LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 10

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

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OPERATION
LTC3785-1
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
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 ≅ (2.5e
ERROR AMP
The error amplifi er is a voltage mode amplifi er with a
reference voltage of 1.225V internally connected to the
non-inverting input. The loop compensation components
are confi gured around the amplifi er to provide loop com-
pensation for the converter. The RUN/SS pin will clamp the
error amp output, V
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
divider as the error amplifi er 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 fi xed 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
10
SENSE
pin can be skewed to achieve the threshold. Since
SENSE
pin, which usually has a similar voltage
C
, to provide a soft-start function.
10
/RT)Hz.
C
)
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
from V
are typically selected to be the same as those used in the
error amplifi er feedback divider. If the voltage on V
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
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 insuffi cient. A capaci-
tor can be placed from V
deglitch fi ltering, 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
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
limit has been chosen, R
following formula:
where R
and I
Once the voltage between I
threshold, current will be sourced out of FB to take control
R
ILSET
LIMIT
OUT
DS(ON)A
=
= current limit in Amps.
to GND. The values used for this resistor divider
R
DS ON A
= R
(
6000
DS(ON)
)
•
LSET
I
LIMIT
SENSE
ILSET
SENSE
of N-channel MOSFET switch A
pin. Once the desired current
SENSE
SVIN
Ω
can be determined by the
to GND to add additional
via a resistor divider tap
and I
must rise 1.5% above
SSW1
SVIN
exceeds the
and I
SENSE
SSW1
37851f
.