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LT3724IFE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT3724
10
3724fd
OPERATIONS
either the start of the next cycle or until the bootstrapped
capacitor is fully charged.
MOSFET Driver
The LT3724 contains a high speed boosted driver to turn
on and off an external N-channel MOSFET switch. The
MOSFET driver derives its power from the boost capacitor
which is referenced to the SW pin and the source of the
MOSFET. The driver provides a large pulse of current to
turn on the MOSFET fast and minimize transition times.
Multiple MOSFETs can be paralleled for higher current
operation.
To eliminate the possibility of shoot through between the
MOSFET and the internal SW pull-down switch, an adap-
tive nonoverlap circuit ensures that the internal pull-down
switch does not turn on until the gate of the MOSFET is
below its turn on threshold.
Low Current Operation (Burst Mode Operation)
To increase low current load efficiency, the LT3724 is
capable of operating in Linear Technology’s proprietary
Burst Mode operation where the external MOSFET operates
intermittently based on load current demand. The Burst
Mode function is disabled by connecting the BURST_EN
pin to V
CC
and enabled by connecting the pin to SGND.
When the required switch current, sensed via the V
C
pin
voltage, is below 15% of maximum, Burst Mode operation
is employed and that level of sense current is latched onto
the IC control path. If the output load requires less than
this latched current level, the converter will overdrive the
output slightly during each switch cycle. This overdrive
condition is sensed internally and forces the voltage on the
V
C
pin to continue to drop. When the voltage on V
C
drops
150mV below the 15% load level, switching is disabled,
and the LT3724 shuts down most of its internal circuitry,
reducing total quiescent current to 100µA. When the
converter output begins to fall, the V
C
pin voltage begins
to climb. When the voltage on the V
C
pin climbs back to
the 15% load level, the IC returns to normal operation and
switching resumes. An internal clamp on the V
C
pin is set
at 100mV below the output disable threshold, which limits
the negative excursion of the pin voltage, minimizing the
converter output ripple during Burst Mode operation.
During Burst Mode operation, the V
IN
pin current is 20µA
and the V
CC
current is reduced to 80µA. If no external drive
is provided for V
CC
, all V
CC
bias currents originate from the
V
IN
pin, giving a total V
IN
current of 100µA. Burst current
can be reduced further when V
CC
is driven using an output
derived source, as the V
CC
component of V
IN
current is
then reduced by the converter duty cycle ratio.
Start-Up
The following section describes the start-up of the supply
and operation down to 4V once the step-down supply is
up and running. For the protection of the LT3724 and the
switching supply, there are internal undervoltage lockout
(UVLO) circuits with hysteresis on V
IN
, V
CC
and V
BOOST
,
as shown in the Electrical Characteristics table. Start-up
and continuous operation require that all three of these
undervoltage lockout conditions be satisfied because
the TG MOSFET driver is disabled during any UVLO fault
condition. In startup, for most applications, V
CC
is powered
from V
IN
through the high voltage linear regulator of the
LT3724. This requires V
IN
to be high enough to drive the
V
CC
voltage above its undervoltage lockout threshold.
V
CC
, in turn, has to be high enough to charge the BOOST
capacitor through an external diode so that the BOOST
voltage is above its undervoltage lockout threshold. There
is an NPN switch that pulls the SW node to ground each
cycle during the TG power MOSFET off-time, ensuring the
BOOST capacitor is kept fully charged. Once the supply
is up and running, the output voltage of the supply can
backdrive V
CC
through an external diode. Internal circuitry
disables the high voltage regulator to conserve V
IN
supply
current. Output voltages that are too low or too high to
backdrive V
CC
require additional circuitry such as a voltage
doubler or linear regulator. Once V
CC
is backdriven from
a supply other than V
IN
, V
IN
can be reduced to 4V with
normal operation maintained.
(Refer to Functional Diagram)
LT3724
11
3724fd
OPERATIONS
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltage ramp minimizes output voltage overshoot, reduces
inrush current from the V
IN
supply, and facilitates supply
sequencing. A capacitor, C
SS
, connected between V
OUT
of
the supply and the C
SS
pin of the IC, programs the slew
rate. The capacitor provides a current to the C
SS
pin which
is proportional to the dV/dt of the output voltage. The
soft-start circuit overrides the control loop and adjusts the
inductor current until the output voltage slew rate yields a
2µA current through the soft-start capacitor. If the current is
greater than 2µA, then the current threshold set by the DC
control voltage, V
C
, is decreased and the inductor current
is lowered. This in turn lowers the output current and the
output voltage slew rate is decreased. If the current is less
than 2µA, then the current threshold set by the DC control
voltage, V
C
, is increased and the inductor current is raised.
This in turn increases the output current and the output
voltage slew rate is increased. Once the output voltage is
within 5% of its regulation voltage, the soft-start circuit
is disabled and the main control regulates the output. The
soft-start circuit is reactivated when the output voltage
drops below 70% of its regulation voltage.
Slope/Antislope Compensation
The IC incorporates slope compensation to eliminate
potential subharmonic oscillations in the current control
loop. The IC’s slope compensation circuit imposes an
artificial ramp on the sensed current to increase the rising
slope as duty cycle increases.
Unfortunately, this additional ramp typically affects the
sensed current value, thereby reducing the achievable
current limit value by the same amount as the added ramp
represents. As such, the current limit is typically reduced
as the duty cycle increases. The LT3724, however, contains
antislope compensation circuitry to eliminate the current
limit reduction associated with slope compensation. As the
slope compensation ramp is added to the sensed current,
a similar ramp is added to the current limit threshold. The
end result is that the current limit is not compromised so
the LT3724 can provide full power regardless of required
duty cycle.
Shutdown
The LT3724 includes a shutdown mode where all the
internal IC functions are disabled and the V
IN
current is
reduced to less than 10µA. The shutdown pin can be used
for undervoltage lockout with hysteresis, micropower shut-
down or as a general purpose on/off control of the converter
output. The shutdown function has two thresholds. The
first threshold, a precision 1.23V threshold with 120mV
of hysteresis, disables the converter from switching. The
second threshold, approximately a 0.7V referenced to
SGND, completely disables all internal circuitry and reduces
the V
IN
current to less than 10µA. See the Application
Information section for more information.
(Refer to Functional Diagram)
LT3724
12
3724fd
The basic LT3724 step-down (buck) application, shown
in the Typical Application on the front page, converts a
larger positive input voltage to a lower positive or negative
output voltage. This Application Information section assists
selection of external components for the requirements of
the power supply.
R
SENSE
Selection
The current sense resistor, R
SENSE
, monitors the inductor
current of the supply (See Typical Application on front
page). Its value is chosen based on the maximum required
output load current. The LT3724 current sense amplifier
has a maximum voltage threshold of, typically, 150mV.
Therefore, the peak inductor current is 150mV/R
SENSE
.
The maximum output load current, I
OUT(MAX)
, is the peak
inductor current minus half the peak-to-peak ripple cur-
rent, ∆I
L
.
Allowing adequate margin for ripple current and external
component tolerances, R
SENSE
can be calculated as fol-
lows:
R
SENSE
=
100mV
I
OUT(MAX)
Typical values for R
SENSE
are in the range of 0.005Ω
to 0.05Ω.
Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
The minimum inductance value is calculated as follows:
L ≥ V
OUT
•
V
IN(MAX)
– V
OUT
f
SW
• V
IN(MAX)
• ∆I
L
f
SW
is the switch frequency (200kHz).
The typical range of values for ∆I
L
is (0.2 • I
OUT(MAX)
) to
(0.5 • I
OUT(MAX)
), where I
OUT(MAX)
is the maximum load
current of the supply. Using ∆I
L
= 0.3 • I
OUT(MAX)
yields a
good design compromise between inductor performance
versus inductor size and cost. Higher values of ∆I
L
will
increase the peak currents, requiring more filtering on
the input and output of the supply. If ∆I
L
is too high,
the slope compensation circuit is ineffective and current
mode instability may occur at duty cycles greater than
50%. Lower values of ∆I
L
require larger and more costly
magnetics. A value of ∆I
L
= 0.3 • I
OUT(MAX)
produces a
±15% of I
OUT(MAX)
ripple current around the DC output
current of the supply.
Some magnetics vendors specify a volt-second product
in their datasheet. If they do not, consult the magnetics
vendor to make sure the specification is not being exceeded
by your design. The volt-second product is calculated as
follows:
Volt-second (µsec)=
(V
IN(MAX)
– V
OUT
) • V
OUT
V
IN(MAX)
• f
SW
The magnetics vendors specify either the saturation cur-
rent, the RMS current or both. When selecting an inductor
based on inductor saturation current, use the peak cur-
rent through the inductor, I
OUT(MAX)
+ ∆I
L
/2. The inductor
saturation current specification is the current at which
the inductance, measured at zero current, decreases by
a specified amount, typically 30%.
When selecting an inductor based on RMS current rating,
use the average current through the inductor, I
OUT(MAX)
.
The RMS current specification is the RMS current at which
the part has a specific temperature rise, typically 40°C,
above 25°C ambient.
After calculating the minimum inductance value, the volt-
second product, the saturation current and the RMS current
for your design, select an off-the-shelf inductor. A list of
magnetics vendors can be found at www.linear.com, or
contact the Linear Technology Application Department.
For more detailed information on selecting an inductor,
please see the “Inductor Selection” section of Linear
Technology Application Note 44.
Step-Down Converter: MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFET include on resistance(R
DS(ON)
), re-
verse transfer capacitance (C
RSS
), maximum drain source
voltage (V
DSS
), total gate charge (Q
G
), and maximum
continuous drain current.
APPLICATIONS INFORMATION
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LT3724IFE#PBF

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Buy LT3724IFE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators High Voltage Non-Synch Controller
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