LTC3604 Datasheet, PDF (16/24 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3604
APPLICATIONS INFORMATION
Each time a power MOSFET gate is switched from low
to high to low again, a packet of charge dQ moves
from V_IN to ground. The resulting dQ/dt is a current
out of INTV_CC that is typically much larger than the DC
control bias current. In continuous mode, I_GATECHG
= f(Q_T + Q_B), where Q_T and Q_B are the gate charges
of the internal top and bottom power MOSFETs and f
is the switching frequency. For estimation purposes,
(Q_T + Q_B) on the LTC3604 is approximately 1nC.
To calculate the total power loss from the LDO load,
simply add the gate charge current and quiescent cur-
rent and multiply by V_IN:
P_LDO = (I_GATECHG + I_Q) • V_IN
3. Other “hidden” losses such as transition loss, cop-
per trace resistances, and internal load currents can
account for additional efficiency degradations in the
overall power system. Transition loss arises from the
brief amount of time the top power MOSFET spends in
the saturated region during switch node transitions. The
LTC3604 internal power devices switch quickly enough
that these losses are not significant compared to other
sources.
Other losses, including diode conduction losses during
dead time and inductor core losses, generally account
for less than 2% total additional loss.
Thermal Considerations
The LTC3604 requires the exposed package backplane
metal (PGND pin on the QFN, SGND pin on the MSOP
package) to be well soldered to the PC board to provide
good thermal contact. This gives the QFN and MSOP
packages exceptional thermal properties, compared to
other packages of similar size, making it difficult in normal
operation to exceed the maximum junction temperature
of the part. In many applications, the LTC3604 does not
dissipate much heat due to its high efficiency and low
thermal resistance package backplane. However, in applica-
tions in which the LTC3604 is running at a high ambient
temperature, high input voltage, high switching frequency,
and maximum output current, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 150°C,
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both power switches will be turned off until temperature
decreases approximately 10°C.
Thermal analysis should always be performed by the user
to ensure the LTC3604 does not exceed the maximum
junction temperature.
The temperature rise is given by:
T_RISE = P_Dθ_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.
Consider the example in which an LTC3604EUD is operat-
ing with I_OUT = 2.5A, V_IN = 12V, f = 1MHz, V_OUT = 1.8V,
and an ambient temperature of 25°C. From the Typical
Performance Characteristics section the R_DS(ON) of the top
switch is found to be nominally 130mΩ while that of the
bottom switch is nominally 100mΩ yielding an equivalent
power MOSFET resistance R_SW of:
R_DS(ON)TOP • 1.8/12 + R_DS(ON)BOT • 10.2/12 = 105mΩ.
From the previous section, I_GATECHG is ~1mA when f =
1MHz, and the spec table lists the maximum I_Q to be 1mA.
Therefore, the total power dissipation due to resistive
losses and LDO losses is:
P_D = I_OUT2 • R_SW + V_IN • (I_{GATECHG +} I_Q)
P_D = (2.5A)² • (0.105Ω) + 12V • 2mA = 680mW
The QFN 3mm × 3mm package junction-to-ambient thermal
resistance, θ_JA, is around 45°C/W. Therefore, the junction
temperature of the regulator operating in a 25°C ambient
temperature is approximately:
T_J = 0.680W • 45°C/W + 25°C = 56°C
Remembering that the above junction temperature is
obtained from an R_DS(ON) at 25°C, we might recalculate the
junction temperature based on a higher R_DS(ON) since it
increases with temperature. Redoing the calculation assum-
ing that R_SW increased 15% at 56°C yields a new junction
temperature of 66°C. If the application calls for a signifi-
cantly higher ambient temperature and/or higher switching
frequency, care should be taken to reduce the temperature
rise of the part by using a heat sink or air flow.
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