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| 型号： | LT3971IDD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/28页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1.2A 10DFN |
| 标准包装： | 121 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.19 V ~ 30 V |
| 输入电压： | 4.3 V ~ 38 V |
| PWM 型： | 电流模式，Burst Mode? |
| 频率 - 开关： | 200kHz ~ 2MHz |
| 电流 - 输出： | 1.2A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-WFDFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 10-DFN（3x3） |
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LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
For lower start-up voltage, the boost diode can be tied to
V IN ; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and this reduces the minimum input voltage to ap-
proximately 400mV above V OUT . At higher load currents,
the inductor current is continuous and the duty cycle is
limited by the maximum duty cycle of the LT3971, requiring
a higher input voltage to maintain regulation.
Enable Pin
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred μA. And the part
may be able to switch at cold or for V IN(EN ) thresholds less
than 7V. Figure 6 shows the magnitude of the increased
input current in a typical application with different pro-
grammed V IN(EN) .
When operating in Burst Mode for light load currents, the
current through the V IN(EN) resistor network can easily be
greater than the supply current consumed by the LT3971.
Therefore, the V IN(EN) resistors should be large to minimize
their effect on efficiency at low loads.
V IN(EN) =
+ 1
The LT3971 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the
EN comparator is 1.01V, with 30mV of hysteresis. The EN
pin can be tied to V IN if the shutdown feature is not used.
Adding a resistor divider from V IN to EN programs the
LT3971 to regulate the output only when V IN is above a
desired voltage (see Figure 5). Typically, this threshold,
V IN(EN) , is used in situations where the input supply is cur-
rent limited, or has a relatively high source resistance. A
switching regulator draws constant power from the source,
so source current increases as source voltage drops. This
looks like a negative resistance load to the source and can
cause the source to current limit or latch low under low
source voltage conditions. The V IN(EN) threshold prevents
the regulator from operating at source voltages where the
problems might occur. This threshold can be adjusted by
setting the values R3 and R4 such that they satisfy the
following equation:
R3
R4
where output regulation should not start until V IN is above
V IN(EN) . Due to the comparator’s hysteresis, switching will
not stop until the input falls slightly below V IN(EN) .
500
400
300
200
100
0
500
400
300
200
100
12V V IN(EN) Input Current
0 1 2 3 4 5 6 7 8 9 10 11 12
INPUT VOLTAGE (V)
V IN(EN) = 12V
R3 = 11M
R4 = 1M
6V V IN(EN) Input Current
V IN
LT3971
0
0
1
2 3 4
5
6
R3
EN
1V
+
–
SHDN
V IN(EN) = 6V
R3 = 5M
R4 = 1M
INPUT VOLTAGE (V)
3971 F06
R4
3971 F05
Figure 5. Programmed Enable Threshold
Figure 6. Input Current vs Input Voltage
for a Programmed V IN(EN) of 6V and 12V
3971fd
17
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