參數(shù)資料
型號(hào)： LT1302- 5
廠(chǎng)商： Linear Technology Corporation
英文描述： Micropower High Output Current Step-Up Fixed 5V DC/DC Converters(微功耗,高輸出電流,5V固定輸出電壓,步升DC/DC變換器)
中文描述： 微高輸出電流升壓型固定5V直流/直流轉(zhuǎn)換器（微功耗，高輸出電流，5V的固定輸出電壓，步升直流/直流變換器）
文件頁(yè)數(shù)： 7/16頁(yè)
文件大小： 313K
代理商： LT1302- 5
7
LT1302/LT1302-5
OPERATIOU
CMP1 stays on and the peak switch current is regulated
by the voltage on the V
C
pin (A1’s output). V
C
drives the
base of Q1. As the V
C
voltage rises, Q2 conducts less
current, resulting in less drop across R5. Q4’s peak
current must then increase in order for A2 to trip. This
current mode control results in good stability and immu-
nity to input voltage variations. Because this is a linear,
closed-loop system, frequency compensation is required.
A series RC from V
C
to ground provides the necessary
pole-zero combination.
The LT1302-5 incorporates feedback resistors R1 and
R2 into the device. Output voltage is set at 5.05V in Burst
Mode, dropping to 4.97V in current mode.
APPLICATIO
S I
FOR
ATIO
U
Inductor Selection
Inductors used with the LT1302 must fulfill two require-
ments. First, the inductor must be able to handle current
of 2.5A to 3A without runaway saturation. Rod or drum
core units usually saturate gradually and it is acceptable to
exceed manufacturers’ published saturation currents by
20% or so. Second, it should have low DCR, under 0.05
so that copper loss is kept low. Inductance value is not
critical. Generally, for low voltage inputs down to 2V, a
10
μ
H inductor is recommended (such as Coilcraft DO3316-
103). For inputs above 4V to 5V use a 22
μ
H unit (such as
Coilcraft DO3316-223). Switching frequency can reach up
to 400kHz so the core material should be able to handle
high frequency without loss. Ferrite or molypermalloy
cores are a better choice than powdered iron. If EMI is a
concern a toroidal inductor is suggested, such as Coiltronics
CTX20-4.
For a boost converter, duty cycle can be calculated by the
following formula:
W
U
U
DC=1–
V
V
IN
OUT
A special situation exists where the V
OUT
/V
IN
differential is
high, such as a 2V-to-12V converter. The required duty
cycle is higher than the LT1302 can provide, so the
converter must be designed for discontinuousoperation.
This means that inductor current goes to zero during the
switch off-time. In the 2V-to-12V case, inductance must
be low enough so that current in the inductor can reach
2A in a single cycle. Inductor value can be defined by:
L
V
V
t
A
IN
SW
2
ON
≤
(
)
×
With the 2V input a value of 3.3
μ
H is acceptable. Since the
inductance is so low, usually a smaller core size can be
used. Efficiency will not be as high as for the continuous
case since peak currents will necessarily be higher.
Table 1 lists inductor suppliers along with appropriate part
numbers.
Table 1. Recommended Inductors
VENDOR
Coilcraft
PART NO.
DO3316-103
DO3316-153
DO3316-223
CTX10-2
CTX20-4
LPT4545-100LA
LPT4545-200LA
CD105-100
CD105-150
CDR125-220
VALUE(
μ
H)
10
15
22
10
20
10
20
10
15
22
PHONE NO.
(708) 639-6400
Coiltronics
(407) 241-7876
Dale
(605) 665-9301
Sumida
(708) 956-0666
Capacitor Selection
The output capacitor should have low ESR for proper
performance. A high ESR capacitor can result in “mode-
hopping” between current mode and Burst Mode at high
load currents because the output voltage will increase by
I
SW
×
ESR when the inductor current is flowing into the
diode. Figure 4 shows output voltage of an LT1302-5
boost converter with two 220
μ
F AVX TPS capacitors at the
output. Ripple voltage at a 510mA load is about 30mV
P-P
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