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參數(shù)資料
| 型號： | SC4624SETRT |
| 廠商： | Semtech Corporation |
| 英文描述： | Low Input Voltage, High Efficiency, 4A Integrated FET Synchronous Step down DC/DC Regulator |
| 中文描述： | 低輸入電壓，高效率及4A集成FET的同步降壓直流/直流穩(wěn)壓器 |
| 文件頁數(shù)： | 13/20頁 |
| 文件大?。?/td> | 738K |
| 代理商： | SC4624SETRT |
13
2007 Semtech Corp.
www.semtech.com
SC4624
POWER MANAGEMENT
Application information (Cont.)
After the high side PMOS turn on around 30ns, the OCP
comparator will compare between V2 and V1.
When the
converter detects an over current condition (V2 > V1) as
shown in Figure 16, the SC4624 proceeds into the cycle by
cycle protection mode (Point B to Point C), which responds
to minor over current cases and the output voltage is
monitored.
If the over current and low output voltage (set at 60% of
nominal output voltage) occur at the same time, the SS
pin is pull low by an internal switch and the comp pin is
pulled low and the devices stops switching. Assume start
from FB = 0V, FB and SS voltage rise forward 0.5V. Once
SS voltage exceeds 0.4V, the hiccup comparator becomes
enabled. The hiccup period is around 2
17
/F
OSC
. (Point C to
Point D).
For example, with a switching frequency application of
550kHz, the hiccup period is around 238ms. (refer to
Figure 7).
A poor layout will make OCP trip point shift and is not
easily to calculate by R
. This is because it is affected
by ground bounce, spiker voltage between Vin pin and PH
pin, and internal parameter tolerance. Users can refer to
Figure 14, it shows how to set maximum output current
by R
ISET
.
C
D
Iout
0
Imax
Vout
0
.
6 * Vout
Vo
A
0
B
Figure 16. Over Current Protection Characteristic
Inductor Selection
For a typical SC4624 application, the inductor selection
is mainly based on its value, saturation current and DC
resistance. The inductor should be able to handle the
peak current without saturating and its copper resistance
in the winding should be as low as possible to minimize its
resistive power loss.
The inductor value can be determined according to its
operating point and the switching frequency as follows:
20$;
,
6
,1
287
,1
'
287
,
I
9
9
u
9
u
9
/
u
u
where
fs = switching frequency.
D
I = ratio of the peak to peak inductor current to the
maximum output load current.
The peak to peak inductor current is:
20$;
3
3
,
,
,
u
'
After the required inductor value is selected, the proper
selection of the core material is based on the peak
inductor current and efficiency requirements. The core
must be able to handle the peak inductor current I
without saturation and produce low core loss during the
high frequency operation
and is given as follows:
,
,
,
3
3
,20$;
3($.
The power loss for the inductor includes its core loss and
copper loss. If possible, the winding resistance should be
minimized to reduce any copper loss of the inductor, (the
core loss can be found in the manufacturer’s datasheet).
The inductor’s copper loss can be estimated as follows:
:,1',1*
5
/506
&223(5
3
,
u
where
I
LRMS
is the RMS current in the inductor.
This current can be calculated as follows:
20$;
,
/506
,
,
'
u
u
Output Capacitor Selection
Basically there are two major factors to consider in select-
ing the type and quantity of the output capacitors. The
first one is the required ESR (Equivalent Series Resis
-
tance) which should be low enough to reduce the voltage
deviation from its nominal one during its load changes.
The second one is the required capacitance, which should
be high enough to hold up the output voltage. Before the
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