LTC1435CS Linear Technology, LTC1435CS Datasheet - Page 10

• Manufacturer Part Number: LTC1435CS
• Description: IC SW REG SYNC STEP-DOWN 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1435CS

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 9 V
• Current - Output: 50mA
• Frequency - Switching: 125kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
LTC1435
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
N-channel equation includes an additional term for tran-
sition losses, which are highest at high input voltages.
For V
proves with larger MOSFETs, while for V
transition losses rapidly increase to the point that the use
of a higher R
vides higher efficiency. The synchronous MOSFET losses
are greatest at high input voltage or during a short circuit
when the duty cycle in this switch is nearly 100%. Refer
to the Foldback Current Limiting section for further
applications information.
The term (1 + ) is generally given for a MOSFET in the
form of a normalized R
voltage MOSFETs. C
characteristics. The constant k = 2.5 can be used to
estimate the contributions of the two terms in the main
switch dissipation equation.
The Schottky diode D1 shown in Figure 1 conducts during
the dead-time between the conduction of the two large
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on and storing charge
during the dead-time, which could cost as much as 1% in
efficiency. A 1A Schottky is generally a good size for 3A
regulators.
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
10
RMS
IN
IN
= 0.005/ C can be used as an approximation for low
C required
. To prevent large voltage transients, a low ESR input
and C
IN
= I
IN
OUT
< 20V the high current efficiency generally im-
OUT
/2. This simple worst-case condition is com-
DS(ON)
Selection
I
RMS
U
RSS
device with lower C
DS(ON)
is usually specified in the MOSFET
I
INFORMATION
MAX
2
U
R losses while the topside
vs Temperature curve, but
V
OUT
W
V
IN
IN
V
= 2V
IN
RSS
V
IN
OUT
DS(ON)
OUT
actual pro-
> 20V the
U
, where
1 2 /
and k
OUT
/
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
capacitor 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
ripple 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
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic 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 recommendations.
RIPPLE(P-P)
C
OUT
V
OUT
L
= ripple current in the inductor. The output ripple
required ESR < 2R
requirement.
I ESR
L
OUT
OUT
is driven by the required effective
4
) is approximated by:
fC
SENSE
L
1
OUT
= 0.4I
OUT
= output capacitance
OUT(MAX)
IN
assuming:
OUT
L
the output
increases
has been