LTC3631IMS8E#TRPBF Linear Technology, LTC3631IMS8E#TRPBF Datasheet - Page 15

• Manufacturer Part Number: LTC3631IMS8E#TRPBF
• Description: IC CONV STP-DWN SYNC ADJ 8-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3631IMS8E#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 45 V
• Current - Output: 100mA
• Voltage - Input: 4.5 ~ 45 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP Exposed Pad, 8-HMSOP, 8-eMSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Frequency - Switching: -

applicaTions inForMaTion
2. I
Thermal Considerations
The LTC3631 does not dissipate much heat due to its high
efficiency and low peak current level. Even in worst-case
conditions (high ambient temperature, maximum peak
current and high duty cycle), the junction temperature will
exceed ambient temperature by only a few degrees.
Design Example
As a design example, consider using the LTC3631 in an
application with the following specifications: V
V
for this example that switching should start when V
than 12V and should stop when V
OUT
The gate charge current results from switching the gate
capacitance of the internal power MOSFET switches.
Each time the gate is switched from high to low to
high again, a packet of charge, dQ, moves from V
ground. The resulting dQ/dt is the current out of V
that is typically larger than the DC bias current.
internal switches, R
switching, the average output current flowing through
the inductor is “chopped” between the high side PMOS
switch and the low side NMOS switch. Thus, the series
resistance looking back into the switch pin is a function
of the top and bottom switch R
duty cycle (DC = V
The R
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain the I
R
average output current:
Other losses, including C
losses and inductor core losses, generally account for
less than 2% of the total power loss.
2
SW
R losses are calculated from the resistances of the
= 3.3V, I
R
I
2
SW
R Loss = I
to R
DS(ON)
= (R
L
OUT
and multiply the result by the square of the
DS(ON)TOP
for both the top and bottom MOSFETs can
= 100mA, f = 250kHz. Furthermore, assume
O
2
(R
OUT
SW
SW
)DC + (R
/V
, and external inductor R
+ R
IN
IN
) as follows:
L
)
and C
IN
DS(ON)BOT
DS(ON)
2
is less than 8V.
R losses, simply add
OUT
ESR dissipative
values and the
)(1 – DC)
IN
IN
is greater
L
. When
= 24V,
IN
to
IN
First, calculate the inductor value that gives the required
switching frequency:
Next, verify that this value meets the L
For this input voltage and peak current, the minimum
inductor value is:
Therefore, the minimum inductor requirement is satisfied,
and the 47μH inductor value may be used.
Next, C
be sized for a current rating of at least:
Due to the low peak current of the LTC3631, decoupling
the V
applications.
C
requirement. For a 2% (67mV) output voltage ripple at no
load, C
A 9.7µF capacitor gives this typical output voltage ripple at
no load. Choose a 10µF capacitor as a standard value.
The output voltage can now be programmed by choosing
the values of R1 and R2. Choose R2 = 240k and calculate
R1 as:
OUT
I
L =
L
C
R1=
RMS
MIN
OUT
IN
will be selected based on the output voltage ripple
Ê
Ë Á
IN
OUT
supply with a 1µF capacitor is adequate for most



= 100mA •
250kHz • 225mA
=
=
and C
0.8V
V
OUT
24V • 100ns
225mA • 4 • 10
can be calculated from:
2 67mV –



225mA
OUT
3.3V
– 1

 • R2 = 750k

are selected. For this design, C
3.3V
24V
3.3V
160
≅ 10µH
ˆ
¯ ˜
•
–6
• 1–



Ê
Ë Á
3.3V
24V
3.3V
24V
– 1 ≅ 35mA
ˆ
¯ ˜
LTC3631
MIN
@ 47µH
requirement.
RMS
IN
15
should
3631fc