LTC3805
10
3805fg
operation
First, consider operation without overcurrent protection.
For some maximum converter output current, the voltage
on the I
TH
pin rises to and is clamped at approximately 1.9V.
This corresponds to a 100mV limit on the voltage at the
I
SENSE
pin. As the output current is further increased, the
duty cycle is reduced as the output voltage sags. However,
the peak current in the external MOSFET is limited by the
100mV threshold at the I
SENSE
pin.
As the output current is increased further, eventually,
the duty cycle is reduced to the 6% minimum. Since the
external MOSFET is always turned on for this minimum
amount of time, the current comparator no longer limits
the current through the external MOSFET based on the
100mV threshold. If the output current continues to in-
crease, the current through the MOSFET could rise to a
level that would damage the converter.
To prevent damage, the overcurrent pin, OC, is also
connected to the current sense resistor, and a fault is
triggered if the voltage on the OC pin exceeds 100mV. To
protect itself, the converter stops operating as described
in the next section. External resistors can be used to ad-
just the overcurrent
threshold to voltages higher or lower
than
100mV as described in the Applications Information
section.
Soft-Start and Fault Timeout Operation
The soft-start and fault timeout of the LTC3805 uses either
a fixed internal timer or an external timer programmed
by a capacitor from the SSFLT pin to GND. The internal
soft-start and fault timeout times are minimums and can
be increased by placing a capacitor from the SSFLT pin
to GND. Operation is shown in Figure 1.
Leave the SSFLT pin open to use the internal soft-start and
fault timeout. The internal soft-start is complete in about
1.8ms. In the event of an overcurrent as detected by the
OC pin exceeding 100mV, the LTC3805 shuts down and
an internal timing circuit waits for a fault timeout of about
4.25ms and then restarts the converter.
Add a capacitor C
SS
from the SSFLT pin to GND to increase
both the soft-start time and the time for fault timeout. Dur-
ing soft-start, C
SS
is charged with a 6µA current. When the
LTC3805 comes out of shutdown, the LTC3805 quickly
charges C
SS
to about 0.7V at which point GATE begins
switching. From that point, GATE continues
switching with
increasing duty cycle until the SSFLT pin reaches about
2.25V at which point soft-start is over and closed-loop
regulation begins. The voltage on the SSFLT pin addition-
ally further charges to about 4.75V.
C
SS
also performs the timeout function in the event of a
fault. After a fault, C
SS
is slowly discharged from about
4.75V to about 0.7V by a 2µA current. When the voltage
on the SSFLT pin reaches 0.7V the converter attempts to
restart. More detail on programming the external soft-start
fault timeout is described in the Applications Information
section.
Powering the LTC3805
A built-in shunt regulator from the V
CC
pin to GND limits
the voltage on the V
CC
pin to approximately 9.5V as long
as the shunt regulator is not forced to sink more than
25mA. The shunt regulator is always active, even when
the LTC3805 is in shutdown, since it serves the vital func-
tion of protecting the V
CC
pin from overvoltage. The shunt
regulator permits the use of a wide variety of powering
schemes for the LTC3805 even from high voltage sources
that exceed the LTC3805’s absolute maximum ratings.
Further details
on powering schemes are described in the
Applications Information section.
Adjustable Slope Compensation
The
LTC3805 injects a 10µA peak current ramp out of
its I
SENSE
pin which can be used, in conjunction with an
external resistor, for slope compensation in designs that
require it. This current ramp is approximately linear and
begins at zero current at 6% duty cycle, reaching peak
current at 80% duty cycle. Additional details are provided
in the Applications Information section.
