LM2614ATLX NSC [National Semiconductor], LM2614ATLX Datasheet - Page 13

• Manufacturer Part Number: LM2614ATLX
• Description: 400mA Sub-Miniature Adjustable DC-DC Converter Optimized for RF Power Amplifiers
• Manufacturer: NSC [National Semiconductor]

Application Information
SETTING THE OUTPUT VOLTAGE
The LM2614 can be used with external feedback resistors
and an analog signal to vary the output voltage. Select an
output voltage from 1.0V to 3.6V by setting the voltage on
the VCON as directed in Table 1.
Refer to Figure 12 for the relation between VOUT and
VCON.
When the control voltage is between 1.85V and 0V, the
output voltage will vary in a monotonic fashion with respect
For low-cost applications, an unshielded inductor is sug-
gested. For noise critical applications, a toroidal or shielded
inductor should be used. A good practice is to lay out the
board with footprints accommodating both types for design
flexibility. This allows substitution of a low-noise shielded
inductor, in the event that noise from low-cost unshielded
models is unacceptable.
The saturation current rating is the current level beyond
which an inductor loses its inductance. Different manufactur-
ers specify the saturation current rating differently. Some
specify saturation current point to be when inductor value
falls 30% from its original value, others specify 10%. It is
VCON
VCON = 0V
VCON (V)
DO1608C-103
P1174.103T
ELL6RH100M
CDRH5D18-100
P0770.103T
Part Number
TABLE 1. Output Voltage Selection
>
0V
(Circuit in Figure 4)
VOUT vs VCON
FIGURE 12.
V
FB
(1+R1/R2)−VCON (R1/R2)
V
FB
VOUT (V)
TABLE 2. Suggested Inductors and Their Suppliers
(1+R1/R2)
Coilcraft
Pulse
Panasonic
Sumida
Pulse
Vendor
20036721
13
to the voltage on the control pin as per the equation in Table
1. Select the value of R2 to allow at least 100 times the
feedback pin bias current to flow through it.
EXTERNAL COMPENSATION
The LM2614 uses external components connected to the
EANEG and EAOUT pins to compensate the regulator (Fig-
ure 4). Typically, all that is required is a series connection of
one capacitor (C4) and one resistor (R3). A capacitor (C5)
can be connected across the EANEG and EAOUT pins to
improve the noise immunity of the loop. C5 reacts with R3 to
give a high frequency pole. C4 reacts with the high open loop
gain of the error amplifier and the resistance at the EANEG
pin to create the dominant pole for the system, while R3 and
C4 react to create a zero in the frequency response. The
pole rolls off the loop gain, to give a bandwidth somewhere
between 10kHz and 50kHz, this avoids a 100kHz parasitic
pole contributed by the current mode controller. Typical val-
ues in the 220pF to 1nF (C4) range are recommended to
create a pole on the order of 10Hz or less.
The next dominant pole in the system is formed by the output
capacitance (C2) and the parallel combination of the load
resistance and the effective output resistance of the regula-
tor. This combined resistance (Ro) is dominated by the small
signal output resistance, which is typically in the range of 3Ω
to 15Ω. The exact value of this resistance, and therefore this
load pole depends on the steady state duty cycle and the
internal ramp value. Ideally we want the zero formed by R3
and C4 to cancel this load pole, such that R3=RoC2/C4. Due
to the large variation in Ro, this ideal case can only be
achieved at one operating condition. Therefore a compro-
mise of about 5Ω for Ro should be used to determine a
starting value for R3. This value can then be optimized on
the bench to give the best transient response to load
changes and changes in VCON, under all conditions. Typical
values are 10pF for C5 and 220pF to 470pF for C4, to
ensure good response from dropout conditions to V
(min).
INDUCTOR SELECTION
Use a 10µH inductor with saturation current rating higher
than the peak current rating of the device. The inductor’s
resistance should be less than 0.3Ω for good efficiency.
Table 2 lists suggested inductors and suppliers.
always better to look at the inductance versus current curve
and make sure the inductor value doesn’t fall below 30% at
the peak current rating of the LM2614. Beyond this rating,
the inductor loses its ability to limit current through the PWM
switch to a ramp. This can cause poor efficiency, regulation
errors or stress to DC-DC converters like the LM2614. Satu-
ration occurs when the magnetic flux density from current
through the windings of the inductor exceeds what the in-
ductor’s core material can support with a corresponding
magnetic field.
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