LTC3630A
17
3630afc
For more information www.linear.com/LTC3630A
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, ∆Q, moves from V
IN
to
ground. The resulting ∆Q/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 cur
ves. 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
In most applications, the LTC3630A does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3630A is running at high ambient tempera
-
ture 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 LTC3630A 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 LTC3630A 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 LTC3630A in dropout at an
input voltage of 5V, a load current of 500mA and an ambi
-
ent 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 •
= 106.4°C
which is below the maximum junction temperature of
150°C.
Note that the while the LTC3630A 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 LTC3630A.
Design Example
As a design example, consider using the LTC3630A in an
application with the following specifications: V
IN
= 24V,
V
IN(MAX)
= 80V, 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
