RT9194
Application Information
Capacitors Selection
Careful selection of the external capacitors for RT9194 is
highly recommended in order to remain high stability and
performance.
Regarding the supply voltage capacitor, connecting a
capacitor which is 1μF between V
CC
and ground is a must.
The capacitor improves the supply voltage stability to
provide chip normally operating.
Regarding the input capacitor, connecting a capacitor which
100μF between V
IN
and ground is recommended to
increase stability. With large value of capacitance could
result in better performance for both PSRR and line
transient response.
When driving external pass element, connecting a
capacitor 100μF between V
OUT
and ground is
recommended for stability. With larger capacitance can
reduce noise and improve load transient response and
PSRR.
Output Voltage Setting
The RT9194 develop a 0.8V reference voltage; especially
suit for low voltage application. As shown in application
circuit, the output voltage could easy set the output
voltage by R1 & R2 divider resistor.
Power Good Function
The RT9194 has the power good function with delay. The
power good output is an open drain output. Connect a
100kΩ pull up resistor to V
OUT
to obtain an output voltage.
When the output voltage arrive 90% of normal value the
power good will output voltage with 3ms delay time.
When the output voltage falling arrive 75% of normal value
the power good will turn off with less than 1ms delay time.
But, there are two exceptions. One is the enable pull low
the power good will turn off quickly. The second is the
V
CC
falling arrive POR value (4V typ.) the power good
also will turn off quickly.
Chip Enable Operation
Pull the EN pin low to drive the device into shutdown mode.
During shutdown mode, the standby current drops to
5μA
(MAX)
. The external capacitor and load current determine
MOSFET Selection
The RT9194 are designed to driver external N-Channel
MOSFET pass element. MOSFET selection criteria
include threshold voltage V
GS
(V
TH
), maximum
continuous drain current ID, 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).
Calculate the required R
DS(ON)
from the following formula:
V
−
V
OUT
NMOSFET R
DS(ON)
=
IN
I
LOAD
For example, the MOSFET operate up to 2A when the
input voltage is 1.5V and set the output voltage is 1.2V,
R
ON
= (1.5V-1.2V) / 2A = 150mΩ, the MOSFET's
R
ON
have to select lower than 150mΩ. A Philip PHD3055E
MOSFET with an R
DS(ON)
of 120mΩ(typ.) is a close
match.
And carry on consider the thermal resistance from
junction to ambient
θ
(JA)
of the MOSFET's package. The
power dissipation calculate by :
P
D
= (V
IN
−
V
OUT
) x I
LOAD
The thermal resistance from junction to ambient
θ
(JA)
calculate by :
(T
−
T )
θ
(JA)
=
J A
P
D
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.
DS9194-08 March 2007
the output voltage decay rate. Drive the EN pin high to
turn on the device again.
Under Voltage Protection
RT9194 equips the V
OUT
under-voltage (UV) protection
function. The UV protection circuits will start monitoring
the power status after the PGOOD pin goes high. If the
output voltage drops to below 75% of its setting value,
the PGOOD and DRI pins will be pulled low and latch
RT9194. The UV latch status will be released only when
V
CC
or Enable pin goes low and returns high again, which
will also cause RT9194 to re-activate.
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