LTC3568_15 Datasheet, PDF (13/18 Pages) Linear Technology – 1.8A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3568
Applications Information
the losses in LTC3568 circuits: 1) LTC3568 V_IN current,
2) switching losses, 3) I²R losses, 4) other losses.
1. The V_IN current is the DC supply current given in the
electrical characteristics which excludes MOSFET driver
and control currents. V_IN current results in a small loss
that increases with V_IN, even at no load.
2. The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current re-
sults from switching the gate capacitance of the power
MOSFETs. Each time a 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 V_IN that is typically much larger than the DC bias
current. In continuous mode, I_GATECHG = f_O(QT + QB),
where QT and QB are the gate charges of the internal
top and bottom MOSFET switches. The gate charge
losses are proportional to V_IN and thus their effects
will be more pronounced at higher supply voltages.
3. I²R Losses are calculated from the DC resistances of
the internal switches, R_SW, and external inductor, RL. In
continuous mode, the average output current flowing
through inductor L is “chopped” between the internal
top and bottom switches. Thus, the series resistance
looking into the SW pin is a function of both top and
bottom MOSFET R_DS(ON) and the duty cycle (DC) 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 Characteristics
curves. Thus, to obtain I²R losses:
I²R losses = I_OUT2(R_SW + RL)
4. Other “hidden” losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important
to include these “system” level losses in the design of a
system. The internal battery and fuse resistance losses
can be minimized by making sure that C_IN has adequate
charge storage and very low ESR at the switching fre-
quency. Other losses including diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3568 does not dis-
sipate much heat due to its high efficiency. However, in
applications where the LTC3568 is running at high ambient
temperature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximum junction temperature of the part. If the junction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the LTC3568 from exceeding the maximum junc-
tion 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
junction temperature of the part. The temperature rise is
given by:
T_RISE = 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, T_J, is given by:
T_J = T_RISE + T_AMBIENT
As an example, consider the case when the LTC3568 is in
dropout at an input voltage of 3.3V with a load current of
1.8A with a 70°C ambient temperature. From the Typical
Performance Characteristics graph of Switch Resistance,
the R_DS(ON) resistance of the P‑channel switch is 0.125Ω.
Therefore, power dissipated by the part is:
P_D = I² • R_DS(ON) = 405mW
The DFN package junction-to-ambient thermal resistance,
θ_JA is 43°C/W. Therefore, the junction temperature of the
regulator operating in a 70°C ambient temperature is
approximately:
T_J = 0.405 • 43 + 70 = 87.4°C
Remembering that the above junction temperature is
obtained from an R_DS(ON) at 70°C, we might recalculate
the junction temperature based on a higher R_DS(ON) since
it increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
3568fa
13