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| 型号： | LT1578IS8-2.5#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/28页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK 2.5V 1.5A 8SOIC |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 固定 |
| 输出数： | 1 |
| 输出电压： | 2.5V |
| 输入电压： | 4 V ~ 15 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 200kHz |
| 电流 - 输出： | 1.5A |
| 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： | 8-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-SOIC |
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LT1578/LT1578-2.5
APPLICATIO N S I N FOR M ATIO N
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1578 is used to set output
voltage and provide several overload protection features.
The first part of this section deals with selecting resistors
to set output voltage and the remaining part talks about
foldback frequency and current limiting created by the FB
pin. Please read both parts before committing to a final
design. The fixed 2.5V LT1578-2.5 has internal divider
resistors and the FB pin, renamed SENSE, is connected
directly to the 2.5V output.
The suggested value for the output divider resistor (see
Figure 2) from FB to ground (R2) is 5k or less, and a
formula for R1 is shown below. The output voltage error
caused by ignoring the input bias current on the FB pin is
less than 0.25% with R2 = 5k. Please read the following
if divider resistors are increased above the suggested
values.
R 1 = R 2 ( V OUT ? 1 . 21 )
1 . 21
More Than Just Voltage Feedback
The feedback pin is used for more than just output voltage
sensing. It also reduces switching frequency and current
limit when output voltage is very low (see the Frequency
Foldback graph in Typical Performance Characteristics).
This is done to control power dissipation in both the IC and
the external diode and inductor during short-circuit con-
ditions. A shorted output requires the switching regulator
to operate at very low duty cycles, and the average current
through the diode and inductor is equal to the short-circuit
current limit of the switch (typically 2A for the LT1578,
folding back to less than 0.77A). Minimum switch on time
limitations would prevent the switcher from attaining a
sufficiently low duty cycle if switching frequency were
maintained at 200kHz, so frequency is reduced by about
5:1 when the feedback pin voltage drops below 0.7V (see
Frequency Foldback graph). This does not affect operation
with normal load conditions; one simply sees a gear shift
in switching frequency during start-up as the output
voltage rises.
LT1578
TO FREQUENCY
SHIFTING
1.4V
Q1
V SW
OUTPUT
5V
ERROR
AMPLIFIER
+
1.21V
R1
–
R3
1k
R4
1k
FB
+
R5
5k
Q2
TO SYNC CIRCUIT
R2
5k
V C
GND
1578 F02
Figure 2. Frequency and Current Limit Foldback
8
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