LTC1435A Linear Technology, LTC1435A Datasheet - Page 10

• Manufacturer Part Number: LTC1435A
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
LTC1435A
C
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle V
V
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
This formula has a maximum at V
I
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on only 2000 hours
of life. This makes it advisable to further derate the capaci-
tor or to choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet size or height requirements in the design. Always
consult the manufacturer if there is any question.
The selection of C
series resistance (ESR). Typically, once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple ( V
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. With I
will be less than 100mV at max V
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR(size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
10
RMS
RIPPLE(P-P)
IN
IN
C
C required
. To prevent large voltage transients, a low ESR input
and C
OUT
IN
V
= I
OUT
L
= ripple current in the inductor. The output ripple
OUT
required ESR < 2R
OUT
/2. This simple worst-case condition is com-
requirement.
Selection
I ESR
L
I
RMS
OUT
U
OUT
is driven by the required effective
I
INFORMATION
MAX
4
) is approximated by:
L
U
= 0.4I
fC
SENSE
1
OUT
V
OUT
OUT(MAX)
OUT
IN
W
assuming:
= output capacitance
V
IN
IN
V
= 2V
IN
the output ripple
V
OUT
OUT
OUT
L
U
increases
has been
, where
1 2 /
OUT
/
In surface mount applications multiple capacitors may have
to be paralleled to meet the ESR or RMS current handling
requirements of the application. Aluminum electrolytic and
dry tantalum capacitors are both available in surface mount
configurations. In the case of tantalum, it is critical that the
capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface mount tantalum, available in case heights ranging
from 2mm to 4mm. Other capacitor types include Sanyo
OS-CON, Nichicon PL series and Sprague 593D and 595D
series. Consult the manufacturer for other specific recom-
mendations.
INTV
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 maxi-
mum junction temperature rating for the LTC1435A to be
exceeded. The IC supply current is dominated by the gate
charge supply current when not using an output derived
EXTV
ing frequency as discussed in the Efficiency Considerations
section. The junction temperature can be estimated by using
the equations given in Note 1 of the Electrical Character-
istics. For example, the LTC1435A is limited to less than
17mA from a 30V supply:
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in continuous mode at maximum V
The LTC1435A contains an internal P-channel MOSFET
switch connected between the EXTV
switch closes and supplies the INTV
EXTV
drops below 4.5V. This allows the MOSFET driver and
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC1435A. The INTV
15mA and must be bypassed to ground with a minimum
of 2.2 F tantalum or low ESR electrolytic. Good bypassing
EXTV
T
J
CC
CC
CC
CC
= 70 C + (17mA)(30V)(100 C/W) = 126 C
Regulator
pin is above 4.8V, and remains closed until EXTV
source. The gate charge is dependent on operat-
Connection
CC
CC
CC
pin can supply up to
and INTV
power whenever the
CC
IN
.
pins. The
CC