ML4863 Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4863 Datasheet PDF : 10 Pages
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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 (VIN(MAX))
The mainimum input voltage (VIN(MIN))
The maximum output current (IOUT(MAX))
The maximum output ripple (DVOUT)
As a general design rule, the output ripple should be kept
below 100mV to ensure stability.
2.
Select a sense resistor, RSENSE, using equation 1.
3a. Determine the inductance required for the
optimum output ripple using equation 2.
3b. Determine the minimum inductor current rating
required. The peak inductor current is calculated
using the following formula:
IL PEAK
=
235mV
RSENSE
+
VIN (MAX)
(2.5 10-6 )
LP
(4)
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.
3d. Specify the coupled inductor's turns ratio:
Np : Ns = 1:1
4a. Calculate the minimum output capacitance
required using:
C = IOUT (MAX)
VOUT + VIN (MAX)
VOUT
2.5 10-6
DVOUT
(5)
4b. Establish the maximum allowable ESR for the
ouput capacitor:
RESR
<
DVOUT RSENSE
150mV
(6)
5.
As a final design check, evaluate the system
stability (no compensation, single pole response)
by using the following equation:
! "$# ∆VOUT ≤ (6 × 10−6 ) ×
RSENSE × (VOUT + VIN (MIN) )
LP
(7)
where RSENSE and LP are the actual values to be
used.
ML4863
See Table 1 for suggested component manufacturers.
Component Manufacturer
Part
Number
Sense
Dale
Resistors IRC
LRC Series
WSL Series
Phone
(402) 563-6506
(512) 992-7900
Inductors
Coilcraft
Coiltronics
Dale
R4999
(847) 639-6400
OCTA-PAC Series (561)241-7876
LPE-6562 Series (605) 665-9301
LPT-4545 series
Capacitors AVX
Sprague
TPS series
593D Series
(207) 282-5111
(207) 324-4140
MOSFETs National
Motorola
Siliconix
NDS94XX
NDS99XX
MMDF Series
MMSF Series
Littlefoot Series
(800) 272-9954
(602) 897-5056
(408) 988-8000
Table 1. Component Suppliers
DESIGN EXAMPLE
1.
Specify the application by defining:
VIN(MAX) = 6V
VIN(MIN) = 4V
IOUT(MAX) = 500mA
DVOUT = 100mV
2.
Select the sense resistor, RSENSE, using Equation 1:
RSENSE
=
4
5+ 4
×
150mV + 4V
500mA 20 × 6 × 0.5
× 0.85
(1a)
RSENSE = 138mW @ 120mW
3a. Determine the inductance required using
equation 2.
LP = (25 × 10−6 ) × 6 × 0.12 = 18µH
(2a)
3b. Determine the minimum inductor current rating
required.
(4a)
IL PEAK
=
235mV
120mΩ
+
6 × (2.5 × 10−6 )
18 × 10–6
=
2.79A
7
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