NCV8505 Series
http://onsemi.com
10
APPLICATION NOTES
SETTING THE DELAY TIME
The delay time is controlled by the Reset Delay Low
Voltage, Delay Switching Threshold, and the Delay Charge
Current. The delay follows the equation:
tDELAY +ƪCDELAY(Vdt *Reset Delay Low Voltage)ƫ
Delay Charge Current
Example:
Using CDELAY = 33 nF.
Assume reset Delay Low Voltage = 0.
Use the typical value for Vdt = 1.8 V (2.5 V, 3.3 V, and
5.0 V options).
Use the typical value for Delay Charge Current = 4.2 mA.
tDELAY +ƪ33 nF(1.8 *0)ƫ
4.2 mA+14 ms
STABILITY CONSIDERATIONS
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: start−up
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
manufacturers data sheet usually provides this information.
The value for the output capacitor COUT shown in Figure 18
should work for most applications, however it is not
necessarily the optimized solution.
Figure 18. Test and Application Circuit Showing
Output Compensation
VIN VOUT
COUT**
33 mF
RRST
RESET
CIN*
0.1 mFNCV8505
*CIN required if regulator is located far from the power supply
filter.
**COUT required for stability. Capacitor must operate at minimum
temperature expected.
CALCULATING POWER DISSIPATION IN A
SINGLE OUTPUT LINEAR REGULATOR
The maximum power dissipation for a single output
regulator (Figure 19) is:
PD(max) +[VIN(max) *VOUT(min)]IOUT(max) (1)
)VIN(max)IQ
where:
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
IOUT(max) is the maximum output current for the
application, and
IQ is the quiescent current the regulator consumes at
IOUT(max).
Once the value of PD(max) is known, the maximum
permissible value of RqJA can be calculated:
RqJA +150oC*TA
PD(2)
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.
SMART
REGULATOR®
IQ
Control
Features
IOUT
IIN
Figure 19. Single Output Regulator with Key
Performance Parameters Labeled
VIN VOUT
}
HEAT SINKS
A heat sink 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 (3)
where:
RqJC = the junction−to−case thermal resistance,
RqCS = the case−to−heatsink thermal resistance, and
RqSA = the heatsink−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 heat sink data sheets
of heat sink manufacturers.