Datasheet

LTC3631
15
3631fe
For more information www.linear.com/LTC3631
applicaTions inForMaTion
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
IN
to
ground. The resulting dQ/dt is the current out of V
IN
that is typically larger than the DC bias current.
2. I
2
R losses are calculated from the resistances of the
internal switches, R
SW
, and external inductor R
L
. When
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
DS(ON)
values and the
duty cycle (DC = V
OUT
/V
IN
) as follows:
R
SW
= (R
DS(ON)TOP
)DC + (R
DS(ON)BOT
)(1 – DC)
The R
DS(ON)
for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteris
-
tics curves. Thus, to obtain the I
2
R losses, simply add
R
SW
to R
L
and multiply the result by the square of the
average output current:
I
2
R Loss = I
O
2
(R
SW
+ R
L
)
Other losses, including C
IN
and C
OUT
ESR dissipative
losses and inductor core losses, generally account for
less than 2% of the total power loss.
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
IN
= 24V,
V
OUT
= 3.3V, I
OUT
= 100mA, f = 250kHz. Furthermore, as-
sume for this example that switching should start when V
IN
is greater than 12V and should stop when V
IN
is less than 8V.
First, calculate the inductor value that gives the required
switching frequency:
L =
3.3V
250kHz • 225mA
Ê
Ë
Á
ˆ
¯
˜
• 1–
3.3V
24V
Ê
Ë
Á
ˆ
¯
˜
@ 47µH
Next, verify that this value meets the L
MIN
requirement.
For this input voltage and peak current, the minimum
inductor value is:
L
MIN
=
24V • 100ns
225mA
≅ 10µH
Therefore, the minimum inductor requirement is satisfied,
and the 47μH inductor value may be used.
Next, C
IN
and C
OUT
are selected. For this design, C
IN
should
be sized for a current rating of at least:
I
RMS
= 100mA •
3.3V
24V
•
24V
3.3V
– 1 ≅ 35mA
RMS
Due to the low peak current of the LTC3631, decoupling
the V
IN
supply with a 1µF capacitor is adequate for most
applications.
C
OUT
will be selected based on the output voltage ripple
requirement. For a 2% (67mV) output voltage ripple at no
load, C
OUT
can be calculated from:
C
OUT
=
225mA • 4 • 10
–6
2 67mV –
3.3V
160






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:
R1=
V
OUT
0.8V
– 1






• R2 = 750k