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参数资料
| 型号： | LTC1475CMS8#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 0.75 A SWITCHING REGULATOR, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 18/20页 |
| 文件大小： | 237K |
| 代理商： | LTC1475CMS8#TR |
7
LTC1474/LTC1475
APPLICATIONS INFORMATION
WU
U
ments. Lower peak currents have the advantage of lower
output ripple (
VOUT = IPEAK ESR), lower noise, and less
stress on alkaline batteries and other circuit components.
Also, lower peak currents allow the use of inductors with
smaller physical size.
Peak currents as low as 10mA can be programmed with
the appropriate sense resistor. Increasing RSENSE above
10
, however, gives no further reduction of IPEAK.
For RSENSE values less than 1, it is recommended that
the user parallel standard resistors (available in values
≥
1
) instead of using a special low valued shunt resistor.
Although a single resisor could be used with the desired
value, these low valued shunt resistor types are much
more expensive and are currently not available in case
sizes smaller than 1206. Three or four 0603 size standard
resistors require about the same area as one 1206 size
current shunt resistor at a fraction of the cost.
At higher supply voltages and lower inductances, the peak
currents may be slightly higher due to current comparator
overshoot and can be predicted from the second term in
the following equation:
I
R
VV
L
PEAK
SENSE
IN
OUT
=
+
()
()
01
025
25 10 7
.
(2)
Note that RSENSE only sets the maximum inductor current
peak. At lower dI/dt (lower input voltages and higher
inductances), the observed peak current at loads less than
IMAX may be less than this calculated peak value due to the
voltage comparator tripping before the current ramps up
high enough to trip the current comparator. This effect
improves efficiency at lower loads by keeping the I2R
losses down (see Efficiency Considerations section).
Inductor Value Selection
Once RSENSE and IPEAK are known, the inductor value can
be determined. The minimum inductance recommended
as a function of IPEAK and IMAX can be calculated from:
L
VV t
II
MIN
OUT
D OFF
PEAK
MAX
≥
+
()
075
.
(3)
where tOFF = 4.75s.
MAXIMUM OUTPUT CURRENT (mA)
0
R
SENSE
(
)
5
4
3
2
1
0
50
100
150
200
1474/75 F02
250
300
FOR LOWEST NOISE
FOR BEST EFFICIENCY
Figure 2. RSENSE Selection
The basic LTC1474/LTC1475 application circuit is shown
in Figure 1, a high efficiency step-down converter. External
component selection is driven by the load requirement and
begins with the selection of RSENSE. Once RSENSE is
known, L can be chosen. Finally D1, CIN and COUT are
selected.
RSENSE Selection
The current sense resistor (RSENSE) allows the user to
program the maximum inductor/switch current to opti-
mize the inductor size for the maximum load. The LTC1474/
LTC1475 current comparator has a maximum threshold of
100mV/(RSENSE + 0.25). The maximum average output
current IMAX is equal to this peak value less half the peak-
to-peak ripple current
IL.
Allowing a margin for variations in the LTC1474/LTC1475
and external components, the required RSENSE can be
calculated from Figure 2 and the following equation:
RSENSE = (0.067/IMAX) – 0.25
(1)
for 10mA < IMAX < 200mA.
For IMAX above 200mA, RSENSE is set to zero by shorting
Pins 6 and 7 to provide the maximum peak current of
400mA (limited by the fixed internal sense resistor). This
400mA default peak current can be used for lower IMAX if
desired to eliminate the need for the sense resistor and
associated decoupling capacitor. However, for optimal
performance, the peak inductor current should be set to no
more than what is needed to meet the load current require-
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