RT9008
7
DS9008-01 April 2011 www.richtek.com
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.
OUT REF
R1
VV (1)
R2
=+
Where V
REF
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 (C
CC
) connecting
a ceramic capacitor 1μF between the V
CC
and GND is a
must.
The capacitor C
CC
improves the supply voltage stability to
provide chip normal operation.
As to the input capacitor ,C
IN,
connecting a 100μF between
the V
IN,
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 (C
OUT
) is recommended
for stability. With larger capacitor, the RT9008 can reduce
noise the improve load transient response and PSRR.
MOSFET Selection and Thermal Consideration
The RT9008 is designed to drive an external N-MOSFET
pass element. MOSFET selection criteria include
threshold voltage V
GS
(V
TH
), maximum continuous drain
current I
D,
on-resistance R
DS(ON),
maximum drain-to-source
voltage V
DS
and package thermal resistance θ
JA
.
The most critical specification is the MOSFET R
DS(ON).
The maximum allowed R
DS(ON)
can be calculated by the
following formula :
IN OUT
DS(ON)
LOAD
VV
R
I
−
=
For example, if the maximum load current is 2A, the input
voltage is 1.5V and the output voltage is 1.2V, then R
DS(ON)
= (1.5V − 1.2V)/2A = 150mΩ.
The MOSFET's R
DS(ON)
have to be selected to be lower
than 150mΩ. A Philips PHD3055E MOSFET with an
R
DS(ON)
of 120mΩ (typ.) is a good choice.
After that, consider the thermal resistance from junction
to ambient θ
JA
of the MOSFET's package. The power
dissipation is calculated by :
P
D
= (V
IN
− V
OUT
) x I
LOAD
The thermal resistance from junction to ambient θ
JA
can
be calculated by :
D
AJ
(JA)
P
)T(T −
=
θ
In this example, P
D
= (1.5V − 1.2V) x 2A = 0.6W. The
PHD3055E's θ
JA
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 :
P
D(MAX)
= (T
J(MAX)
− T
A
) / θ
JA
