LTC3630 Datasheet, PDF (18/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous

LTC3630
APPLICATIONS INFORMATION
R_SW to R_L and multiply the result by the square of the
average output current:
I²R Loss = I_O2(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
3630fb
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