LTC1874EGN Linear Technology, LTC1874EGN Datasheet - Page 8

• Manufacturer Part Number: LTC1874EGN
• Description: IC CTRLR DC/DC DUAL STPDN 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1874EGN

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 9.8 V
• Frequency - Switching: 550kHz
• Voltage - Input: 2.5 ~ 9.8 V
• Operating Temperature: -65°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC1874
I
occurs at the maximum input voltage.
In Burst Mode operation on an LTC1874 controller, the
ripple current is normally set such that the inductor
current is continuous during the burst periods. Therefore,
the peak-to-peak ripple current must not exceed:
This implies a minimum inductance of:
A smaller value than L
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value of inductor is known, an off the shelf
inductor can be selected. The inductor should be rated for
the calculated peak current. Some manufacturers specify
both peak saturation current and peak RMS current. Make
sure that the RMS current meets your continuous load
requirements. Also, you may want to compare the DC
resistance of different inductors in order to optimize the
efficiency.
Inductor core losses are usually not specified and you will
need to evaluate them yourself. Usually, the core losses
are not a problem because the inductors operate with
relatively low magnetic flux swings. The best way to
evaluate the core losses is by measuring the converters
efficiency. Converter efficiency will reveal the difference in
both DC current losses and core losses.
Off the shelf inductors are available from numerous manu-
facturers. Some of the most common manufacturers are
Coilcraft, Coiltronics, Panasonic, Toko, Tokin, Murata and
Sumida.
8
RIPPLE
(Use V
I
L
RIPPLE
MIN
= 0.4(I
IN(MAX)
f
V
IN
R
R
OUT(MAX)
0 03
SENSE
0 03
.
SENSE
.
V
= V
V
OUT
U
IN
). Remember, the maximum I
)
MIN
V
V
OUT
U
IN
could be used in the circuit;
V
V
D
D
W
U
RIPPLE
Power MOSFET Selection
The main selection criteria for the power MOSFET are the
threshold voltage V
reverse transfer capacitance C
Since the controller is designed for operation down to low
input voltages, a logic level threshold MOSFET (R
guaranteed at V
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the controller is less
than the absolute maximum V
The required minimum R
erned by its allowable power dissipation. For applications
that may operate the controller in dropout, i.e., 100% duty
cycle, at its worst case the required R
where P
temperature dependency of R
given for a MOSFET in the form of a normalized R
temperature curve, but p = 0.005/ C can be used as an
approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less than
100% and the controller is in continuous mode, the
R
where DC is the maximum operating duty cycle of the
controller.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
MOSFET duty cycle. At high input voltages the diode
conducts most of the time. As V
diode conducts only a small fraction of the time. The most
stressful condition for the diode is when the output is
DS(ON)
R
R
DS ON
DS ON
(
(
P
is governed by:
is the allowable power dissipation and p is the
)
)
DC
100
DC I
GS
%
= 2.5V) is required for applications that
OUT
GS(TH)
P
I
OUT MAX
P
2
1
, the “on” resistance R
DS(ON)
(
DS(ON)
P
p
GS
RSS
)
P
2
IN
of the MOSFET is gov-
rating, typically 8V.
and total gate charge.
1
approaches V
. (1 + p) is generally
DS(ON)
p
is given by:
DS(ON)
OUT
DS(ON)
DS(ON)
the
vs
,