LTC7138
9
7138f
For more information www.linear.com/LTC7138
Start-Up and Shutdown
If the voltage on the RUN pin is less than 0.7V, the LTC7138
enters a shutdown mode in which all internal circuitry is
disabled, reducing the DC supply current to 1.4µA. When the
voltage on the RUN pin exceeds 1.21V, normal operation of
the main control loop is enabled. The RUN pin comparator
has 110mV of internal hysteresis, and therefore must fall
below 1.1V to disable the main control loop.
An internal 1ms soft-start function limits the ramp rate of
the output voltage on start-up to prevent excessive input
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 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 1ms soft-start,
then the output voltage will be limited by the ramp rate on
the SS pin. The internal and external soft-start functions
are reset on start-up, after an undervoltage or overvoltage
event on the input supply, and after an overtemperature
shutdown.
Peak Inductor Current Programming
The peak current comparator nominally limits the peak
inductor current to 610mA. 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. The valley current threshold
tracks the peak current threshold setting, and is typically
60% of the peak current.
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 network, R
ISET
and 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
degraded load step transient response.
For applications requiring higher output current, the
LTC7138 provides a feedback comparator output pin (FBO)
for combining the output current of multiple LTC7138s.
operaTion
By connecting the FBO pin of a master LTC7138 to the V
FB
pin of one or more slave LTC7138s, the output currents
can be combined to source 400mA times the number of
LTC7138s.
Dropout Operation
When the input supply decreases toward the output sup
-
ply, the duty cycle increases to maintain regulation. The
P-channel MOSFET switch in the LTC7138 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 is greater
than the maximum load current when not in dropout.
Input Voltage and Overtemperature Protection
When using the LTC7138, care must be taken not to exceed
any of the ratings specified in the Absolute Maximum Rat
-
ings section. As an added safeguard, however, the LTC7138
incorporates an overtemperature shutdown feature. If the
junction temperature reaches approximately 180°C, the
LTC7138 will enter thermal shutdown mode. The power
switch 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 LTC7138 additionally implements protection features
which inhibit switching when the input voltage is not within
a programmable operating range. By use of a resistive
divider from the input supply to ground, the RUN and
OVLO pins serve as a precise input supply voltage moni
-
tor. 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. Furthermore, if the input voltage falls below
3.5V typical (3.8V maximum), an internal undervoltage
detector disables switching.
When switching is disabled, the LTC7138 can safely sustain
input voltages up to the absolute maximum rating of 140V.
Input supply undervoltage or overvoltage events trigger a
soft-start reset, which results in a graceful recovery from
an input supply transient.
(Refer to Block Diagram)
