LTC3630
(Refer to Block Diagram)
OPERATION
An internal 0.8ms soft-start function limits the ramp rate
oftheoutputvoltageonstart-uptopreventexcessiveinput
supply droop. If a longer ramp time and consequently less
supply droop is desired, a capacitor can be placed from
the SS pin to ground. The 5ꢀA 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.
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 LTC3630 allows the
duty cycle to increase all the way to 100%. At 100% duty
cycle, the P-channel MOSFET stays on continuously, pro-
viding output current equal to the peak current, which can
be greater than 1A. The power dissipation of the LTC3630
can increase dramatically during dropout operation espe-
cially at input voltages less than 10V. The increased power
dissipation is due to higher potential output current and
increasedP-channelMOSFETon-resistance.SeetheTher-
malConsiderationssectionoftheApplicationsInformation
for a detailed example.
The peak inductor current is not limited by the internal or
external soft-start functions; however, placing a capacitor
from the I pin to ground does provide this capability.
SET
Peak Inductor Current Programming
Input Voltage and Overtemperature Protection
The peak current comparator nominally limits the peak
inductor current to 1.2A. This peak inductor current can
When using the LTC3630, 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 LTC3630 incorporates an overtemperature shutdown
feature.Ifthejunctiontemperaturereachesapproximately
180°C, the LTC3630 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.
be adjusted by placing a resistor from the I
pin to
SET
ground. The 5ꢀA 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 pin
SET
isreducedto1ꢀA.TheI currentisincreasedbackto5ꢀA
SET
on the first switching cycle after exiting sleep mode. The
I
current reduction in sleep mode, along with adding
SET
a filtering capacitor, C , from the I
pin to ground,
The LTC3630 can provide a programmable undervoltage
lockout which can also serve as a precise input voltage
ISET
SET
provides a method of reducing light load output voltage
ripple at the expense of lower efficiency and slightly de-
graded load step transient response.
monitor by using a resistive divider from V to GND with
IN
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 (5ꢀA).
Furthermore, if the input voltage falls below 3.5V typi-
cal (3.7V maximum), an internal undervoltage detector
disables switching.
For applications requiring higher output current, the
LTC3630providesafeedbackcomparatoroutputpin(FBO)
forcombiningtheoutputcurrentofmultipleLTC3630s. By
connecting the FBO pin of a “master” LTC3630 to the V
FB
pin of one or more “slave” LTC3630s, the output currents
can be combined to source much more than 500mA.
When switching is disabled, the LTC3630 can safely sus-
tain input voltages up to the absolute maximum rating of
70V. Input supply undervoltage events trigger a soft-start
reset, which results in a graceful recovery from an input
supply transient.
3630fb
10
Linear Systems [ Linear Systems ]
