LTC3630A Datasheet, PDF (17/24 Pages) Linear Technology – High Efficiency, 76V 500mA Synchronous Step-Down Converter

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. I²R 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 I²R losses, simply add
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 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
•
45°C
W
=
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
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3630af
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