Datasheet

LT3042
19
3042fb
For more information www.linear.com/LT3042
APPLICATIONS INFORMATION
ENABLE/UVLO
The EN/UV pin is used to put the regulator into a mi-
cropower shutdown state. The LT3042
has an accurate
1.24V turn-on threshold on the EN/UV pin with 170mV
of hysteresis. This threshold can be used in conjunction
with a resistor divider from the input supply to define an
accurate undervoltage lockout (UVLO) threshold for the
regulator. The EN/UV pin current (I
EN
) at the threshold from
the Electrical Characteristics table needs to be considered
when calculating the resistor divider network:
V
IN(UVLO)
= 1.24V • 1+
R
EN2
R
EN1
⎛
⎝
⎜
⎞
⎠
⎟
+I
EN
•R
EN2
The EN/UV pin current (I
EN
) can be ignored if R
EN1
is less
than 100k. If unused, tie EN/UV pin to IN.
Programmable Power Good
As illustrated in the Block Diagram, power good thresh
-
old is user programmable using the ratio of two external
resistors, R
PG2
and R
PG1
:
V
OUT(PG _ THRESHOLD)
= 0.3V • 1+
R
PG2
R
PG1
⎛
⎝
⎜
⎞
⎠
⎟
+I
PGFB
•R
PG2
If the PGFB pin increases above 300mV, the open-collector
PG pin de-asserts and becomes high impedance. The
power good comparator has 7mV hysteresis and 5µs of
deglitching. The PGFB pin current (I
PGFB
) from the Electrical
Characteristics table must be considered when determining
the resistor divider network. The PGFB pin current (I
PGFB
)
can be ignored if R
PG1
is less than 30k. If power good
functionality is not used, float the PG pin. Please note that
programmable power good and fast start-up capabilities
are disabled for output voltages below 300mV or when
the device is in shutdown.
Externally Programmable Current Limit
The ILIM pin’s current limit threshold is 300mV. Connecting
a resistor from ILIM to GND sets the maximum current
flowing out of the ILIM pin, which in turn programs the
LT3042’s current limit. With a 125mA • kΩ programming
scale factor, the current limit can be calculated as follows:
Current Limit = 125mA • kΩ/R
ILIM
For example, a 1kΩ resistor programs the current limit to
125mA and a 2kΩ resistor programs the current limit to
62.5mA. For good accuracy, Kelvin connect this resistor
to the LT3042’s GND pin.
In cases where IN-to-OUT differential is greater than 12V,
the LT3042’s foldback circuitry decreases the internal
current limit. As a result, internal current limit may over
-
ride the externally programmed current limit level to keep
the LT3042 within its safe-operating-area (SOA). See the
Internal Current Limit vs Input-to-Output Differential graph
in the Typical Performance Characteristics section.
As shown in the Block Diagram, the ILIM pin sources current
proportional (1:400) to output current; therefore, it also
serves as a current monitoring pin with a 0V to 300mV
range. If external current limit or current monitoring is
not used, tie ILIM to GND.
Output Overshoot Recovery
During a load step from full load to no load (or light
load), the output voltage overshoots before the regulator
responds to turn the power transistor OFF. Given that there
is no load (or very light load) present at the output, it takes
a long time to discharge the output capacitor.
As illustrated in the Block Diagram, the LT3042 incorporates
an overshoot recovery circuitry that turns on a current
sink to discharge the output capacitor in the event OUTS
is higher than SET. This current is typically about 4mA.
No load recovery is disabled for input voltages less than
2.5V or output voltages less than 1.5V.
If OUTS is externally held above SET, the current sink
turns ON in an attempt to restore OUTS to its programmed
voltage. The current sink remains ON until the external
circuitry releases OUTS.
Direct Paralleling for Higher Current
Higher output current is obtained by paralleling multiple
LT3042s. Tie all SET pins together and all IN pins together.
Connect the OUT pins together using small pieces of PCB
trace (used as a ballast resistor) to equalize currents in
the LT3042s. PCB trace resistance in milliohms/inch is
shown in Table 2.
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