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参数资料
| 型号： | LTC1149CS-5#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/20页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16-SOIC |
| 标准包装： | 50 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 250kHz |
| 占空比： | 100% |
| 电源电压： | 最高 48V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | 0°C ~ 70°C |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 包装： | 管件 |
LTC1149
LTC1149-3.3/LTC1149-5
APPLICATIO S I FOR ATIO
25mV minimum current comparator threshold. This tech-
nique is also useful for eliminating audible noise from
certain types of inductors in high current (I OUT > 5A)
applications when they are lightly loaded.
An external offset is put in series with the SENSE – pin to
subtract from the built-in 25mV offset. An example of this
technique is shown in Figure 6. Two 100 ? resistors are
inserted in series with the leads from the sense resistor.
With the addition of R3, a current is generated through R1
circuitry. Turning on the N-channel MOSFET when this
fault is detected will then force the system fuse to blow.
The N-channel MOSFET needs to be sized so it will safely
handle this overcurrent condition. The typical delay from
pulling the C T Pin 6 high to when the NGATE Pin 13 goes
high is 250ns. Under shutdown conditions, the N-channel
is held off and pulling Pin 6 high will not cause the output
to be crowbarred.
A small N-channel FET can be used as an interface between
)
causing an offset of:
V OFFSET = V OUT
R1
R1 + R3
)
the overvoltage detect circuitry and the LTC1149 as shown
in Figure 7.
If V OFFSET > 25mV, the minimum threshold will be cancelled
and Burst Mode operation is prevented from occurring.
Since V OFFSET is constant, the maximum load current is
CROWBAR
5
VN2222LL
6
V CC
C T
LTC1149
also decreased by the same offset. Thus, to get back to the
ACTIVE WHEN CROWBAR = V IN
same I MAX , the value of the sense resistor must be lower:
OFF WHEN CROWBAR = GROUND
1149 F07
R SENSE ≈ 75mV
I MAX
To prevent noise spikes from erroneously tripping the
current comparator, a 1000pF capacitor is needed across
Pins 8 and 9.
Figure 7. Output Crowbar Interface
Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1149 series. These items are also illustrated graphi-
L
R SENSE
cally in the layout diagram of Figure 8. Check the following
LTC1149
SENSE +
9
R2
100 ?
+
C OUT
in your layout:
1. Are the signal and power grounds segregated? The
SENSE –
8
1000pF
R1
100 ?
1149 F06
LTC1149 signal ground Pin 11 must connect separately
to the (–) plate of C OUT . The other ground Pins 12 and
R3
Figure 6. Suppressing Burst Mode Operation
Output Crowbar
An added feature to using an N-channel MOSFET as the
synchronous switch is the ability to crowbar the output
with the same MOSFET. Pulling the timing capacitor Pin 6
above 1.5V when the output voltage is greater than the
desired regulated value, will turn on the N-channel MOSFET.
A fault condition which causes the output voltage to go
above a maximum value can be detected by external
14
14 should return to the source of the N-channel MOSFET,
anode of the Schottky diode and (–) plate of C IN , which
should have as short lead lengths as possible.
2. Does the LTC1149 SENSE – Pin 8 connect to a point
close to R SENSE and the (+) plate of C OUT ? In adjustable
applications, the resistive divider R1, R2 must be
connected between the (+) plate of C OUT and signal
ground.
3. Are the SENSE – and SENSE + leads routed together with
minimum PC trace spacing? The differential decou-
pling capacitor between Pins 8 and 9 should be as close
as possible to the LTC1149. Up to 100 ? may be placed
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