NCV4269A
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12
SENSE INPUT (SI) / SENSE OUTPUT (SO) VOLTAGE
MONITOR
An on−chip comparator is available to provide early
warning to the microprocessor of a possible reset signal
(Figure 4). The output is from an open collector driver with
an internal 20 kW pull up resistor to output Q. The reset signal
typically turns the microprocessor off instantaneously. This
can cause unpredictable results with the microprocessor. The
signal received from the SO pin will allow the microprocessor
time to complete its present task before shutting down. This
function is performed by a comparator referenced to the band
gap voltage. The actual trip point can be programmed
externally using a resistor divider to the input monitor SI
(Figure 21). The values for RSI1 and RSI2 are selected for a
typical threshold of 1.20 V on the SI Pin.
SIGNAL OUTPUT
Figure 22 shows the SO Monitor timing waveforms as a
result of the circuit depicted in Figure 21. As the output
voltage (VQ) falls, the monitor threshold (VSI,Low), is
crossed. This causes the voltage on the SO output to go low
sending a warning signal to the microprocessor that a reset
signal may occur in a short period of time. TWA RNIN G is the
time the microprocessor has to complete the function it is
currently working on and get ready for the reset
shutdown signal. When the voltage on the SO goes low and
the RO stays high the current consumption is typically
560 mA at 1 mA load current.
Figure 22. SO Warning Waveform Time Diagram
VQ
SI
VRO
VSI,Low
TWARNING
SO
STABILITY CONSIDERATIONS
The input capacitor CI in Figure 21 is necessary for
compensating input line reactance. Possible oscillations caused
by input inductance and input capacitance can be damped by
using a resistor of approximately 1.0 W in series with CI.
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: startup delay,
load transient response and loop stability.
The capacitor value and type should be based on cost,
availability, size and temperature constraints. The
aluminum electrolytic capacitor is the least expensive
solution, but, if the circuit operates at low temperatures
(−25°C to −40°C), both the value and ESR of the capacitor
will vary considerably. The capacitor manufacturer’s data
sheet usually provides this information.
The 10 mF output capacitor CQ shown in Figure 21 should
work for most applications; however, it is not necessarily the
optimized solution. Stability is guaranteed at CQ is min
2.2 mF and max ESR is 10 W. There is no min ESR limit
which was proved with MURATA’s ceramic caps
GRM31MR71A225KA01 (2.2 mF, 10 V, X7R, 1206) and
GRM31CR71A106KA01 (10 mF, 10 V, X7R, 1206) directly
soldered between output and ground pins.
CALCULATING POWER DISSIPATION IN A SINGLE
OUTPUT LINEAR REGULATOR
The maximum power dissipation for a single output
regulator (Figure 21) is:
PD(max) +[VI(max) *VQ(min)]I
Q(max) )VI(max)Iq(eq. 4)
where:
VI(max) is the maximum input voltage,
VQ(min) is the minimum output voltage,
IQ(max) is the maximum output current for the application,
and Iq is the quiescent current the regulator consumes at
IQ(max).
Once the value of PD(max) is known, the maximum
permissible value of RqJA can be calculated:
(eq. 5)
RqJA = (150°C – TA) / PD
The value of RqJA can then be compared with those in the
package section of the data sheet. Those packages with
RqJA
’s less than the calculated value in equation 2 will keep
the die temperature below 150°C. In some cases, none of the
packages will be sufficient to dissipate the heat generated by
the IC, and an external heatsink will be required. The current
flow and voltages are shown in the
Measurement Circuit Diagram.
HEATSINKS
A heatsink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air.
Each material in the heat flow path between the IC and the
outside environment will have a thermal resistance. Like
series electrical resistances, these resistances are summed to
determine the value of RqJA:
RqJA +RqJC )RqCS )RqSA (eq. 6)
where:
RqJC = the junction−to−case thermal resistance,
RqCS = the case−to−heat sink thermal resistance, and
RqSA = the heat sink−to−ambient thermal resistance.
RqJC appears in the package section of the data sheet. Like
RqJA, it too is a function of package type. RqCS and RqSA are
functions of the package type, heatsink and the interface
between them. These values appear in data sheets of
heatsink manufacturers. Thermal, mounting, and
heatsinking considerations are discussed in the
ON Semiconductor application note AN1040/D, available
on the ON Semiconductor website.