參數(shù)資料
型號(hào)： LT1507CS8-3.3
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： Octal D-Type Edge-Triggered Flip-Flops With 3-State Outputs 24-SO 0 to 70
中文描述： 3 A SWITCHING REGULATOR, 570 kHz SWITCHING FREQ-MAX, PDSO8
封裝： 0.150 INCH, PLASTIC, SO-8
文件頁(yè)數(shù)： 11/20頁(yè)
文件大小： 343K
代理商： LT1507CS8-3.3
11
LT1507
APPLICATIO
S I
FOR
ATIO
U
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE
ESR (MAX
)
AVX TPS, Sprague 593D
AVX TAJ
D CASE SIZE
AVX TPS, Sprague 593D
AVX TAJ
C CASE SIZE
AVX TPS
AVX TAJ
W
U
U
appropriate for input bypassing because of their high
ripple current ratings and tolerance of turn-on surges.
OUTPUT RIPPLE VOLTAGE
Ripple voltage is determined by the high frequency imped-
ance of the output capacitor and ripple current through the
inductor. Ripple current is triangular (continuous mode)
with a peak-to-peak value of:
I
V
V
V
V
L f
)( )( )
OUT
IN
OUT
IN
P-P
=
(
)(
)
(
Output ripple voltage is also triangular with peak-to-peak
amplitude of:
V
RIPPLE
= (I
P–P
)(ESR) (peak-to-peak)
Example: with V
IN
= 5V, V
OUT
= 3.3V, L = 5
μ
H, ESR = 0.1
;
I
V
A
mV
RIPPLE
P-P
P-P
P-P
=
( .
=
)( .
)
=
=
( . )(
6
. )
5 5 10
500 10
0 45
45
3
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current will be less than the 1.5A rating of
the LT1507, especially with lower inductor values. Induc-
tor ripple current must be taken into account as well as
reduced switch current at high duty cycles. Maximum
switch currentrating (I
P
) of the LT1507 is 1.5A up to 50%
duty cycle (DC), decreasing to 1.35A at 80% duty cycle,
shown graphically in Typical Performance Characteristics
and as a formula below. Current rating decreases with
duty cycle because the LT1507 has internal slope com-
pensation to prevent current mode subharmonic switch-
ing. For more details on subharmonic oscillation read
Application Note 19. Peak guaranteed switch current (I
P
)
is found from:
I
A
V
I
A
V
V
V
P
IN
P
OUT
IN
IN
=
≤
=
≥
1 5
0 5
1 75
0 5
0 5
)
for V
for V
OUT
OUT
RIPPLE CURRENT (A)
0.7 to 1.1
0.4
0.1 to 0.3
0.7 to 0.9
0.1 to 0.3
0.9 to 2.0
0.7 to 1.1
0.36 to 0.24
0.2 (Typ)
1.8 to 3.0
0.5 (Typ)
0.22 to 0.17
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 rugged-
ness is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-onsurges
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 200mA RMS. The formula
to calculate this is:
Output Capacitor Ripple Current (RMS)
I
RMS
(
V
(
V
V
L f V
RIPPLE
OUT
IN
IN
OUT
)
.
)(
–
)
)
=
0 29
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems when ceramic is used for the
output capacitor. Solid tantalum capacitor ESR generates
a loop “zero” at 5kHz to 50kHz that is instrumental in
giving acceptable loop phase margin. Ceramic capacitors
remain capacitive to beyond 300kHz and usually resonate
with their ESL before ESR becomes effective. They are
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