LT8697
applicaTions inForMaTion
The LT8697 can operate at very high duty cycle, thus
maintaining the output voltage in regulation with the input
voltageonlyseveralhundredmVhigher.Thisdropoutvolt-
To avoidoverheatingandpoorefficiency,aninductormust
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
age depends on load current and the R
of the top
addition, the saturation current (typically labeled I ) rat-
DS(ON)
SAT
switch. However, the LT8697 skips off-times in very high
duty cycle conditions, reducing the switching frequency
ing of the inductor must be higher than the load current
plus 1/2 of the inductor ripple current:
below that programmed by R . In this dropout mode, the
T
∆IL
maximum allowable on-time is about 18µs. If this 18µs
on-timethresholdisreached,theLT8697enforcesa400ns
off-time to keep the BST capacitor charged at light loads.
This behavior limits the maximum duty cycle to 97.5%,
butguaranteesgooddropoutperformanceacrossallloads
and any start-up condition.
IL(PEAK) = IOUT(MAX)
+
2
where ΔI is the inductor ripple current as calculated
L
below and I
given application.
is the maximum output load for a
OUT(MAX)
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
For applications that cannot allow deviation from the pro-
grammedswitchingfrequencyatlowV /V
ratios, use
IN OUT
1A and an I of greater than 1.3A. During long duration
SAT
the following formula to set switching frequency:
overload or short-circuit conditions, the inductor RMS
current rating requirement is greater to avoid overheat-
ing of the inductor. To keep the efficiency high, the series
resistance (DCR) should be less than 0.04Ω, and the core
materialshouldbeintendedforhighfrequencyapplications.
VIN(MIN) − 5.8 − VSW(TOP)
VIN(MIN) + V − V
1
fSW(MAX)
=
t
OFF(MIN)
SW(TOP)
SW(BOT)
where V
is the minimum input voltage without
IN(MIN)
skipped cycles, V
The LT8697 limits the peak switch current in order to
protect the switches and the system from overload faults.
and V
are the internal
SW(TOP)
SW(BOT)
switch drops (~0.3V, ~0.15V, respectively at maximum
load), f is the switching frequency (set by R ), and
The top switch current limit (I ) is at least 4.8A at low
LIM
SW
T
duty cycles and decreases linearly to 4A at DC = 0.8. The
t
is the minimum switch off-time. Note that higher
OFF(MIN)
inductorvaluemustthenbesufficienttosupplythedesired
switching frequency will increase the minimum input
voltage below which cycles will be dropped to achieve
higher duty cycle.
maximum output current (I
), which is a function
OUT(MAX)
of the switch current limit (I ) and the ripple current.
LIM
∆IL
Inductor Selection and Maximum Output Current
IOUT(MAX) = ILIM
−
2
The LT8697 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuitconditionstheLT8697safelytoleratesopera-
tion with a saturated inductor through the use of a high
speed peak-current mode architecture.
The peak-to-peak ripple current in the inductor can be
calculated as follows:
5V
5V
∆IL =
• 1−
L • fSW
VIN(MAX)
A good first choice for the inductor value is as follows:
where f is the switching frequency of the LT8697 and
SW
L is the value of the inductor. Therefore, the maximum
output current that the LT8697 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
5.8V+ VSW(BOT)
L =
fSW
where f is the switching frequency in MHz, V
SW
SW(BOT)
is the bottom switch drop (~0.15V) and L is the inductor
sufficient maximum output current (I
) given the
value in μH.
OUT(MAX)
8697p
19
For more information www.linear.com/LT8697
LINEAR_DIMENSIONS [ Linear Dimensions ]
