13
LTC1775
Schottky diode must be placed next to the synchronous
switch to minimize this effect. One also might consider
using a power switch with an integrated Schottky diode, or
omitting the diode altogether in high current applications.
C
IN
and C
OUT
Selection
In continuous mode, the drain current of the top MOSFET
is approximately a square wave of duty cycle V
OUT
/V
IN
. To
prevent large input voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS current is given by:
II
V
V
V
V
RMS O MAX
OUT
IN
IN
OUT
≅−
()
/
1
12
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
O(MAX)
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life. This makes it advisable to further derate
the capacitor or to choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
placed in parallel to meet size or height requirements in the
design.
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple. The output ripple
∆V
OUT
is approximately bounded by:
∆∆V I ESR
fC
OUT L
OUT
≤+
1
8()()( )
Since ∆I
L
increases with input voltage, the output ripple is
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied the capacitance is adequate for
filtering and has the required RMS current rating.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON (organic semiconductor
dielectric) capacitor available from Sanyo has the lowest
product of ESR and size of any aluminum electrolytic at a
somewhat higher price. An additional ceramic capacitor in
parallel with OS-CON capacitors is recommended to re-
duce the effect of their lead inductance.
In surface mount applications, multiple capacitors placed
in parallel may be required to meet the ESR, RMS current
handling and load step requirements. Dry tantalum, spe-
cial polymer and aluminum electrolytic capacitors are
available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only use
types that have been surge tested for use in switching
power supplies. Several excellent surge-tested choices
are the AVX TPS and TPSV or the KEMET T510 series.
Aluminum electrolytic capacitors have significantly higher
ESR, but can be used in cost-driven applications providing
that consideration is given to ripple current ratings and
long term reliability. Other capacitor types include Nichicon
PL, NEC Neocap, Panasonic SP and Sprague 595D series.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5.2V supply which powers the drivers and internal cir-
cuitry within the LTC1775. The INTV
CC
pin can supply a
maximum RMS current of 50mA and must be bypassed to
ground with a minimum of 4.7µF tantalum or low ESR
electrolytic capacitance. Good bypassing is necessary to
supply the high transient currents required by the MOSFET
gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the LTC1775
to exceed its maximum junction temperature rating. Most
of the supply current drives the MOSFET gates unless an
external EXTV
CC
source is used. The junction temperature
can be estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC1775CGN
is limited to less than 14mA from a 30V supply:
T
J
= 70°C + (14mA)(30V)(130°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in continuous mode at high V
IN
. Relief can be
provided by using the EXTV
CC
pin to provide the gate drive
current.
APPLICATIO S I FOR ATIO
WUUU
