LTC3809EMSE-1 Linear Technology, LTC3809EMSE-1 Datasheet - Page 15

• 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
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated fi eld/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
Schottky Diode Selection (Optional)
The schottky diode D in Figure 9 conducts current dur-
ing the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1%
in effi ciency. A 1A Schottky diode is generally a good
size for most LTC3809-1 applications, since it conducts
a relatively small average current. Larger diode results
in additional transition losses due to its larger junction
capacitance. This diode may be omitted if the effi ciency
loss can be tolerated.
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:
IN
C
and C
IN
Re
quiredI
OUT
Selection
RMS
≈
I
MAX
•
V
OUT
•
(
V
IN
V
IN
–
V
OUT
OUT
/V
)
1 2
IN
/
). To
This formula has a maximum value at V
I
monly used for design because even signifi cant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 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 the
size or height requirements in the design. Due to the high
operating frequency of the LTC3809-1, ceramic capacitors
can also be used for C
if there is any question.
The selection of C
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
Setting Output Voltage
The LTC3809-1 output voltage is set by an external
feedback resistor divider carefully placed across the
output, as shown in Figure 3. The regulated output voltage
is determined by:
RMS
Δ
V
OUT
V
= I
OUT
RIPPLE
OUT
=
≈
0 6
/2. This simple worst-case condition is com-
.
I
RIPPLE
increase with input voltage.
V
•
OUT
RIPPLE
⎛
⎜
⎝
OUT
1
•
) is approximated by:
+
IN
⎛
⎜
⎝
ESR
. Always consult the manufacturer
R
R
is driven by the effective series
is the ripple current in the induc-
B
A
⎞
⎟
⎠
+
8
• •
f C
LTC3809-1
1
OUT
OUT
IN
= 2V
⎞
⎟
⎠
is the output
OUT
15
, where
38091fc