RT8011/A
13
DS8011/A-02 March 2011 www.richtek.com
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as :
Efficiency = 100% − (L1+ L2+ L3+ ...) where L1, L2, etc.
are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses: V
DD
quiescent current and I
2
R losses.
The V
DD
quiescent current loss dominates the efficiency
loss at very low load currents whereas the I
2
R loss
dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve
at very low load currents can be misleading since the
actual power lost is of no consequence.
1. The V
DD
quiescent current is due to two components :
the DC bias current as given in the electrical characteristics
and the internal main switch and synchronous switch gate
charge currents. 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 ΔQ moves
from V
DD
to ground. The resulting ΔQ/Δt is the current out
of V
DD
that is typically larger than the DC bias current. In
continuous mode, I
GATECHG
= f(QT+QB) where QT and QB
are the gate charges of the internal top and bottom
switches.
Both the DC bias and gate charge losses are proportional
to V
DD
and thus their effects will be more pronounced at
higher supply voltages.
2. I
2
R losses are calculated from the resistances of the
internal switches, RSW and external inductor RL. In
continuous mode the average output current flowing
through inductor L is “chopped” between the main switch
and the synchronous switch. Thus, the series resistance
looking into the LX pin is a function of both top and bottom
MOSFET R
DS(ON)
and the duty cycle (D) as follows :
RSW = R
DS(ON)
TOP x D + R
DS(ON)
BOT x (1"D) The R
DS(ON)
for both the top and bottom MOSFETs can be obtained
from the Typical Performance Characteristics curves. Thus,
to obtain I
2
R losses, simply add RSW to RL and multiply
the result by the square of the average output current.
Other losses including C
IN
and C
OUT
ESR dissipative
losses and inductor core losses generally account for less
than 2% of the total loss.
Thermal Considerations
In most applications, the RT8011/A does not dissipate
much heat due to its high efficiency. But, in applications
where the RT8011/A 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 RT8011/A 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 junction temperature of the part. The
temperature rise is given by : T
R
= P
D
x θ
JA
Where PD 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
A
+ T
R
Where T
A
is the ambient temperature.
As an example, consider the RT8011/A in dropout at an
input voltage of 3.3V, a load current of 2A and an ambient
temperature of 70°C. From the typical performance graph
of switch resistance, the R
DS(ON)
of the P-Channel switch
at 70°C is approximately 121mΩ. Therefore, power
dissipated by the part is :
P
D
= (I
LOAD
)
2
(R
DS(ON)
) = (2A)
2
(121mΩ) = 0.484W
For the DFN3x3 package, the θ
JA
is 110°C/W. Thus the
junction temperature of the regulator is : TJ = 70°C +
(0.484W) (110°C/W) = 123.24°C Which is below the
maximum junction temperature of 125°C. Note that at
higher supply voltages, the junction temperature is lower
due to reduced switch resistance (R
DS(ON)
).
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
OUT
immediately shifts by an amount
equal to ΔI
LOAD(ESR)
, where ESR is the effective series
