LTC1430
U
W U U
APPLICATIO S I FOR ATIO
OVERVIEW
internal reference. At this point, the MIN comparator
overrides the FB amplifier and forces the loop to full duty
cycle, setbytheinternaloscillatoratabout90%. Similarly,
the MAX comparator monitors the output voltage at 3%
above the internal reference and forces the output to 0%
duty cycle when tripped. These two comparators prevent
extreme output perturbations with fast output transients,
while allowing the main feedback loop to be optimally
compensated for stability.
The LTC1430 is a voltage feedback PWM switching regu-
lator controller (see Block Diagram) designed for use in
high power, low voltage step-down (buck) converters. It
includes an onboard PWM generator, a precision refer-
ence trimmed to ±0.5%, two high power MOSFET gate
drivers and all necessary feedback and control circuitry to
form a complete switching regulator circuit. The PWM
loop nominally runs at 200kHz.
The 16-lead versions of the LTC1430 include a current
limit sensing circuit that uses the upper external power
MOSFET as a current sensing element, eliminating the
need for an external sense resistor.
Current Limit Loop
The 16-lead LTC1430 devices include yet another feed-
back loop to control operation in current limit. The current
limit loop is disabled in 8-lead devices. The ILIM amplifier
monitors the voltage drop across external MOSFET M1
with the IFB pin during the portion of the cycle when G1 is
high.ItcomparesthisvoltagetothevoltageattheIMAX pin.
As the peak current rises, the drop across M1 due to its
RDS(ON) increases. When IFB drops below IMAX, indicating
that M1’s drain current has exceeded the maximum level,
ILIM starts to pull current out of the external soft-start
capacitor, cuttingthedutycycleandcontrollingtheoutput
current level. At the same time, the ILIM comparator
generates a signal to disable the MIN comparator to
prevent it from conflicting with the current limit circuit. If
the internal feedback node drops below about 0.8V, indi-
cating a severe output overload, the circuitry will force the
internal oscillator to slow down by a factor of as much as
100. If desired, the turn on time of the current limit loop
can be controlled by adjusting the size of the soft-start
capacitor, allowing the LTC1430 to withstand short over-
current conditions without limiting.
Also included in the 16-lead version is an internal soft-
start feature that requires only a single external capacitor
to operate. In addition, 16-lead parts feature an adjustable
oscillator which can run at frequencies from 50kHz to
beyond500kHz, allowingaddedflexibilityinexternalcom-
ponent selection. The 8-lead versions do not include
current limit, internal soft-start or frequency adjustability.
THEORY OF OPERATION
Primary Feedback Loop
TheLTC1430sensestheoutputvoltageofthecircuitatthe
output capacitor with the SENSE+ and SENSE– pins and
feeds this voltage back to the internal transconductance
amplifier FB. FB compares the resistor-divided output
voltage to the internal 1.26V reference and outputs an
error signal to the PWM comparator. This is then com-
pared to a fixed frequency sawtooth waveform generated
by the internal oscillator to generate a pulse width modu-
lated signal. This PWM signal is fed back to the external
MOSFETs through G1 and G2, closing the loop. Loop
compensation is achieved with an external compensation
network at COMP, the output node of the FB transconduc-
tance amplifier.
By using the RDS(ON) of M1 to measure the output current,
the current limit circuit eliminates the sense resistor that
wouldotherwiseberequiredandminimizesthenumberof
components in the external high current path. Because
powerMOSFETRDS(ON) isnottightlycontrolledandvaries
with temperature, the LTC1430 current limit is not de-
signed to be accurate; it is meant to prevent damage to the
power supply circuitry during fault conditions. The actual
currentlevelwherethelimitingcircuitbeginstotakeeffect
may vary from unit to unit, depending on the power
MOSFETs used. See Soft-Start and Current Limit for more
details on current limit operation.
MIN, MAX Feedback Loops
Two additional comparators in the feedback loop provide
high speed fault correction in situations where the FB
amplifiermaynotrespondquicklyenough. MINcompares
the feedback signal to a voltage 40mV (3%) below the
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Linear [ Linear ]
