Datasheet

LTC4090/LTC4090-5
20
4090fc
APPLICATIONS INFORMATION
Catch Diode
The catch diode conducts current only during switch-off
time. Average forward current in normal operation can
be calculated from:
I
D(AVG)
=I
HVOUT
•
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
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(MV)
V
F
AT 2A
(MV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B120
B130
B220
B230
DFLS240L
20
30
20
30
40
1
1
2
2
2
500
500
500
500
500
International Rectifi er
10BQ030
20BQ030
30
30
1
2
420 470
470
High Voltage Regulator Output Capacitor Selection
The high voltage regulator output capacitor has two es-
sential functions. Along with the inductor, it fi lters 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 fi nd 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 specifi ed
by the supplier, and should be 0.05Ω 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 frequencies, 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
casual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output.