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RT9008

Low Dropout Linear Regulator Controller with Soft-Start

Richtek Technology 

Application Information

Output Voltage Setting

As shown in application circuit, the output voltage can be

easy set by the external resistor divider of R1 and R2.

VOUT = VREF (1+ RR21)

Where VREF is the feedback reference voltage (0.8V

typical).

Chip Enable Operation

Pull the EN pin low (< 0.4V) to shutdown the device. During

shutdown mode, the standby current is lower than 5μA.

The external capacitor and load current determine the

output voltage decay rate. Drive the EN pin high (>1.4V)

to turn on the device again.

Soft-Start

Soft-Start provides for the monotonic, glitch-free turn-on

of the regulator. Soft-start limits the input inrush current

which may cause a glitch, especially if the source

impedance is high. The soft-start is achieved by the

controller ramping up to the error amplifier reference input.

The RT9008 soft-start time is 190us when the soft-start

capacitor is 1nF, 920μs for 4.7nF and 1.9ms for 10nF.

Capacitors Selection

Careful selection of the external capacitors is highly

recommended for the best performance of the RT9008.

Regarding the supply voltage capacitor (CCC) connecting

a ceramic capacitor 1μF between the VCC and GND is a

must.

The capacitor CCC improves the supply voltage stability to

provide chip normal operation.

As to the input capacitor ,CIN, connecting a 100μF between

the VIN, and GND is recommended to increase stability.

With large capacitor value could result in better

performance for both PSRR and line transient response.

When driving external pass element, a 100μF electrolytic

capacitor on the output capacitor (COUT) is recommended

for stability. With larger capacitor, the RT9008 can reduce

noise the improve load transient response and PSRR.

RT9008

MOSFET Selection and Thermal Consideration

The RT9008 is designed to drive an external N-MOSFET

pass element. MOSFET selection criteria include

threshold voltage VGS (VTH), maximum continuous drain

current ID, on-resistance RDS(ON), maximum drain-to-source

voltage VDS and package thermal resistance θJA.

The most critical specification is the MOSFET RDS(ON).

The maximum allowed RDS(ON) can be calculated by the

following formula :

RDS(ON)

=

VIN − VOUT

ILOAD

For example, if the maximum load current is 2A, the input

voltage is 1.5V and the output voltage is 1.2V, then RDS(ON)

= (1.5V − 1.2V)/2A = 150mΩ.

The MOSFET's RDS(ON) have to be selected to be lower

than 150mΩ. A Philips PHD3055E MOSFET with an

RDS(ON) of 120mΩ (typ.) is a good choice.

After that, consider the thermal resistance from junction

to ambient θJA of the MOSFET's package. The power

dissipation is calculated by :

PD = (VIN − VOUT) x ILOAD

The thermal resistance from junction to ambient θJA can

be calculated by :

θ(JA)

=

(TJ − TA

PD

)

In this example, PD = (1.5V − 1.2V) x 2A = 0.6W. The

PHD3055E's θJA is 75°C/W for its D-PAK package, which

translates to a 45°C temperature rise above ambient. The

package provides exposed backsides that directly transfer

heat to the PCB board.

The RT9008 maximum power dissipation depends on the

thermal resitance of the IC package, PCB layout, the rate

of surroundings airflow and temperature difference between

junction to ambient.

The maximum power dissipation can be calculated by

following formula :

PD(MAX) = (TJ(MAX) − TA) / θJA

DS9008-01 April 2011

www.richtek.com

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