LTC3809EMSE-1 Linear Technology, LTC3809EMSE-1 Datasheet - Page 14

• Manufacturer Part Number: LTC3809EMSE-1
• Description: IC CTRLR DC/DC SYNC 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3809EMSE-1

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz
• Voltage - Input: 2.75 ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3809-1
that do not work well at high input voltages (e.g., V
5V) may work fi ne at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given R
delays are much smaller than for a P-channel MOSFET.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, f
the inductor’s peak-to-peak ripple current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus, highest effi ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specifi ed maximum,
the inductor should be chosen according to:
Burst Mode Operation Considerations
The choice of R
the load current at which the LTC3809-1 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
14
I
L
I
RIPPLE
BURST PEAK
≥
f
OSC RIPPLE
V
IN
(
=
DS(ON)
–
•
V
I
V
V
OUT
OUT
)
IN
DS(ON)
=
, the gate charge and turn-on and turn-off
4
•
1
V
•
•
OUT(MAX)
IN
f
Δ
V
and inductor value also determines
OSC
V
OUT
V
–
IN
R
SENSE MAX
V
DS ON
•
OUT
L
(
. Note that the largest ripple
(
)
OSC
)
, directly determine
IN
>
The corresponding average current depends on the
amount of ripple current. Lower inductor values (higher
I
operation begins.
The ripple current is normally set so that the inductor current
is continuous during the burst periods. Therefore,
This implies a minimum inductance of:
A smaller value than L
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
effi ciency. In general, though, it is a good idea to keep
I
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
fi xed inductor value, but is very dependent on the induc-
tance selected. As inductance increases, core losses go
down. Unfortunately, increased inductance requires more
turns of wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred 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 current
is exceeded. Core saturation results in an abrupt increase
in inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
RIPPLE
RIPPLE
I
L
RIPPLE
MIN
) will reduce the load current at which Burst Mode
comparable to I
≤
≤ I
f
OSC BURST PEAK
BURST(PEAK)
V
•
IN
I
–
V
OUT
(
MIN
BURST(PEAK)
could be used in the circuit,
)
•
V
V
OUT
IN
.
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