LT3431 데이터 시트보기 (PDF) - Linear Technology
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LT3431 Datasheet PDF : 28 Pages
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LT3431
APPLICATIO S I FOR ATIO
importance, the subsequent suggestions in Peak Induc-
tor and Fault Current and EMI will additionally help in the
selection of the inductor value.
Peak-to-peak output ripple voltage is the sum of a triwave
(created by peak-to-peak ripple current (ILP-P) times ESR)
and a square wave (created by parasitic inductance (ESL)
and ripple current slew rate). Capacitive reactance is
assumed to be small compared to ESR or ESL.
( )( ) ( ) VRIPPLE = ILP-P
ESR + ESL Σ dI
dt
where:
ESR = equivalent series resistance of the output
capacitor
ESL = equivalent series inductance of the output
capacitor
dI/dt = slew rate of inductor ripple current = VIN/L
Peak-to-peak ripple current (ILP-P) through the inductor
and into the output capacitor is typically chosen to be
between 20% and 40% of the maximum load current. It is
approximated by:
( )( ) VOUT VIN – VOUT
( )( )( ) ILP-P =
VIN f L
Example: with VIN = 12V, VOUT = 5V, L = 10µH, ESR =
0.080Ω and ESL = 10nH, output ripple voltage can be
approximated as follows:
(5)(12 − 5)
( )( )( ) IP-P = 12 10 •10−6 500 •103 = 0.58A
Σ dI = 12 = 106 •1.2
dt 10 •10−6
( )( ) ( )( )( ) VRIPPLE = 0.58A 0.08 + 10 •10−9 106 1.2
= 0.046 + 0.012 = 58mVP- P
To reduce output ripple voltage further requires an in-
crease in the inductor value with the trade-off being a
10
physically larger inductor with the possibility of increased
component height and cost.
Ceramic Output Capacitor
An alternative way to further reduce output ripple voltage
is to reduce the ESR of the output capacitor by using a
ceramic capacitor. Although this reduction of ESR re-
moves a useful zero in the overall loop response, this zero
can be replaced by inserting a resistor (RC) in series with
the VC pin and the compensation capacitor CC. (See
Ceramic Capacitors in Applications Information.)
Peak Inductor Current and Fault Current
To ensure that the inductor will not saturate, the peak
inductor current should be calculated knowing the maxi-
mum load current. An appropriate inductor should then
be chosen. In addition, a decision should be made whether
or not the inductor must withstand continuous fault
conditions.
If maximum load current is 1A, for instance, a 1A inductor
may not survive a continuous 4A overload condition. Dead
shorts will actually be more gentle on the inductor because
the LT3431 has frequency and current limit foldback.
Peak inductor and switch current can be significantly
higher than output current, especially with smaller induc-
tors and lighter loads, so don’t omit this step. Powdered
iron cores are forgiving because they saturate softly,
whereas ferrite cores saturate abruptly. Other core mate-
rials fall somewhere in between. The following formula
assumes continuous mode of operation, but errs only
slightly on the high side for discontinuous mode, so it can
be used for all conditions.
( ( )()( )( )( ) ) IPEAK
=
IOUT
+
(ILP-P )
2
=
IOUT
+
VOUT VIN – VOUT
2 VIN f L
EMI
Decide if the design can tolerate an “open” core geometry
like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid to
prevent EMI problems. This is a tough decision because
the rods or barrels are temptingly cheap and small and
sn3431 3431fs
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