LTC3785 Datasheet, PDF (10/20 Pages) Linear Technology – 10V, High Effi ciency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
U
OPERATIO
TOPSIDE MOSFET DRIVER SUPPLY (V_BST1, V_BST2)
The external bootstrap capacitors connected to the V_BST1
and V_BST2 pins supply the gate drive voltage for the top-
side MOSFET switches A and D. When the top MOSFET
switch A turns on, the switch node SW1 rises to V_IN and
the V_BST2 pin rises to approximately V_IN + V_CC. When the
bottom MOSFET switch B turns on, the switch node SW1
drops low and the boost capacitor is charged through the
diode connected to V_CC. When the top MOSFET switch D
turns on, the switch node SW2 rises to V_OUT and the V_BST2
pin rises to approximately V_OUT + V_CC. When the bottom
MOSFET switch C turns on, the switch node SW2 drops
low and the boost capacitor is charged through the diode
connected to V_CC. The boost capacitors need to store about
100 times the gate charge required by the top MOSFET
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 ≅ (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 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
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
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
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