LTC3785_15 Datasheet, PDF (10/20 Pages) Linear Technology – 10V, High Efficiency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
Operation
switch A and D. In most applications a 0.1µF to 0.47µF,
X5R or X7R dielectric capacitor is adequate.
Run/Soft-Start (RUN/SS)
The RUN/SS pin serves as the enable to the LTC3785,
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_OSC
≅


25000
R_T


MHz
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 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 10% above the reference.
The undervoltage threshold is 6.5% below the reference
10
with both comparators having 1.5% hysteresis. During
an overvoltage fault, all output switching stops until the
fault ceases. During an undervoltage fault, the IC is com-
manded to run fixed frequency only (disabled Burst Mode
operation). 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 the range is a constant, tightening the UV threshold
will loosen the OV threshold and vice versa.
Forward Current Limit
The LTC3785 is designed to sense the input current by sam-
pling 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
of the voltage loop, resulting in a lower output voltage
to regulate the input current. This fault condition causes
the RUN/SS capacitor to begin discharging. The level of
the discharge current depends on how much the current
exceeds the programmed threshold. Figure 3 is a simpli-
fied diagram of the current sense and fault circuitry. If the
current limit fault duration is long enough to discharge the
RUN/SS capacitor below 1.225V, the fault latch is set and
will cycle the RUN/SS capacitor 16 times (1µA charging
and 1µA discharging of the RUN/SS capacitor) to create an
off time of 32 times the soft-start time before the outputs
are allowed to switch to restart the output voltage. If the
current limit fault level exceeds 150% of the programmed
I_LIMIT level at any time, the I_MAX comparator is tripped and
output switches B and D are turned on to discharge the
inductor current for the remainder of the cycle.
3785fc