ML4863
DESIGN CONSIDERATIONS
DESIGN PROCEDURE
A typical design can be implemented by using the
following procedure.
1.
Specify the application by defining:
The maximum input voltage (V
IN(MAX)
)
The mainimum input voltage (V
IN(MIN)
)
The maximum output current (I
OUT(MAX)
)
The maximum output ripple (DV
OUT
)
As a general design rule, the output ripple should be kept
below 100mV to ensure stability.
2.
3a.
3b.
Select a sense resistor, R
SENSE
, using equation 1.
Motorola
See Table 1 for suggested component manufacturers.
Component Manufacturer
Sense
Resistors
Inductors
Dale
IRC
Coilcraft
Coiltronics
Dale
Capacitors
MOSFETs
AVX
Sprague
National
Part
Number
LRC Series
WSL Series
R4999
Phone
(402) 563-6506
(512) 992-7900
(847) 639-6400
OCTA-PAC Series (561)241-7876
LPE-6562 Series (605) 665-9301
LPT-4545 series
TPS series
593D Series
NDS94XX
NDS99XX
MMDF Series
MMSF Series
Littlefoot Series
(207) 282-5111
(207) 324-4140
(800) 272-9954
(602) 897-5056
(408) 988-8000
Determine the inductance required for the
optimum output ripple using equation 2.
Determine the minimum inductor current rating
required. The peak inductor current is calculated
using the following formula:
I
L PEAK
-
6
.
235mV V
IN ( MAX)
(25
10 )
=
+
R
SENSE
L
P
Siliconix
Table 1. Component Suppliers
(4)
DESIGN EXAMPLE
1.
Specify the application by defining:
V
IN(MAX)
= 6V
V
IN(MIN)
= 4V
I
OUT(MAX)
= 500mA
DV
OUT
= 100mV
Select the sense resistor, R
SENSE
, using Equation 1:
R
SENSE
=
4
150mV
4V
×
+
×
0.85
5
+
4
500mA 20
×
6
×
0.5
3c.
Specify the inductor's DC winding resistance. A
good rule of thumb is to allow 5mW, or less, of
resistance per µH of inductance. For minimum
core loss, choose a high frequency core material
such as Kool-Mu, ferrite, or MPP.
2.
Specify the coupled inductor's turns ratio:
Np : Ns = 1:1
3d.
(1a)
4a.
Calculate the minimum output capacitance
required using:
C
=
I
OUT ( MAX)
R
SENSE
= 138mW
@
120mW
3a.
(5)
Determine the inductance required using
equation 2.
L
P
=
(25
×
10
−
6
)
×
6
×
0.12
= 18µH
3b.
(6)
I
L PEAK
=
235mV 6
×
(25
×
10 )
.
+
=
2.79A
120m
Ω
18
×
10
–6
−
6
V
OUT
+
V
IN ( MAX)
V
OUT
25
10
.
D
V
-
6
OUT
4b.
Establish the maximum allowable ESR for the
ouput capacitor:
R
ESR
<
(2a)
D
V
OUT
R
SENSE
150mV
Determine the minimum inductor current rating
required.
(4a)
5.
As a final design check, evaluate the system
stability (no compensation, single pole response)
by using the following equation:
∆V
OUT
≤
(6
×
10
−
6
)
×
R
!
SENSE
×
(V
OUT
+
V
IN (MIN)
)
L
P
"#
$
(7)
where R
SENSE
and L
P
are the actual values to be
used.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
