RT8004GCP 데이터 시트보기 (PDF) - Richtek Technology

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RT8004GCP

3A, 4MHz, Synchronous Step-Down Regulator

Richtek Technology 

RT8004

The RT8004 contains an internal soft-start clamp that

gradually raises the clamp on COMP after the EN/SS pin

is pulled above 0.8V. The full current range becomes

available on COMP after 1024 switching cycles. If a longer

soft-start period is desired, the clamp on COMP can be

set externally with a resistor and capacitor on the EN/SS

pin as shown in Typical Application Circuit. The soft-start

duration can be calculated by using the following formula:

TSS = RSS x CSS x ln( VIN ) (s)

VIN − 1.8V

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 I2R

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 (DC) as follows :

RSW = RDS(ON)TOP x DC + RDS(ON)BOT x (1−DC)

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 RT8004 does not dissipate much

heat due to its high efficiency. But, in applications where

the RT8004 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 RT8004 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.

www.richtek.com

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DS8004-03 September 2007

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