NCV8501 Series
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12
UNDERSTANDING THE NCV8501 ENABLE PIN
INPUT CURRENT
Figure 21. NCV8501 Enable Function Equivalent
Circuit
VCC
GND
ENABLE
5μA
20K
1.2M
11V 7V
~3.85V
Z1 Z2
R1
R2
D1
P1
N1
1.25VZ3
D2
D3
D4
D5
Internal
power
rail
D6
Internal
reference
(max)
Z1, R1, and Z2 provide ESD and overvoltage protection.
Note that, for ENABLE pin voltages in excess of 10 V, an
external series resistor is required to limit the current into
Z1.
For ENABLE pin voltages less than +7 V, the 5 mA
(maximum value) current source dominates the input
current, as the opposing P1 base current is negligible by
comparison.
For ENABLE pin voltages between +7 V and +11 V, the
input current is given by:
5 mA + ((V
ENABLE
− 7) / 20 kW)
For ENABLE pin voltages in excess of 10 V (Z1
breakover voltage can be as low as 10 V), the input current
is dominated by the external series resistor. For the case
where V
ENABLE
= 12 V; R
EXT
= 10 kW, the input current can
be up to (2 V/10 kW), = 200 mA.
The ENABLE threshold is that voltage required to
achieve ~3.85 V at the base of N1, or approximately (3.85 V
− 2 Vbe). At +20°C, this threshold is ~2.55 V. At −40°C, it
can be as high as 3 V.
If the value of R
EXT
is increased to ~200 kW, to reduce
ENABLE input current, then the worst−case drop across
R
EXT
must be added to 3 V to determine the effective
maximum ENABLE threshold. At V
ENABLE
< 7 V, we only
need to consider the 5 mA current sink.
Max effective threshold = 3 V + (5 mA * 220 kW)
= 3 V + 1.1 V
= 4.1 V
CALCULATING POWER DISSIPATION IN A
SINGLE OUTPUT LINEAR REGULATOR
The maximum power dissipation for a single output
regulator (Figure 22) is:
P
D(max)
+ [V
IN(max)
* V
OUT(min)
]I
OUT(max)
) V
IN(max)
I
Q
(eq. 1)
where:
V
IN(max)
is the maximum input voltage,
V
OUT(min)
is the minimum output voltage,
I
OUT(max)
is the maximum output current for the
application, and
I
Q
is the quiescent current the regulator consumes at
I
OUT(max)
.
Once the value of P
D(max)
is known, the maximum
permissible value of R
q
JA
can be calculated:
R
QJA
+
150
C *
T
A
P
D
(eq. 2)
The value of R
q
JA
can then be compared with those in the
package section of the data sheet. Those packages with
R
q
JA
’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®
I
Q
Control
Features
I
OUT
I
IN
Figure 22. Single Output Regulator with Key
Performance Parameters Labeled
V
IN
V
OUT
}
Figure 23. 16 Lead SOW (Exposed Pad), qJA as a
Function of the Pad Copper Area (2 oz. Cu
Thickness), Board Material = 0.0625, G−10/R−4
40
70
90
100
Thermal Resistance,
Junction to Ambient, R
q
JA
, (°C/W)
0
Copper Area (mm
2
)
200 400 800
80
60
50
600
