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LT8697 参数 Datasheet PDF下载

LT8697图片预览
型号: LT8697
PDF下载: 下载PDF文件 查看货源
内容描述: USB 5V 2.5A输出, 42V输入同步降压用电缆压降补偿 [USB 5V 2.5A Output, 42V Input Synchronous Buck with Cable Drop Compensation]
分类和应用:
文件页数/大小: 28 页 / 481 K
品牌: LINEAR_DIMENSIONS [ Linear Dimensions ]
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LT8697  
operaTion  
The LT8697 is a monolithic, constant frequency, current  
mode step-down DC/DC converter. An oscillator, with  
frequency set using a resistor on the RT pin, turns on the  
internal top power switch at the beginning of each clock  
cycle. Current in the inductor then increases until the top  
switch current comparator trips and turns off the switch.  
The peak inductor current is controlled by the voltage on  
the internalVCnode. When thetop powerswitch turns off,  
thesynchronouspowerswitchturnsonuntilthenextclock  
cycle begins or inductor current falls to zero. If overload  
conditions result in more than 4.2A flowing through the  
bottom switch, the next clock cycle will be delayed until  
switch current returns to a safe level.  
the output voltage to 5.8V. When regulation is determined  
by either the output current limit or the SYS pin, USB5V  
is not regulated to 5V and the output voltage falls below  
its programmed value.  
If the EN/UV pin is low, the LT8697 is shut down and  
draws 1μA from the input. When the EN/UV pin is above  
1V, the switching regulator will become active.  
TheLT8697operatesinforcedcontinuousmode(FCM)for  
fast transient response and full frequency operation over  
a wide load range. If a clock is applied to the SYNC pin  
the part will synchronize to the external clock frequency  
and operate in FCM.  
To improve efficiency across all loads, supply current  
to internal circuitry is sourced from the SYS pin when  
biased at 3.3V or above. Else, the internal circuitry will  
To control the output voltage, the LT8697’s error ampli-  
fier servos the VC node by comparing the voltage on the  
USB5Vpin, divideddownabout5:1, withaninternal0.97V  
reference. When the load current increases, it causes a  
reduction in the feedback voltage relative to the reference.  
This differential error makes the error amplifier raise the  
VC voltage which raises the top switch peak current limit.  
The feedback process continues until the average induc-  
tor current matches the new load current and the output  
voltage is in regulation.  
draw current from V .  
IN  
When in FCM the oscillator operates continuously and  
positive SW transitions are aligned to the clock. Negative  
inductor current is allowed. The LT8697 can sink current  
from the output and return this charge to the input in this  
mode, improving load step transient response. FCM is  
disabled if the V pin is held above 29V or if the SYS pin  
IN  
is held above 7.5V. When FCM is disabled in these ways,  
negative inductor current is not allowed and the LT8697  
skips SW cycles in light load conditions.  
To implementcabledropcompensation,theLT8697drives  
the RCBL pin to 20(V – V ). Current sourced from  
ISP  
ISN  
the RCBL pin is derived from the USB5V pin, creating an  
outputoffsetabovethe5VUSB5VpinvoltagethroughR  
CDC  
Comparators monitoring the USB5V pin voltage will pull  
the PG pin low if the output voltage varies more than 9%  
(typical)fromthesetpoint,orifafaultconditionispresent.  
that is proportional to the load current and the R /R  
CDC CBL  
resistorratio.Thisnegativeoutputimpedancecompensates  
for resistive drops in wiring for remote loads.  
The oscillator reduces the LT8697’s operating frequency  
whenthevoltageattheSYSpinisbelow4V.Thisfrequency  
foldback helps to control the inductor current when the  
output voltage is lower than the programmed value during  
start-up or overcurrent conditions.  
The LT8697 error amp has two additional feedback paths  
that can override the USB5V pin control of the VC node.  
For output current limit, the voltage V – V across  
ISP  
ISN  
the output current sense resistor is not allowed to exceed  
the lower of 48mV or V /20. Also, the SYS pin limits  
ICTRL  
8697p  
11  
For more information www.linear.com/LT8697