lt3070 Linear Technology Corporation, lt3070 Datasheet - Page 21

• Manufacturer Part Number: lt3070
• Description: 5a, Low Noise, Programmable Output, 85mv Dropout Linear Regulator
• Manufacturer: Linear Technology Corporation

ApplicAtions inFormAtion
Load Regulation
The LT3070 provides a Kelvin sense pin for V
the application to correct for parasitic package and PCB
I-R drops. However, LTC recommends that the SENSE pin
terminate in close proximity to the LT3070’s OUT pins.
This minimizes parasitic inductance and optimizes regula-
tion. The LT3070 handles moderate levels of output line
impedance, but excessive impedance between V
C
and adversely affects stability.
Figure 1 in the Pin Functions section illustrates the Kelvin-
Sense connection method that eliminates voltage drops
due to PCB trace resistance. However, note that the voltage
drop across the external PCB traces adds to the dropout
voltage of the regulator. The SENSE pin input bias current
depends on the selected output voltage. SENSE pin input
current varies from 50µA typically at V
typically at V
Short-Circuit and Overload Recovery
Like many IC power regulators, the LT3070 has safe op-
erating area (SOA) protection. The safe area protection
decreases current limit as input-to-output voltage increases
and keeps the power transistor inside a safe operating
region for all values of input-to-output voltage up to the
absolute maximum voltage rating. V
the UVLO threshold for any function. The LT3070 has a
precision current limit specified at ±15% that is active if
V
Under conditions of maximum I
V
3W. If ambient temperature is high enough, die junction
temperature will exceed the 125°C maximum operating
temperature. If this occurs, the LT3070 relies on two
additional thermal safety features. At about 145°C, the
PWRGD output pulls low providing an early warning of an
impending thermal shutdown condition. At 165°C typically,
the LT3070’s thermal shutdown engages and the output is
shut down until the IC temperature falls below the thermal
hysteresis limit. The SOA protection decreases current limit
as the IN-to-OUT voltage increases and keeps the power
dissipation at safe levels for all values of input-to-output
voltage. The LT3070 provides some output current at all
OUT
BIAS
IN
-V
causes excessive phase shift in the feedback loop
is above UVLO.
OUT
the device’s power dissipation peaks at about
OUT
= 1.8V.
LOAD
BIAS
OUT
and maximum
= 0.8V to 300µA
must be above
OUT
, allowing
OUT
and
values of input-to-output voltage up to the absolute maxi-
mum voltage rating. See the Current Limit vs V
the Typical Performance Characteristics.
During start-up, after the BIAS voltage has cleared its UVLO
threshold and V
at the rate of current limit charging C
With a high input voltage, a problem can occur where the
removal of an output short will not allow the output volt-
age to recover. Other regulators with current limit foldback
also exhibit this phenomenon, so it is not unique to the
LT3070. The load line for such a load may intersect the
output current curve at two points: normal operation and
the SOA restricted load current settings. A common situ-
ation is immediately after the removal of a short circuit,
but with a static load ≥ 1A. In this situation, removal of the
load or reduction of I
and allow V
Reverse Voltage
The LT3070 incorporates a circuit that detects if V
creases below V
a typical threshold of about (V
threshold is exceeded, this detector circuit turns off the
drive to the internal NMOS pass transistor, thereby turning
off the output. The output pulls low with the load current
discharging the output capacitance. This circuit’s intent
is to limit and prevent back-feed current from OUT to IN
if the input voltage collapses due to a fault or overload
condition.
Thermal Considerations
The LT3070’s maximum rated junction temperature of
125°C limits its power handling capability and is domi-
nated by the output current multiplied by the input/output
voltage differential:
I
The LT3070’s internal power and thermal limiting circuitry
protect it under overload conditions. For continuous nor-
mal load conditions, do not exceed the maximum junction
temperature of 125°C. Give careful consideration to all
sources of thermal resistance from junction to ambient.
This includes junction to case, case-to-heat sink interface,
OUT
• (V
IN
OUT
– V
to return to normal regulation.
IN
OUT
OUT
is increasing, output voltage increases
. This reverse-voltage detector has
)
OUT
to <1A will clear this condition
IN
– V
OUT
OUT
) = –6mV. If the
.
LT3070
IN
curve in

IN
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