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.
above V = 29V the LT8697 disables forced continuous
IN
mode so the part can pulse skip to maintain regulation at
any low V
to V ratio. For V < 29V, use the following
OUT
IN IN
equation to find the minimum output voltage (V
)
OUT(MIN)
where the LT8697 can regulate the output current limit:
V
V
= 0.1 • f • t
• (V – V
+
OUT(MIN)
SW
ON(MIN)
– V
SENSE L
IN
SW(TOP)
) – V
SW(BOT)
– V
SW(BOT)
Setting the Current Limit
where f
is the switching frequency, t
is the
SW
ON(MIN)
minimum on-time, V
and V
are the in-
SW(TOP)
SW(BOT)
In addition to regulating the output voltage, the LT8697
includes a current regulation loop for setting the average
outputcurrentlimit.TheLT8697measuresthevoltagedrop
ternal switch drops (~0.3V and ~0.15V) respectively at
maximum load), V
is voltage across the R
at
SENSE
SENSE
the programmed output current and V is the resistive
L
across an external current sense resistor R
using
SENSE
drop across the inductor ESR at the programmed output
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.
current. If the calculated V
is negative or is less
OUT(MIN)
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
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:
VCTRL
RSENSE • 20.3
ILIM
=
output current can be regulated into V
= 0V for any
OUT
input voltage up to 42V. For a 2MHz application in a worst-
case condition, the programmed output current can be
The preceding I equation is valid for V
– V <
ISN
LIM
ISP
48mV. At 48mV V
, the internal current limit loop
SENSE
regulated into V
= 0.3V or higher. Refer to Figure 6
OUT
takes over output current regulation from the ICTRL pin.
The maximum programmable output current (I
is therefore found by the following equation:
to see how the front page application circuit responds to
)
LIM(MAX)
a short directly on the regulator output without a cable.
1.0
48mV
RSENSE
V
V
V
V
= OPEN, V = 27V
IN
CTRL
CTRL
CTRL
CTRL
ILIM(MAX)
=
= 0.5V, V = 16V
IN
= OPEN, V = 16V
IN
0.8
0.6
0.4
0.2
0
= 0.5V, V = 27V
IN
The internal 2μA pull-up on the ICTRL pin allows this pin
to be floated if unused, in which case the I
be the output current limit.
would
LIM(MAX)
When in forced continuous mode, the LT8697’s ability to
regulate the output current is limited by its t . In this
ON(MIN)
scenario, atverylowoutputvoltagetheoutputcurrentcan
exceedtheprogrammedoutputcurrentlimitandislimited
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,
2
2.5
1
1.5
3
I
(A)
OUT
8697 F06
ꢁigure 6. Output Current Regulation
vs VOUꢂ at fSW = 2MHz, RSENSE = 18mΩ
8697p
17
For more information www.linear.com/LT8697
LINEAR_DIMENSIONS [ Linear Dimensions ]
