RT8011APF datasheet(13/17 Pages) RICHTEK

13 / 17 page

RT8011/A

DS8011/A-00 August 2006

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: VDD quiescent current and I

2R losses.

The VDD quiescent current loss dominates the efficiency

loss at very low load currents whereas the I2R 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 VDD 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 VDD to ground. The resulting

ΔQ/Δt is the current out

of VDD that is typically larger than the DC bias current. In

continuous mode, IGATECHG= 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 VDD and thus their effects will be more pronounced at

higher supply voltages.

2. I2R 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 RDS(ON) and the duty cycle (D) as follows :

RSW = RDS(ON)TOP x D + RDS(ON)BOT x (1"D) The RDS(ON)

for both the top and bottom MOSFETs can be obtained

from the Typical Performance Characteristics curves. Thus,

to obtain I2R losses, simply add RSW to RL and multiply

the result by the square of the average output current.

Other losses including CIN and COUT 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 : TR = PD x

θJA Where PD is

the power dissipated by the regulator and

θJA is the thermal

resistance from the junction of the die to the ambient

temperature. The junction temperature, TJ, is given by :

TJ = TA + TR Where TA 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 RDS(ON) of the P-Channel switch

at 70

°C is approximately 121mΩ. Therefore, power

dissipated by the part is :

PD = (ILOAD)

2 (RDS(ON)) = (2A)2 (121m

Ω) = 0.484W

For the DFN3x3 package, the

θJA 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 (RDS(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, VOUT immediately shifts by an amount

equal to

ΔILOAD(ESR), where ESR is the effective series

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