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参数资料
| 型号： | LT1956-5EGN |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 570 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件页数： | 4/24页 |
| 文件大小： | 437K |
| 代理商： | LT1956-5EGN |
12
LT1956/LT1956-5
APPLICATIO S I FOR ATIO
WU
UU
Output capacitor ripple current (RMS):
I
VV
V
Lf V
RIPPLE RMS
OUT
IN
OUT
IN
()
.–
=
()(
)
()( )( )
029
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available. They are generally chosen for their
good high frequency operation, small size and very low
ESR (effective series resistance). Their low ESR reduces
output ripple voltage but also removes a useful zero in the
loop frequency response, common to tantalum capaci-
tors. To compensate for this, a resistor RC can be placed
in series with the VC compensation capacitor CC. Care
must be taken however, since this resistor sets the high
frequency gain of the error amplifier, including the gain
at the switching frequency. If the gain of the error
amplifier is high enough at the switching frequency,
output ripple voltage (although smaller for a ceramic
output capacitor) may still affect the proper operation of
the regulator. A filter capacitor CF in parallel with the
RC/CC network is suggested to control possible ripple at
the VC pin. The LT1956 can be stabilized using a 22F
ceramic output capacitor and VC components values of
CC = 10nF, RC = 2.2k and CF = 1nF.
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT1956 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
IC
I
VV
V
RIPPLE RMS IN
OUT
IN
OUT
IN
()
–
=
()
2
Ceramic capacitors are ideal for input bypassing. At 500kHz
switching frequency, the energy storage requirement of
the input capacitor suggests that values in the range of
2.2
F to 10F are suitable for most applications. If opera-
tion is required close to the minimum input required by the
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
output ripple voltage. To get low ESR takes
volume, so
physically smaller capacitors have high ESR. The ESR
range for typical LT1956 applications is 0.05
to 0.2. A
typical output capacitor is an AVX type TPS, 100
F at 10V,
with a guaranteed ESR less than 0.1
. This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical, and values from
22
F to greater than 500F work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22
F solid
tantalum capacitor, it will have high ESR, and output ripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE
ESR (MAX,
)
RIPPLE CURRENT (A)
AVX TPS, Sprague 593D
0.1 to 0.3
0.7 to 1.1
AVX TAJ
0.7 to 0.9
0.4
D CASE SIZE
AVX TPS, Sprague 593D
0.1 to 0.3
0.7 to 1.1
C CASE SIZE
AVX TPS
0.2 (typ)
0.5 (typ)
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
specially tested for surge capability, but surge ruggedness
is not a critical issue with the
output capacitor. Solid
tantalum capacitors fail during very high
turn-on surges,
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular with a typical value of 200mARMS. The formula
to calculate this is:
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