LM2574HVM-12 National Semiconductor, LM2574HVM-12 Datasheet - Page 19

• Manufacturer Part Number: LM2574HVM-12
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,BIPOLAR,SOP,14PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2574HVM-12

• Rohs Compliant: NO

Application Hints
The data sheet thermal resistance curves and the thermal
model in Switchers Made Simple software (version 3.3)
can estimate the maximum junction temperature based on
operating conditions. ln addition, the junction temperature
can be estimated in actual circuit operation by using the fol-
lowing equation.
T
With the switcher operating under worst case conditions and
all other components on the board in the intended enclosure,
measure the copper temperature (T
be done by temporarily soldering a small thermocouple to
the pc board copper near the IC, or by holding a small ther-
mocouple on the pc board copper using thermal grease for
good thermal conduction.
The thermal resistance (
Note: Pin numbers are for the 8-pin DIP package.
For an input voltage of 8V or more, the maximum available
output current in this configuration is approximately 100 mA.
At lighter loads, the minimum input voltage required drops to
approximately 4.7V.
The switch currents in this buck-boost configuration are
higher than in the standard buck-mode design, thus lowering
the available output current. Also, the start-up input current
of the buck-boost converter is higher than the standard buck-
mode regulator, and this may overload an input power
source with a current limit less than 0.6A. Using a delayed
turn-on or an undervoltage lockout circuit (described in the
next section) would allow the input voltage to rise to a high
enough level before the switcher would be allowed to turn
on.
Because of the structural differences between the buck and
the buck-boost regulator topologies, the buck regulator de-
sign procedure section can not be used to to select the in-
ductor or the output capacitor. The recommended range of
inductor values for the buck-boost design is between 68 µH
and 220 µH, and the output capacitor values must be larger
than what is normally required for buck designs. Low input
voltages or high output currents require a large value output
capacitor (in the thousands of micro Farads).
The peak inductor current, which is the same as the peak
switch current, can be calculated from the following formula:
j-cu
j-cu
j
= T
= 42˚C/W for the N-8 package
= 52˚C/W for the M-14 package
cu
+ (
j-cu
x P
D
)
j-cu
) for the two packages is:
(Continued)
cu
FIGURE 11. Inverting Buck-Boost Develops −12V
) near the IC. This can
19
The power dissipation (P
it can be estimated by using the formula:
Where I
(adjustable version use the supply current vs. duty cycle
curve).
Additional Applications
INVERTING REGULATOR
Figure 11 shows a LM2574-12 in a buck-boost configuration
to generate a negative 12V output from a positive input volt-
age. This circuit bootstraps the regulator’s ground pin to the
negative output voltage, then by grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it to −12V.
Where f
rent operating conditions, the minimum V
worst case. Select an inductor that is rated for the peak cur-
rent anticipated.
Also, the maximum voltage appearing across the regulator is
the absolute sum of the input and output voltage. For a −12V
output, the maximum input voltage for the LM2574 is +28V,
or +48V for the LM2574HV.
The Switchers Made Simple version 3.3) design software
can be used to determine the feasibility of regulator designs
using different topologies, different input-output parameters,
different components, etc.
NEGATIVE BOOST REGULATOR
Another variation on the buck-boost topology is the negative
boost configuration. The circuit in Figure 12 accepts an input
voltage ranging from −5V to −12V and provides a regulated
−12V output. Input voltages greater than −12V will cause the
output to rise above −12V, but will not damage the regulator.
osc
S
is obtained from the typical supply current curve
= 52 kHz. Under normal continuous inductor cur-
D
) for the IC could be measured, or
DS011394-19
IN
represents the
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