SC4524SETRT データシートの表示（PDF） - Semtech Corporation
部品番号
コンポーネント説明
メーカー
SC4524SETRT Datasheet PDF : 21 Pages
| |||
SC4524
POWER MANAGEMENT
Applications Information
duty-cycle invariant and are guaranteed higher than 2.3A.
The maximum load current is therefore conservatively
10mF X5R ceramic capacitor is adequate. For high voltage
applications, a small ceramic (1mF or 2.2mF) can be
placed in parallel with a low ESR electrolytic capacitor to
satisfy both the ESR and bulk capacitance requirements.
,
287 0$;
= ,/0
- D,/
= $ - D,/
(5) Output Capacitor
If D,/ = ¼,/0 , then
,
287 0$;
=,
-
D,
/
/0
=,
-
,
/0
/0
= ¼,/0 .
The saturation current of the inductor should be 20-30%
higher than the peak current limit (2.3A). Low-cost
powder iron cores are not suitable for high-frequency
switching power supplies due to their high core losses.
Inductors with ferrite cores should be used.
Power Line Input Capacitor
A buck converter draws pulse current with peak-to-peak
amplitude equal to its output current IOUT from its input
supply. An input capacitor placed between the supply
and the buck converter filters the AC current and keeps
the current drawn from the supply to a DC constant. The
input capacitance CIN should be high enough to filter the
pulse input current. Its equivalent series resistance (ESR)
should be low so that power dissipated in the capacitor
does not result in significant temperature rise and
degrade reliability. For a buck converter, the RMS ripple
current in the input capacitor is
,506 &,1 = ,287 ' - ' .
(6)
Power
dissipated
in
the
input
capacitor
is
,
506 &,1
¼ (65 .
Equation (6)
has a maximum value of
,287
( at
'
=
),
corresponding to the worst-case power dissipation
,287 ¼(65
in CIN.
Multi-layer ceramic capacitors, which have very low ESR
(a few mW) and can easily handle high RMS ripple current,
are the ideal choice for input filtering. A single 4.7mF or
The output ripple voltage DVOUT of a buck converter can
be expressed as
D9287
=
D,/ ÌÌÍË(65 +
I&287
ÜÜÝÛ
(7)
where COUT is the output capacitance.
Inductor ripple current DIL increases as D decreases
(Equation (3)). The output ripple voltage is therefore the
highest when VIN is at its maximum. The first term in (7)
results from the ESR of the output capacitor while the
second term is due to the charging and discharging of
COUT by the inductor ripple current. Substituting DIL =
0.69A, f = 500kHz and COUT = 22mF ceramic with ESR =
2mW in (7),
D9287 = $ ¼ PW + PW
= P9 + P9 = P9
Depending on operating frequency and the type of
capacitor, ripple voltage resulting from charging and
discharging of COUT may be higer than that due to ESR. A
10mF to 47mF X5R ceramic capacitor is found adequate
for output filtering in most applications. Ripple current
in the output capacitor is not a concern because the
inductor current of a buck converter directly feeds COUT,
resulting in very low ripple current. Avoid using Z5U and
Y5V ceramic capacitors for output filtering because these
types of capacitors have high temperature and high
voltage coefficients.
Freewheeling Diode
Use of Schottky barrier diodes as freewheeling rectifiers
reduces diode reverse recovery input current spikes,
easing high-side current sensing in the SC4524. These
diodes should have an average forward current rating
between 1A and 2A and a reverse blocking voltage of at
least a few volts higher than the input voltage. For
switching regulators operating at low duty cycles (i.e. low
ã 2006 Semtech Corp.
11
www.semtech.com
Share Link: 