Datasheet

LTC3785
10
3785fc
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.
O
SCILLA
TOR
The frequency of operation is set through a resistor from
the RT pin to ground where:
f
R
MHz
OSC
T
≅






25000
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 1.5% 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.
F
OR
WARD 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.