LP2954 Datasheet, PDF (8/13 Pages) National Semiconductor (TI) – 5V and Adjustable Micropower Low-Dropout Voltage Regulators

Application Hints (Continued)
the value of R2 in cases where the regulator must work with
no load (see MINIMUM LOAD ). I_FB will produce a typical 2%
error in V_OUT which can be eliminated at room temperature
by trimming R1. For better accuracy, choosing R2 = 100 kΩ
will reduce this error to 0.17% while increasing the resistor
program current to 12 µA. Since the typical quiescent current
is 120 µA, this added current is negligible.
*See External Capacitors
P_Total = (V_IN −5) I_L+ (V_IN) I_G
FIGURE 1. Basic 5V Regulator Circuit
DS011128-5
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T_R(max). This is calculated
by using the formula:
T_R(max) = T_J(max) − T_A(max)
where: T_J(max) is the maximum allowable junction
temperature
T_A(max) is the maximum ambient temperature
Using the calculated values for T_R(max) and P(max), the re-
quired value for junction-to-ambient thermal resistance,
θ_(J-A), can now be found:
θ_(J-A) = T_R(max)/P(max)
If the calculated value is 60˚ C/W or higher , the regulator
may be operated without an external heatsink. If the calcu-
lated value is below 60˚ C/W, an external heatsink is re-
quired. The required thermal resistance for this heatsink can
be calculated using the formula:
θ_(H-A) = θ_(J-A) − θ_(J-C) − θ_(C-H)
where:
θ_(J-C) is the junction-to-case thermal resistance, which is
specified as 3˚ C/W maximum for the LP2954.
θ_(C-H) is the case-to-heatsink thermal resistance, which is
dependent on the interfacing material (if used). For details
and typical values, refer to (Note 2) listed at the end of the
ELECTRICAL CHARACTERISTICS section.
θ_(H-A) 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_(H-A) calculated from the
above listed formula.
PROGRAMMING THE OUTPUT VOLTAGE
The regulator may be pin-strapped for 5V operation using its
internal resistive divider by tying the Output and Sense pins
together and also tying the Feedback and 5V Tap pins to-
gether.
Alternatively, it may be programmed for any voltage between
the 1.23V reference and the 30V maximum rating using an
external pair of resistors (see Figure 2). The complete equa-
tion for the output voltage is:
where V_REF is the 1.23V reference and I_FB is the Feedback
pin bias current (−20 nA typical). The minimum recom-
mended load current of 1 µA sets an upper limit of 1.2 MΩ on
DS011128-36
* See Application Hints
** Drive with TTL-low to shut down
FIGURE 2. Adjustable Regulator
DROPOUT DETECTION COMPARATOR
This comparator produces a logic “LOW” whenever the out-
put 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 (refer to block diagrams on page 1).
The 5% low trip level remains constant regardless of the pro-
grammed output voltage. An out-of-regulation condition can
result from low input voltage, current limiting, or thermal lim-
iting.
Figure 3 gives a timing diagram showing the relationship be-
tween the output voltage, the ERROR output, and input volt-
age as the input voltage is ramped up and down to a regula-
tor programmed for 5V 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 com-
parator 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 100 kΩ to 1 MΩ. This
resistor is not required if the output is unused.
When V_IN ≤ 1.3V, the error flag pin becomes a high imped-
ance, allowing the error flag voltage to rise to its pull-up volt-
age. Using V_OUT as the pull-up voltage (rather than an exter-
nal 5V source) will keep the error flag voltage below 1.2V
(typical) in this condition. The user may wish to divide down
the error flag voltage using equal-value resistors (10 kΩ sug-
gested) to ensure a low-level logic signal during any fault
condition, while still allowing a valid high logic level during
normal operation.
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