΔIRIPPLE
ΔVRIPPLE = ESR× ΔIRIPPLE
+
... (2)
8×F × COUT
S
Figure 5 Voltage Divider
Copyright © 2010
Rev.1.3, 2011-04-22
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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