LTC3700EMS Linear Technology, LTC3700EMS Datasheet - Page 11

• Manufacturer Part Number: LTC3700EMS
• Description: IC DC/DC CNTRLR STPDN LDO 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3700EMS

• Topology: Step-Down (Buck) (1), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 2
• Frequency - Switching: 550kHz
• Voltage/current - Output 1: Adj to 0.8V, 5A
• Voltage/current - Output 2: Adj to 0.8V, 150mA
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 2.4 V ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Current - Output: 1A
• Voltage - Output: 0.8 ~ 5 V
• Voltage - Input: 2.65 ~ 9.8 V
• Internal Switch(s): No
• Synchronous Rectifier: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
In Burst Mode operation on the LTC3700, 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 for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will in-
crease. Ferrite designs have very low core losses and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
(Use V
I
L
RIPPLE
MIN
IN(MAX)
®
f
V
IN
cores. Actual core loss is independent of core
R
R
0 03
SENSE
0 03
SENSE
.
.
V
= V
OUT
U
IN
)
MIN
V
INFORMATION
V
OUT
U
IN
could be used in the circuit;
V
V
D
D
W
U
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool M . Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC3700. The main selection criteria for
the power MOSFET are the threshold voltage V
the “on” resistance R
C
Since the LTC3700 is designed for operation down to low
input voltages, a sublogic 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 buck is less than the
absolute maximum V
The required minimum R
erned by its allowable power dissipation. For applications
that may operate the LTC3700 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.
Kool M is a registered trademark of Magnetics, Inc.
RSS
R
DS ON
and total gate charge.
(
P
is the allowable power dissipation and p is the
)
DC
100
GS
%
= 2.5V) is required for applications that
DS(ON)
GS
I
OUT MAX
rating, typically 8V.
DS(ON)
(
, reverse transfer capacitance
DS(ON)
P
)
P
2
of the MOSFET is gov-
1
. (1 + p) is generally
DS(ON)
p
LTC3700
is given by:
GS(TH)
DS(ON)
11
DS(ON)
and
3700f
vs