LTC3785-1 Datasheet, PDF (10/20 Pages) Linear Technology – 10V, High Effi ciency, Buck-Boost Controller with Power Good

LTC3785-1
OPERATION
RUN/SOFT-START (RUN/SS)
The RUN/SS pin serves as the enable to the LTC3785-1,
soft-start function, and fault programming. A 1μA current
source charges the external capacitor. Once the RUN/SS
voltage is above a diode drop(~0.7V) the IC is enabled. Once
the IC is enabled, the RUN/SS voltage minus a diode drop
(RUN/SS – 0.7V) clamps the output of the error amp (V_C)
to limit duty cycle. The range of the duty cycle clamping is
approximately 0.7V to 1.7V. The RUN/SS pin is clamped
to approximately 2.2V. If current limit is reached the pin
will begin to discharge with a current determined by the
magnitude of inductor current overcurrent limit, but not
to exceed 10μA. This function will be described in more
detail in the “Forward Current Limit” section.
OSCILLATOR
The frequency of operation is set through a resistor from
the RT pin to ground where f ≅ (2.5e¹⁰/RT)Hz.
ERROR AMP
The error amplifier is a voltage mode amplifier with a
reference voltage of 1.225V internally connected to the
non-inverting input. The loop compensation components
are configured around the amplifier to provide loop com-
pensation for the converter. The RUN/SS pin will clamp the
error amp output, V_C, to provide a soft-start function.
UNDERVOLTAGE AND OVERVOLTAGE PROTECTION
The LTC3785-1 incorporates overvoltage (OV) and
undervoltage (UV) functions for fault protection and
transient limitation. Both comparators are connected
to the V_SENSE pin, which usually has a similar voltage
divider as the error amplifier without the compensation.
The overvoltage threshold is 7.5% above the reference.
The undervoltage threshold is 7.5% below the reference
with both comparators having 1% hysteresis. During an
overvoltage fault, all output switching stops until the fault
ceases. During an undervoltage fault, the IC is commanded
to run fixed frequency only (disabled Burst Mode opera-
tion). If the design requires a tightened threshold to one
of the comparator thresholds the voltage divider on the
V_SENSE pin can be skewed to achieve the threshold. Since
10
the range is a constant, tightening the UV threshold will
loosen the OV threshold and vice versa.
POWER GOOD COMPARATOR
The PGOOD pin is an open-drain output which indicates
the status of the buck-boost converter output. The output
voltage is monitored at V_SENSE via a resistor divider tap
from V_OUT to GND. The values used for this resistor divider
are typically selected to be the same as those used in the
error amplifier feedback divider. If the voltage on V_SENSE
either falls 7.5% below (UV condition) or rises 7.5%
above (OV condition) the regulation voltage, the PGOOD
open-drain output will pull low signaling the output is
out of regulation. Once an out of regulation condition is
triggered, the voltage on V_SENSE must rise 1.5% above
the UV threshold or fall 2% below the OV threshold before
the pull-down will turn off. In addition, there is a 15μs
deglitch delay to help prevent false trips due to voltage
transients caused by line or load steps. Depending upon
the application, this delay may be insufficient. A capaci-
tor can be placed from V_SENSE to GND to add additional
deglitch filtering, ensuring PGOOD doesn’t trip during
a transient. The PGOOD output will also pull low during
shutdown and input undervoltage lockout to indicate these
fault conditions.
FORWARD CURRENT LIMIT
The LTC3785-1 is designed to sense the input current by
sampling the voltage across MOSFET A during the on time of
the switch (TG1 = High). The sense pins are I_SVIN and I_SSW1.
A current sense resistor can be used if increased accuracy
is required. The current limit threshold can be programmed
with a resistor on the I_LSET pin. Once the desired current
limit has been chosen, R_ILSET can be determined by the
following formula:
R_ILSET =
6000
R_DS(ON)A • I_LIMIT
Ω
where R_DS(ON)A = R_DS(ON) of N-channel MOSFET switch A
and I_LIMIT = current limit in Amps.
Once the voltage between I_SVIN and I_SSW1 exceeds the
threshold, current will be sourced out of FB to take control
37851f