LTC1876 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1876
Linear Technology
'LTC1876' PDF : 36 Pages View PDF
LTC1876
APPLICATIO S I FOR ATIO
Once the value of L is known, select an inductor that can
handle at least 1A without saturating. In addition, ensure
that the inductor has a low DCR (copper wire resistance)
to minimize I2R power losses.
Auxiliary Regulator’s Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice, as
they have extremely low ESR and are available in very
small packages. X5R dielectrics are preferred, followed by
X7R, as these materials retain the capacitance over wide
voltage and temperature ranges. A 4.7µF to 10µF output
capacitor is sufficient for most applications, but systems
with very low output current may need only a 1µF or 2.2µF
output capacitor. Solid tantalum or OS-CON capacitors
can be used, but they will occupy more board area than a
ceramic and will have a higher ESR. Always use a capacitor
with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, and should be placed as close as
possible to the AUXVIN pin. A 1µF to 4.7µF input capacitor
is sufficient for most applications. Table 2 shows a list of
several ceramic capacitor manufacturers. Consult the
manufacturers for detailed information on their entire
selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
The decision to use either low ESR (ceramic) capacitors or
higher ESR (tantalum or OS-CON) capacitors can affect
the stability of the overall system. The ESR of any capaci-
tor, along with the capacitance itself, contributes a zero to
the system. For the tantalum and OS-CON capacitors, this
zero is located at a lower frequency due to the higher value
of the ESR, while the zero of a ceramic capacitor is a much
higher frequency and can generally be ignored.
A phase lead zero can be intentionally introduced by
placing a capacitor (C3) in parallel with the resistor (R8)
between VOUT3 and AUXVFB as shown in Figure 11. The
frequency of the zero is determined by the following
equation.
fZ
=
2π
1
•R8 • C3
By choosing the appropriate values for the resistor and
capacitor, the zero frequency can be designed to slightly
improve the phase margin of the overall converter. The
typical target value for the zero frequency is between
50kHz to 150kHz.
LTC1876
AUXVFB
VOUT3
R8
C3
R7
1876 F11
Figure 11. Adding a Phase Lead Zero
Auxiliary Regulator’s Diode Selection
A Schottky diode is recommended for use with the auxil-
iary regulator. The ON Semiconductor MBR0520 is a very
good choice. Where the input to output voltage differential
exceeds 20V, use the MBR0530 (a 30V diode). These
diodes are rated to handle an average forward current of
0.5A. In applications where the average forward current of
the diode exceeds 0.5A, a Microsemi UPS5817 rated at 1A
is recommended.
Driving AUXSD Above 10V
The maximum voltage allowed on the AUXSD pin is 10V.
In some applications if the applied voltage on this pin is
going to exceed 10V, then a series resistor can be con-
nected to this pin. The value for this resistor is given by:
RSERIES
=
(VAUXSD – 10)
(60 •10–6)
By placing this series resistor, it ensures that the voltage
seen by the pin will not exceed 10V.
1876fa
27
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