NCV4276
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4
Setting the Output Voltage (Adjustable Version)
The output voltage range of the adjustable version can be
set between 2.5 V and 20 V (Figure 4). This is
accomplished with an external resistor divider feeding
back the voltage to the IC back to the error amplifier by the
voltage adjust pin VA. The internal reference voltage is set
to a temperature stable reference voltage of 2.5 V.
The output voltage is calculated from the following
formula. Ignoring the bias current into the VA pin,
Q[(R1 R2) Vref ]R2
Use R2 < 50 k to avoid significant output voltage errors
due to VA bias current.
Connecting VA directly to Q without R1 and R2 creates
an output voltage of 2.5 V.
Designers should consider the tolerance of R1 and R2
during the design phase.
The input voltage range for operation (pin I) of the
adjustable version is between (Q + 0.5 V) and 40 V. Internal
bias requirement s dictate a mini mum input vol tage of 4.3 V.
The dropout voltage for output voltages less than 4 V is
(4.3 V - Q).
Calculating Power Dissipation
in a Single Output Linear Regulator
The maximum power dissipation for a single output
regulator (Figure 3) 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,
IQ is the quiescent current the regulator
consumes at IOUT(max).
Once the value of PD(max) is known, the maximum
permissible value of RΘJA can be calculated:
RJA 150°CTA
PD(2)
The value of RΘJA can then be compared with those in
the package section of the data sheet. Those packages with
RΘJA’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 3. 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 RΘJA:
RJA RJCRCS RSA (3)
where
RΘJC is the junction-to-case thermal resistance,
RΘCS is the case-to-heatsink thermal resistance,
RΘSA is the heatsink-to-ambient thermal
resistance.
RΘJC appears in the package section of the data sheet.
Like RΘJA, it too is a function of package type. RΘCS and
RΘSA are functions of the package type, heatsink and the
interface between them. These values appear in heat sink
data sheets of heat sink manufacturers.