LTC1149CS-5#TRPBF Linear Technology, LTC1149CS-5#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LTC1149CS-5#TRPBF
• Description: IC SW REG STEP-DOWN 5V 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1149CS-5#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 50mA
• Frequency - Switching: 250kHz
• Voltage - Input: 0 ~ 48 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC1149
LTC1149-3.3/LTC1149-5
the N-channel MOSFET operating in continuous mode are
given by:
where is the temperature dependency of R
that there is no transition loss term in the N-channel
dissipation equation because the drain-to-source voltage
is always low when the N-channel MOSFET is turning on
or off. The remaining I
input voltage or during a short circuit, when the N-channel
duty cycle is nearly 100%. Fortunately, low R
N-channel MOSFETs are readily available which reduce
losses to the point that heat sinking is not required, even
during continuous short-circuit operation.
The Schottky diode D1 shown in Figure 1 only conducts
during the dead-time between the conduction of the two
power MOSFETs. D1’s sole purpose in life is to prevent the
body diode of the N-channel MOSFET from turning on and
storing charge during the dead-time, which could cost as
much as 1% in efficiency (although there are no other
harmful effects if D1 is omitted). Therefore, D1 should be
selected for a forward voltage of less than 0.7V when
conducting I
Finally, both MOSFETs and D1 must be selected for
breakdown voltages higher than the maximum V
C
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle V
prevent large voltage transients, a low ESR input 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
10
RMS
IN
N-Ch Duty Cycle =
N-Ch P
C
and C
IN
= I
Required I
MAX
OUT
D
/2. This simple worst-case condition is com-
=
MAX
Selection
V
IN
.
RMS
V
– V
IN
U U
OUT
V
2
R losses are the greatest at high
IN
I
(I
MAX
V
– V
MAX
IN
OUT
[V
)
2
(1 +
OUT
W
IN
(V
V
IN
N
IN
= 2V
)R
– V
DS(ON)
DS(ON)
OUT
OUT
OUT
U
/V
IN
, where
)]
DS(ON)
.
IN
. Note
1/2
. To
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 be
paralleled to meet size or height requirements in the
design. An additional 0.1 F ceramic capacitor may also be
required on V
The selection of C
series resistance (ESR). The ESR of C
than twice the value of R
LTC1149 series:
Optimum efficiency is obtained by making the ESR equal
to R
efficiency degrades by less than 1%. If the ESR is greater
than 2R
will prematurely trigger Burst Mode operation, resulting in
disruption of continuous mode and an efficiency hit which
can be several percent.
Manufacturers such as Nichicon, Chemicon and Sprague
should be considered for high performance capacitors.
The OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR for its size, 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 capacitance, ESR, or RMS
current handling requirements of the application. Alumi-
num 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 tantalums, available
in case heights ranging from 2mm to 4mm. For example,
if 200 F/10V is called for in an application requiring 3mm
height, two AVX 100 F/10V (P/N TPSD 107K010) could be
used. Consult the manufacturer for other specific recom-
mendations.
At low supply voltages, a minimum value of C
suggested to prevent an abnormal low frequency operat-
ing mode (see Figure 4). When C
RIPPLE(P-P)
C
OUT
SENSE
SENSE
Required ESR < 2R
. As the ESR is increased up to 2R
requirement.
, the voltage ripple on the output capacitor
IN
for high frequency decoupling.
OUT
is driven by the required effective
SENSE
SENSE
for proper operation of the
OUT
OUT
is too small, the
must be less
OUT
SENSE
has been
OUT
, the
is