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LT3048IDC-3.3#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT3048 Series
10
3048fa
For more information www.linear.com/LT3048
OPERATION
APPLICATIONS INFORMATION
The LT3048 combines a boost regulator with a low drop-
out linear regulator to produce a programmable output
from
a lower input voltage. The LT3048-15, LT3048-12,
LT3048-5, LT3048-3.3 regulate to 15V, 12V, 5V and 3.3V,
respectively. With EN below 0.4V, all circuits are turned
off, Q2 disconnects the load from the input, and current
consumption is less than 1µA. Driving EN high enables the
oscillator and all bias circuits including voltage reference
and regulation amplifiers.
The boost regulator uses peak current mode operation,
providing cycle-by-cycle current regulation and limiting,
fast transient response, and good stability. The error am
-
plifier, A1, regulates BSTOUT (and LDOIN) to 1.1V above
LDOOUT or V
IN
, whichever is higher.
Regulating 1.1V between LDOIN and LDOOUT allows the
linear regulator to provide good transient response and
ripple rejection, while maintaining good efficiency. This
regulation loop also minimizes total power dissipation
in fault conditions. If the output is overloaded, the linear
regulator will enter current limit, LDOOUT voltage will
decrease. By reducing the boost regulator output voltage,
the voltage across Q2 will be reduced, limiting dissipation
in Q2. Thermal shutdown provides additional protection.
Capacitor Selection
A 4.7μF ceramic capacitor should be sufficient
for the
boost
regulator output bypass. Low ESR (equivalent
series resistance) ceramic capacitors should be used
at the outputs of the regulator to minimize the output
ripple voltage. Use only X5R or X7R dielectrics, as these
materials retain their capacitance over wider voltage and
temperature ranges better than other dielectrics.
Table 1. Ceramic Capacitor Manufacturers
MANUFACTURER PHONE WEB
Taiyo Yuden (408) 573-4150 www.t-yuden.com
AVX (803) 448-9411 www.avxcorp.com
Murata (714) 852-2001 www.murata.com
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and FB pin. Choose the 1% resistors
according to:
R1= R2
V
OUT
1.235V
− 1
R2 should be 200k or less to avoid bias current errors.
Inductor Selection
A 5.6μH inductor will suffice for most LT3048 applications.
See Table 2 for recommended inductor values. Smaller
value inductors can be used at the cost of reduced maximum
load current. Figure 2 shows the maximum load current as
a function of output voltage. The inductor’s RMS current
rating must be greater than the maximum input current.
To keep efficiency high, the DCR (series resistance) should
be minimized. Table 3 lists several vendors.
Table 2. Recommended Inductor Values
PART # INDUCTANCE
LT3048 5.6μH
LT3048-15 10μH
LT3048-12 10μH
LT3048-5 4.7μH
LT3048-3.3 3.3μH
Table 3. Inductor Manufacturers
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.toko.com
Würth Elektronik www.we-online.com
LT3048 Series
11
3048fa
For more information www.linear.com/LT3048
APPLICATIONS INFORMATION
Figure 3. Recommended PCB Layout
Reducing Output Noise With A Bypass Capacitor
The LT3048 relies on the power supply rejection of the
linear regulator to reduce switching regulator noise at
LDOOUT. The linear regulator also contributes thermal
noise to the output. The thermal noise can be reduced,
and transient response improved, by adding a capacitor
between LDOOUT and BYP. A typical value is 1nF. This
capacitor increases start-up time of the regulator.
Recommended PCB Layout
Figure 3 shows the recommended layout for LT3048 cir
-
cuits. Most important is careful placement of the BSTOUT
bypass
capacitor C2. High frequency AC current flows in a
loop formed by C2, internal power transistor Q1 and boost
diode D2. Keep this loop small. Also be sure to place an
unbroken ground plane below this loop, on the highest
copper layer below the surface. This prevents the AC loop
from coupling to LDOOUT and other nearby circuitry. Keep
the SW node as small as possible.
LDO Stability and Output Capacitance
The LT3048 linear regulator requires an output capacitor
for stability. It is designed to be stable with most low
ESR capacitors (typically ceramic, tantalum or low ESR
electrolytic). A minimum output capacitor of 1μF
with an
ESR of 1Ω or less is recommended to prevent oscilla-
tions. Larger
values of output capacitance decrease peak
C2
C4
C3
3048 F03
C1
L1
Figure 2. LT3048 Typical and Guaranteed Load
Current vs. Output Voltage
LDOOUT VOLTAGE (V)
0
LOAD CURRENT (mA)
45
35
25
15
40
30
20
10
0
5
8 164 12 20
3048 F02
226 142 10 18
V
IN
= 2.7V
V
IN
= 3.3V
V
IN
= 3.6V
GUARANTEED
L = 5.6µH
DCR = 120mΩ
deviations and provide improved transient response for
larger load current changes. Bypass capacitors, used to
decouple individual components powered by the LT3048,
increase the effective output capacitor value.
Give consideration to the use of ceramic capacitors as
they are manufactured with a variety of dielectrics, each
with different behavior across temperature and applied
voltage. The most common dielectrics used are specified
with EIA codes of Z5U, Y5V, X5R and X7R. Typical volt
-
age and
temperature coefficients are shown in Figures4
and
5. The X5R and X7R dielectrics have more stable
characteristics and are most suitable for use as the output
capacitor. The X7R type has better stability across tem
-
perature, while
the X5R is less expensive and is available
in higher values. Care still must be exercised when using
X5R and X7R capacitors; the codes only specify operating
temperature range and maximum capacitance change
over temperature. Capacitance change due to DC bias
with X5R and X7R capacitors can be significant enough
to drop capacitor values below appropriate levels. Capaci
-
tor DC bias characteristics tend to improve as case size
increases,
but expected capacitance at operating voltage
should be verified
.
LT3048 Series
12
3048fa
For more information www.linear.com/LT3048
APPLICATIONS INFORMATION
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress. In a
ceramic capacitor, the stress can be induced by vibrations
in the system or thermal transients.
Figure 6. Connecting the Inductor to a Separate Voltage Supply
BSTOUTSW
LDOUT
LT3048
5.6µH
FB
V
IN
EN
1nF
1µF
1µF
3048 F06
4.7µF
PV
IN
1V TO 23V
IN
2.7V TO 4.8V
OUT
5V
BYPOFF ON
GND
301k
100k
Figure 4. Ceramic Capacitor Temperature Characteristics
Figure 5. Ceramic Capacitor DC Bias Characteristics
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3048 F04
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3048 F05
20
0
–20
–40
–60
–80
–100
0
4
8
10
2 6
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Wide Input Voltage Applications
The LT3048 can be used in wide input voltage applica-
tions. The
input supply voltage to the LT3048 must be
between
2.7V and 4.8V and capable of at least 10mA.
The inductor can run off a separate voltage supply (PV
IN
).
This technique allows the output to be powered from 1V
to 23V (see Figure 6).
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT3048IDC-3.3#TRPBF

Mfr. #:
Buy LT3048IDC-3.3#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Low Noise Bias Generator in 2mm 2mm DFN
Lifecycle:
New from this manufacturer.
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