Datasheet

LTC3630
18
3630fc
APPLICATIONS INFORMATION
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
In most applications, the LTC3630 does not dissipate much
heat due to its high efficiency. But, in applications where
the LTC3630 is running at high ambient temperature with
low supply voltage and high duty cycles, such as dropout,
the heat dissipated may exceed the maximum junction
temperature of the part.
To prevent the LTC3630 from exceeding the maximum
junction temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum junc-
tion temperature of the part. The temperature rise from
ambient to junction is given by:
T
R
= P
D
• θ
JA
where P
D
is the power dissipated by the regulator and θ
JA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature is given by:
T
J
= T
A
+ T
R
Generally, the worst-case power dissipation is in dropout
at low input voltage. In dropout, the LTC3630 can provide
a DC current as high as the full 1.2A peak current to the
output. At low input voltage, this current flows through a
higher resistance MOSFET, which dissipates more power.
As an example, consider the LTC3630 in dropout at an input
voltage of 5V, a load current of 500mA and an ambient
temperature of 85°C. From the Typical Performance graphs
of Switch On-Resistance, the R
DS(ON)
of the top switch
at V
IN
= 5V and 100°C is approximately 1.9Ω. Therefore,
the power dissipated by the part is:
P
D
= (I
LOAD
)
2
• R
DS(ON)
= (500mA)
2
• 1.9Ω = 0.475W
For the MSOP package the θ
JA
is 45°C/W. Thus, the junc-
tion temperature of the regulator is:
T
J
= 85°C+0.475W •
45
°
C
W
= 106.4°C
which is below the maximum junction temperature of
150°C.
Note that the while the LTC3630 is in dropout, it can provide
output current that is equal to the peak current of the part.
This can increase the chip power dissipation dramatically
and may cause the internal overtemperature protection
circuitry to trigger at 180°C and shut down the LTC3630.
Design Example
As a design example, consider using the LTC3630 in an
application with the following specifications: V
IN
= 24V,
V
IN(MAX)
= 70V, V
OUT
= 3.3V, I
OUT
= 500mA, f = 200kHz.
Furthermore, assume for this example that switching
should start when V
IN
is greater than 12V.
First, calculate the inductor value that gives the required
switching frequency:
L =
3.3V
200kHz • 1.2A






• 1–
3.3V
24V






≅ 10µH
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 • 150ns
1.2A
≅ 3µH
Therefore, the minimum inductor requirement is satisfied
and the 10μH 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
= 500mA •
3.3V
24V
•
24V
3.3V
– 1≅ 175mA
RMS