LM2619ATLX National Semiconductor, LM2619ATLX Datasheet - Page 12

• Manufacturer Part Number: LM2619ATLX
• Description: Conv DC-DC Single Step Down 2.8V to 5.5V 10-Pin uSMD T/R
• Manufacturer: National Semiconductor
• Type: Step Downr

Specifications of LM2619ATLX

• Package: 10uSMD
• Number Of Outputs: 1
• Minimum Input Voltage: 2.8 V
• Maximum Input Voltage: 5.5 V
• Switching Frequency: 500 to 1000 KHz
• Operating Supply Voltage: 2.8 to 5.5 V
• Maximum Output Current: 0.5 A
• Output Type: Adjustable
• Output Voltage: 1.5 to 3.6 V
• Switching Regulator: Yes
• Efficiency: 96(Typ) %
• Operating Temperature: -25 to 125 °C

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Application Information
SETTING THE OUTPUT VOLTAGE
The LM2619 can be used with external feedback resistors to
set the output voltage.Select the value of R2 to allow atleast
100 times the feedback pin bias current to flow through it.
V
EXTERNAL COMPENSATION
The LM2619 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 3
to 15 . 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 5
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
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 LM2619. 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 LM2619. Satu-
ration occurs when the magnetic flux density from current
OUT
= V
FB
DO1608C-103
ELL6SH100M
ELL6RH100M
CDRH5D18-100
P0770.103T
Part Number
(1+R1/R2)
for Ro should be used to determine a
TABLE 1. Suggested Inductors and Their Suppliers
Coilcraft
Panasonic
Panasonic
Sumida
Pulse
Vendor
12
starting value for R3. This value can then be optimized on
the bench to give the best transient response to load
changes, under all conditions. Typical values are 10pF for
C5, 220pF to 1nF for C4 and 22K to 100K for R3.
A
R
M
D = VOUT/VIN , D’ = 1-D , duty cycle
M
during PFET on time
R
of error amp
R
where R
power stage
f
f
C2
f
control
f
C5
f
f
where f
equation for crossover frequency assumes that f
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 0.3
Table 1 lists suggested inductors and suppliers.
through the windings of the inductor exceeds what the in-
ductor’s core material can support with a corresponding
magnetic field.
CAPACITOR SELECTION
Use a 10µF ceramic input capacitor. Use X7R or X5R types,
do not use Y5V.
Use of tantalum capacitors is not recommended.
Ceramic capacitors provide an optimal balance between
small size, cost, reliability and performance for cell phones
and similar applications. A 22µF ceramic output capacitor is
recomended for applications that require increased toler-
ance to heavy load transients. A 10µF ceramic output ca-
pacitor can be used in applications where the worst case
load transient step is less than 200mA. Use of a 10µF output
capacitor trades off smaller size for an increase in output
voltage ripple, and undershoot during load transients. Table
2 lists suggested capacitors and suppliers.
P1
P2
P3
P4
Z1
X
O
f
p
o
c
1
= R2/(R1+ R2)
= ( • (R
= 1 , Transresistance of output stage
= (R1 i R2) + 5k
= (F • L1) / (D’ • (M
= 1/( 2 •
= R
= 1/(2 •
= 1/(2 •
= 20000 , Open loop gain of error amplifier
= 362000 A/s , Corrective ramp slope
= 1/(2 •
= (VIN - VOUT)/L1 , slope of current through inductor
847-639-6400
714-373-7366
714-373-7366
847-956-0666
858-674-8100
o
/ (2 •
X
Phone
o
gives the approximate crossover frequency.This
is the effective small signal output resistance of
o
• R3 • C5) , high frequency pole due to R3 and
i Rload)/R
• (Rload i R
• A
• R3 • C4) , zero due to R3 and C4
• L1) , high frequency pole from current mode
O
• R
, effective resistance at inverting input
p
c
f
/M
)/(2 •
• C4) , low frequency pole
o
) • C2) , pole due to Rload,Ro and
1
)+
1
⁄
2
• R
- D)
p
847-639-1469
714-373-7323
714-373-7323
847-956-0702
858-674-8262
• C4)
for good efficiency.
FAX
P2
= f
Z1
.