LTC3637_15 Datasheet, PDF (10/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
OPERATION (Refer to Block Diagram)
the SS pin to ground. The 5µA current that is sourced
out of this pin will create a smooth voltage ramp on the
capacitor. If this ramp rate is slower than the internal
0.8ms soft-start, then the output voltage will be limited
by the ramp rate on the SS pin instead. The internal and
external soft-start functions are reset on start-up and after
an undervoltage or overvoltage event on the input supply.
The peak inductor current is not limited by the internal or
external soft-start functions; however, placing a capacitor
from the I_SET pin to ground does provide this capability.
Peak Inductor Current Programming
The peak current comparator nominally limits the peak
inductor current to 2.4A. This peak inductor current can
be adjusted by placing a resistor from the I_SET pin to
ground. The 5µA current sourced out of this pin through
the resistor generates a voltage that adjusts the peak cur-
rent comparator threshold.
During sleep mode, the current sourced out of the I_SET pin
is reduced to 1µA. The I_SET current is increased back to 5µA
on the first switching cycle after exiting sleep mode. The
I_SET current reduction in sleep mode, along with adding
a filtering capacitor, C_ISET, from the I_SET pin to ground,
provides a method of reducing light load output voltage
ripple at the expense of lower efficiency and slightly de-
graded load step transient response.
Dropout Operation
When the input supply decreases toward the output sup-
ply, the duty cycle increases to maintain regulation. The
P-channel MOSFET top switch in the LTC3637 allows
the duty cycle to increase all the way to 100%. At 100%
duty cycle, the P-channel MOSFET stays on continuously,
providing output current equal to the peak current, which
can be greater than 2A. The power dissipation of the
LTC3637 can increase dramatically during dropout opera-
tion especially at input voltages less than 10V. The increased
power dissipation is due to higher potential output current
and increased P-channel MOSFET on-resistance. See
the Thermal Considerations section of the Applications
Information for a detailed example.
Input Voltage and Overtemperature Protection
When using the LTC3637, care must be taken not to
exceed any of the ratings specified in the Absolute Maxi-
mum Ratings section. As an added safeguard, however,
the LTC3637 incorporates an overtemperature shutdown
feature. If the junction temperature reaches approximately
180°C, the LTC3637 will enter thermal shutdown mode.
Both power switches will be turned off and the SW node
will become high impedance. After the part has cooled
below 160°C, it will restart. The overtemperature level is
not production tested.
The LTC3637 additionally implements protection features
which inhibit switching when the input voltage is not within
a programmed operating range. By using a resistive di-
vider from the input supply to ground, the RUN and OVLO
pins can serve as a precise input supply voltage monitor.
Switching is disabled when either the RUN pin falls below
1.1V or the OVLO pin rises above 1.21V, which can be
configured to limit switching to a specific range of input
supply voltage. Pulling the RUN pin below 700mV forces
a low quiescent current shutdown (3µA). Furthermore, if
the input voltage falls below 3.5V typical (3.7V maximum),
an internal undervoltage detector disables switching.
When switching is disabled, the LTC3637 can safely sustain
input voltages up to the absolute maximum rating of 80V.
Input supply undervoltage or overvoltage events trigger a
soft-start reset, which results in a graceful recovery from
an input supply transient.
High Input Voltage Considerations
When operating with an input voltage to output voltage dif-
ferential of more than 65V, a minimum output load current
of 10mA is required to maintain a well-regulated output
voltage under all operating conditions, including shutdown
mode. If this 10mA minimum load is not available, then
the minimum output voltage that can be maintained by
the LTC3637 is limited to V_IN – 65V.
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