6
LTC1649
APPLICATIONS INFORMATION
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OVERVIEW
T
he LTC1649 is a voltage feedback PWM switching regu-
lator controller (see Block Diagram) designed for use in
high power, low input voltage step-down (buck) convert-
ers. It includes an onboard PWM generator, a precision
reference trimmed to ±0.5%, two high power MOSFET
gate drivers and all necessary feedback and control cir-
cuitry to form a complete switching regulator circuit. Also
included is an internal charge pump which provides 5V
gate drive to the external MOSFETs with input supply
voltage as low as 2.7V. The LTC1649 runs at an internally
fixed 200kHz clock frequency and requires an external
resistor divider to set the output voltage.
The LTC1649 includes 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. Also included is an internal soft start
feature that requires only a single external capacitor to
operate.
THEORY OF OPERATION
Primary Feedback Loop
The LTC1649 senses the output voltage of the circuit at the
output capacitor through a resistor divider connected to
the FB pin 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 compared to a fixed frequency sawtooth waveform
generated by the internal oscillator to generate a pulse
width modulated 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 compen-
sation network at COMP, the output node of the FB
transconductance amplifier.
MIN, MAX Feedback Loops
Two additional comparators in the feedback loop provide
high speed fault correction in situations where the FB
amplifier may not respond quickly enough. MIN compares
the feedback signal to a voltage 40mV (3%) below the
internal reference. At this point, the MIN comparator
overrides the FB amplifier and forces the loop to full duty
cycle, set by the internal oscillator at about 93%. 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.
Current Limit Loop
The LTC1649 includes yet another feedback loop to con-
trol operation in current limit. The I
LIM
amplifier monitors
the voltage drop across external MOSFET Q1 with the I
FB
pin during the portion of the cycle when G1 is high. It
compares this voltage to the voltage at the I
MAX
pin. As the
peak current rises, the drop across Q1 due to its R
DS(ON)
increases. When I
FB
drops below I
MAX
, indicating that Q1’s
drain current has exceeded the maximum level, I
LIM
starts
to pull current out of the external soft start capacitor,
cutting the duty cycle and controlling the output current
level. At the same time, the I
LIM
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, indicating a se-
vere 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 LTC1649 to withstand brief overcurrent con-
ditions without limiting.
By using the R
DS(ON)
of Q1 to measure the output current,
the current limit circuit eliminates the sense resistor that
would otherwise be required and minimizes the number of
components in the external high current path. Because
power MOSFET R
DS(ON)
is not tightly controlled and varies
with temperature, the LTC1649 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
current level where the limiting circuit begins to take effect
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.
