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NCV4279D1

5.0 V Micropower 150 mA LDO Linear Regulator with DELAY, Adjustable RESET, and Sense Output

ON Semiconductor 

NCV4279

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. The

output is from an open collector driver. 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 18). The values for RSI1 and RSI2

are selected for a typical threshold of 1.20 V on the SI Pin.

SIGNAL OUTPUT

Figure 19 shows the SO Monitor timing waveforms as a

result of the circuit depicted in Figure 18. As the output

voltage (VQ) falls, the monitor threshold (VSILOW), 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. TWARNING is the

time the microprocessor has to complete the function it is

currently working on and get ready for the reset

shutdown signal.

VQ

SI

VSILOW

VRO

SO

TWARNING

Figure 19. SO Warning Waveform Time Diagram

STABILITY CONSIDERATIONS

The input capacitor CI in Figure 18 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. A tantalum or

aluminum electrolytic capacitor is best, since a film or

ceramic capacitor with almost zero ESR can cause

instability. 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 value for the output capacitor CQ shown in Figure 18

should work for most applications; however, it is not

necessarily the optimized solution. Stability is guaranteed at

values CQ = 10 mF and an ESR = 10 W within the operating

temperature range. Actual limits are shown in a graph in the

typical data section.

CALCULATING POWER DISSIPATION IN A SINGLE

OUTPUT LINEAR REGULATOR

The maximum power dissipation for a single output

regulator (Figure 18) is:

PD(max) + [VI(max) * VQ(min)] IQ(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:

RqJA = (150°C – TA) / PD

(eq. 5)

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.

http://onsemi.com

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