参数资料
型号: LT1956EGN-5#TRPBF
厂商: Linear Technology
文件页数: 16/28页
文件大小: 0K
描述: IC REG BUCK 5V 1.5A 16SSOP
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: 5V
输入电压: 5.5 V ~ 60 V
PWM 型: 电流模式
频率 - 开关: 500kHz
电流 - 输出: 1.5A
同步整流器:
工作温度: -40°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 16-SSOP(0.154",3.90mm 宽)
包装: 带卷 (TR)
供应商设备封装: 16-SSOP
LT1956/LT1956-5
APPLICATIO S I FOR ATIO
R FB
LT1956
L1
INPUT
IN
2.38V
+
V SW
OUTPUT
STANDBY
R HI
5.5 μ A
SHDN
+
TOTAL
SHUTDOWN
+
C1
C2
R LO
0.4V
GND
1956 F04
Figure 4. Undervoltage Lockout
R LO [ V IN ? 2 . 38 ( ? V / V OUT + 1 ) + ? V ]
2 . 38 ? R LO ( 5 . 5 μ A )
[ ( ) ]
2 . 38 – 25 k ( )
25 k ( )
(
)
bypassed with a 1000pF capacitor to prevent coupling
problems from the switch node. If hysteresis is desired in
the undervoltage lockout point, a resistor R FB can be
added to the output node. Resistor values can be calcu-
lated from:
R HI =
R FB = ( R HI ) ( V OUT / ? V )
25k suggested for R LO
V IN = Input voltage at which switching stops as input
voltage descends to trip level
? V = Hysteresis in input voltage level
Example: output voltage is 5V, switching is to stop if input
voltage drops below 12V and should not restart unless
input rises back to 13.5V. ? V is therefore 1.5V and
V IN = 12V. Let R LO = 25k.
25 k 12 ? 2 . 38 1 . 5 / 5 + 1 + 1 . 5
R HI =
5 . 5 μ A
10 . 41
= = 116 k
2 . 24
R FB = 116 k 5 / 1 . 5 = 387 k
SYNCHRONIZING
The SYNC input must pass from a logic level low, through
the maximum synchronization threshold with a duty cycle
between 10% and 90%. The input can be driven directly
from a logic level output. The synchronizing range is equal
to initial operating frequency up to 700kHz. This means
that minimum practical sync frequency is equal to the
worst-case high self-oscillating frequency (570kHz), not
the typical operating frequency of 500kHz. Caution should
be used when synchronizing above 662kHz because at
higher sync frequencies the amplitude of the internal slope
compensation used to prevent subharmonic switching is
reduced. This type of subharmonic switching only occurs
at input voltages less than twice output voltage. Higher
inductor values will tend to eliminate this problem. See
Frequency Compensation section for a discussion of an
entirely different cause of subharmonic switching before
assuming that the cause is insufficient slope compensa-
tion. Application Note 19 has more details on the theory
of slope compensation.
At power-up, when V C is being clamped by the FB pin (see
Figure 2, Q2), the sync function is disabled. This allows the
frequency foldback to operate in the shorted output con-
dition. During normal operation, switching frequency is
controlled by the internal oscillator until the FB pin reaches
0.8V, after which the SYNC pin becomes operational. If no
synchronization is required, this pin should be connected
to ground.
1956f
16
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