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LT8697 Datasheet(PDF) 17 Page - Linear Dimensions Semiconductor

Part No. LT8697
Description  USB 5V 2.5A Output, 42V Input Synchronous Buck with Cable Drop Compensation
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Maker  LINEAR_DIMENSIONS [Linear Dimensions Semiconductor]
Homepage  http://www.lineardimensions.com
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LT8697 Datasheet(HTML) 17 Page - Linear Dimensions Semiconductor

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LT8697
17
8697p
For more information www.linear.com/LT8697
applicaTions inForMaTion
bedesignedtolimitloadcurrent.Also,anelectronicswitch
may be necessary to prevent an output overcurrent condi-
tion on the USB5V output from bringing down the SYS
output. See the Inductor Selection and Maximum Output
Current discussion below to determine how much total
load current can be drawn from the outputs for a given
LT8697 application.
Setting the Current Limit
In addition to regulating the output voltage, the LT8697
includes a current regulation loop for setting the average
outputcurrentlimit.TheLT8697measuresthevoltagedrop
across an external current sense resistor RSENSE using
the ISP and ISN pins. This resistor should be connected
in series with the load current after the output capacitor.
The current loop modulates the cycle-by-cycle top switch
switch current limit such that the average voltage across
the ISP–ISN pins does not exceed its regulation point.
The LT8697 current limit can be programmed by forcing
a voltage on the ICTRL pin between 0V and 1V. Program
the current limit using the following equation:
ILIM =
VCTRL
RSENSE • 20.3
The preceding ILIM equation is valid for VISP – VISN <
48mV. At 48mV VSENSE, the internal current limit loop
takes over output current regulation from the ICTRL pin.
The maximum programmable output current (ILIM(MAX))
is therefore found by the following equation:
ILIM(MAX) =
48mV
RSENSE
The internal 2μA pull-up on the ICTRL pin allows this pin
to be floated if unused, in which case the ILIM(MAX) would
be the output current limit.
When in forced continuous mode, the LT8697’s ability to
regulate the output current is limited by its tON(MIN). In this
scenario, at very low output voltage the output current can
exceed the programmed output current limit and is limited
by the bottom switch current limit of 4.5A plus 1/2 the
ripple current. To help mitigate this effect, at low output
voltage the LT8697 folds back the switching frequency
by 10:1 to allow regulation at very low duty cycle. Also,
above VIN = 29V the LT8697 disables forced continuous
mode so the part can pulse skip to maintain regulation at
any low VOUT to VIN ratio. For VIN < 29V, use the following
equation to find the minimum output voltage (VOUT(MIN))
where the LT8697 can regulate the output current limit:
VOUT(MIN) = 0.1 • fSW • tON(MIN) • (VIN – VSW(TOP) +
VSW(BOT)) – VSW(BOT) – VSENSE – VL
where fSW is the switching frequency, tON(MIN) is the
minimum on-time, VSW(TOP) and VSW(BOT) are the in-
ternal switch drops (~0.3V and ~0.15V) respectively at
maximum load), VSENSE is voltage across the RSENSE at
the programmed output current and VL is the resistive
drop across the inductor ESR at the programmed output
current. If the calculated VOUT(MIN) is negative or is less
than the IR drop across the resistive short on the output
at the programmed current limit, then the LT8697 can
regulate the output current limit.
In practical applications, the resistances of the cable,
inductor and sense resistor are more than adequate to
allow the LT8697 to regulate to the output current limit for
any switching frequency and input voltage. For a 400kHz
application in a worst-case condition, the programmed
output current can be regulated into VOUT = 0V for any
input voltage up to 42V. For a 2MHz application in a worst-
case condition, the programmed output current can be
regulated into VOUT = 0.3V or higher. Refer to Figure 6
to see how the front page application circuit responds to
a short directly on the regulator output without a cable.
IOUT (A)
1
0.4
0.6
3
8697 F06
0.2
0
1.5
2
2.5
1.0
0.8
VCTRL = OPEN, VIN = 27V
VCTRL = 0.5V, VIN = 16V
VCTRL = OPEN, VIN = 16V
VCTRL = 0.5V, VIN = 27V
Figure 6. Output Current Regulation
vs VOUT at fSW = 2MHz, RSENSE = 18mΩ


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