LP2957A_15 Datasheet, PDF (11/24 Pages) Texas Instruments – LP2957/LP2957A 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
www.ti.com
SNVS102C – JUNE 1998 – REVISED APRIL 2013
Table 2. (From Thermalloy)
Thermasil III
1.3
Thermasil II
1.5
Thermalfilm (0.002) with Grease
2.2
θ_(HA) is the heatsink-to-ambient thermal resistance. It is this specification (listed on the heatsink manufacturers
data sheet) which defines the effectiveness of the heatsink. The heatsink selected must have a thermal
resistance which is equal to or lower than the value of θ_(HA)calculated from the above listed formula.
ERROR COMPARATOR
This comparator produces a logic “LOW” whenever the output falls out of regulation by more than about 5%. This
figure results from the comparator's built-in offset of 60 mV divided by the 1.23V reference. An out-of-regulation
condition can result from low input voltage, current limiting, or thermal limiting.
Figure 31 gives a timing diagram showing the relationship between the output voltage, the ERROR output, and
input voltage as the input voltage is ramped up and down to the regulator without snap-on/snap-off output.
The ERROR signal becomes low at about 1.3V input. It goes high at about 5V input, where the output equals
4.75V. Since the dropout voltage is load dependent, the input voltage trip points will vary with load current. The
output voltage trip point does not vary.
The comparator has an open-collector output which requires an external pull-up resistor. This resistor may be
connected to the regulator output or some other supply voltage. Using the regulator output prevents an invalid
“HIGH” on the comparator output which occurs if it is pulled up to an external voltage while the regulator input
voltage is reduced below 1.3V. In selecting a value for the pull-up resistor, note that while the output can sink
400 µA, this current adds to battery drain. Suggested values range from 100k to 1 MΩ. The resistor is not
required if the output is unused.
*In shutdown mode, ERROR will go high if it has been pulled up to an external supply. To avoid this invalid response,
pull up to regulator output.
**Exact value depends on dropout voltage, which varies with load current.
Figure 31. ERROR Output Timing
If a single pull-up resistor is connected to the regulator output, the error flag may briefly rise up to about 1.3V as
the input voltage ramps up or down through the 0V to 1.3V region.
In some cases, this 1.3V signal may be mis-interpreted as a false high by a µP which is still “alive” with 1.3V
applied to it.
To prevent this, the user may elect to use two resistors which are equal in value on the error output (one
connected to ground and the other connected to the regulator output).
If this two-resistor divider is used, the error output will only be pulled up to about 0.6V (not 1.3V) during power-up
or power-down, so it can not be interpreted as a high signal. When the regulator output is in regulation (4.8V to
5V), the error output voltage will be 2.4V to 2.5V, which is clearly a high signal.
OUTPUT ISOLATION
The regulator output can be connected to an active voltage source (such as a battery) with the regulator input
turned off, as long as the regulator ground pin is connected to ground. If the ground pin is left floating,
damage to the regulator can occur if the output is pulled up by an external voltage source.
Copyright © 1998–2013, Texas Instruments Incorporated
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