LTC4090-5_15 Datasheet, PDF (20/30 Pages) Linear Technology – USB Power Manager with 2A High Voltage Bat-Track Buck Regulator

LTC4090/LTC4090-5
APPLICATIONS INFORMATION
Catch Diode
The catch diode conducts current only during switch-off
time. Average forward current in normal operation can
be calculated from:
( ) _{ID(AVG) =IHVOUT •}
V_HVIN – V_HVOUT
V_HVIN
where I_HVOUT is the output load current. The only reason to
consider a diode with a larger current rating than necessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the high voltage regulator
will keep the switch off when V_HVIN > 40V which allows
the use of 40V rated Schottky even when V_HVIN ranges
up to 60V. Table 3 lists several Schottky diodes and their
manufacturers.
Table 3. Diode Vendors
V_R
PART NUMBER
(V)
On Semiconductor
MBRM120E
20
MBRM140
40
Diodes Inc.
B120
20
B130
30
B220
20
B230
30
DFLS240L
40
International Rectifier
10BQ030
30
20BQ030
30
I_AVE
V_F AT 1A V_F AT 2A
(A)
(MV)
(MV)
1
530
595
1
550
1
500
1
500
2
500
2
500
2
500
1
420
470
2
470
High Voltage Regulator Output Capacitor Selection
The high voltage regulator output capacitor has two
essential functions. Along with the inductor, it filters the
square wave generated at the switch pin to produce the
DC output. In this role it determines the output ripple, and
low impedance at the switching frequency is important.
The second function is to store energy in order to satisfy
transient loads and stabilize the LTC4090/LTC4090-5’s
control loop. Ceramic capacitors have very low equiva-
lent series resistance (ESR) and provide the best ripple
performance. A good starting value is:
C_OUT
=
100
V_OUT f_SW
where f_SW is in MHz, and C_OUT is the recommended
output capacitance in µF. Use X5R or X7R types. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
higher value capacitor if the compensation network is also
adjusted to maintain the loop bandwidth. A lower value of
output capacitor can be used to save space and cost but
transient performance will suffer. See the High Voltage
Regulator Frequency Compensation section to choose an
appropriate compensation network.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
Low ESR is important, so choose one that is intended for
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05W or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the high voltage switching regulator due to
their piezoelectric nature. When in Burst Mode operation,
the LTC4090/LTC4090-5’s switching frequency depends
on the load current, and at very light loads the LTC4090/
LTC4090-5 can excite the ceramic capacitor at audio fre-
quencies, generating audible noise. Since the LTC4090/
LTC4090-5 operate at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a ca-
sual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output.
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4090fd