LTC3630A Datasheet, PDF (10/24 Pages) Linear Technology – High Efficiency, 76V 500mA Synchronous Step-Down Converter

LTC3630A
Operation (Refer to Block Diagram)
supply droop is desired, a capacitor can be placed from
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 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 1.2A. 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.
For applications requiring higher output current, the
LTC3630A provides a feedback comparator output pin
(FBO) for combining the output current of multiple
LTC3630As. By connecting the FBO pin of a “master”
LTC3630A to the V_FB pin of one or more “slave” LTC3630As,
the output currents can be combined to source much
more than 500mA.
which can be greater than 1A. The power dissipation of
the LTC3630A can increase dramatically during dropout
operation 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 LTC3630A, 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 LTC3630A incorporates an overtemperature shutdown
feature. If the junction temperature reaches approximately
180°C, the LTC3630A 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 LTC3630A can provide a programmable undervoltage
lockout which can also serve as a precise input voltage
monitor by using a resistive divider from V_IN to GND with
the tap connected to the RUN pin. Switching is enabled
when the RUN pin voltage exceeds 1.21V and is disabled
when dropping below 1.1V. Pulling the RUN pin below
700mV forces a low quiescent current shutdown (5µA).
Furthermore, if the input voltage falls below 3.5V typi-
cal (3.7V maximum), an internal undervoltage detector
disables switching.
When switching is disabled, the LTC3630A can safely
sustain input voltages up to the absolute maximum rating
of 80V. Input supply undervoltage events trigger a soft-
start reset, which results in a graceful recovery from an
input supply transient.
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 LTC3630A allows
the duty cycle to increase all the way to 100%. At 100%
duty cycle, the P-channel MOSFET stays on continu-
ously, providing output current equal to the peak current,
High Input Voltage Considerations
When operating with an input voltage to output voltage
differential of more than 65V, a minimum output load
current of 5mA is required to maintain a well-regulated
output voltage under all operating conditions, including
shutdown mode. If this 5mA minimum load is not available,
then the minimum output voltage that can be maintained
by the LTC3630A is limited to V_IN – 65V.
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