LTC7138
6
7138f
For more information www.linear.com/LTC7138
pin FuncTions
SW (Pin 1): Switch Node Connection to Inductor and
Catch Diode Cathode. This pin connects to the drain of
the internal power MOSFET switch.
V
IN
(Pin 3): Main Supply Pin. A ceramic bypass capacitor
should be tied between this pin and GND.
FBO (Pin 5): Feedback Comparator Output. Connect to the
V
FB
pins of additional LTC7138s to combine the output
current. The typical pull-up current is 20µA. The typical pull-
down impedance is 70Ω. See Applications Information.
V
PRG2
, V
PRG1
(Pins 6, 7): Output Voltage Selection. Short
both pins to ground for a resistive divider programmable
output voltage. Short V
PRG1
to SS and short V
PRG2
to
ground for a 5V output voltage. Short V
PRG1
to ground
and short V
PRG2
to SS for a 3.3V output voltage. Short
both pins to SS for a 1.8V output voltage.
GND (Pin 8, Exposed Pad Pin 17): Ground. The exposed
pad must be soldered to the PCB ground plane for rated
electrical and thermal performance.
V
FB
(Pin 9): Output Voltage Feedback. When configured
for an adjustable output voltage, connect to an external
resistive divider to divide the output voltage down for
comparison to the 0.8V reference. For the fixed output
configuration, directly connect this pin to the output.
SS (Pin 10): Soft-Start Control Input. A capacitor to
ground at this pin sets the output voltage ramp time. A
50µA current initially charges the soft-start capacitor until
switching begins, at which time the current is reduced to
its nominal value of 5µA. The output voltage ramp time
from zero to its regulated value is 1ms for every 6.25nF
of capacitance from SS to GND. If left floating, the ramp
time defaults to an internal 1ms soft-start.
I
SET
(Pin 11): Peak Current Set Input. A resistor from this
pin to ground sets the peak current comparator threshold.
Leave floating for the maximum peak current (610mA typi
-
cal) or short to ground for minimum peak current (170mA
typical). The valley current is typically 60% of the peak
current set by this pin. The maximum output current is
75% of the peak current. The 5µA current that is sourced
out of this pin when switching is reduced to 1µA in sleep.
Optionally, a capacitor can be placed from this pin to GND
to trade off efficiency for light load output voltage ripple.
See Applications Information.
OVLO (Pin 12): Overvoltage Lockout Input. Connect to
the input supply through a resistor divider to set the over
-
voltage lockout level. A voltage on this pin above 1.21V
disables the internal MOSFET switch. Normal operation
resumes when the voltage on this pin decreases below
1.10V. Exceeding the OVLO lockout threshold triggers a
soft-start
reset, resulting in a graceful recovery from an
input supply transient. This pin must be grounded if the
OVLO is not used.
RUN (Pin 14): Run Control Input. A voltage on this pin
above 1.21V enables normal operation. Forcing this pin
below 0.7V shuts down the LTC7138, reducing quiescent
current to approximately 1.4µA. Optionally, connect to
the input supply through a resistor divider to set the
undervoltage lockout.
ANODE (Pin 16): Catch Diode Anode Sense. This pin is the
anode connection for the catch diode. An internal sense
resistor is connected between this pin and the exposed
pad ground.
