RT9018A/B
9
DS9018A/B-07 September 2010 www.richtek.com
Application Information
Adjustable Mode Operation
The output voltage of RT9018A/B is adjustable from 0.8V
to (V
IN
- V
DROP
) by external voltage divider resisters as
shown in Typical Application Circuit (Figure 2). The value
of resisters R1 and R2 should be more than 10kΩ to
reduce the power loss. The V
DD
must be greater than
(V
OUT
+ 1.5V).
Enable
The RT9018A/B goes into shutdown mode when the EN
pin is in the logic low condition. During this condition, the
pass transistor, error amplifier, and band gap are turned
off, reducing the supply current to 10μA typical. The
RT9018A/B goes into operation mode when the EN pin is
in the logic high condition. If the EN pin is floating, NOTE
that the RT9018A/B internal initial logic level. For RT9018A,
the EN pin function pulls high level internally. So the
regulator will be turn on when EN pin is floating. For
RT9018B, the EN pin function pulls low level internally.
So the regulator will be turn off when EN pin is floating.
Output Capacitor
The RT9018A/B is specifically designed to employ ceramic
output capacitors as low as 10μF. The ceramic capacitors
offer significant cost and space savings, along with high
frequency noise filtering.
Input Capacitor
Good bypassing is recommended from input to ground to
help improve AC performance. A 10μF input capacitor or
greater located as close as possible to the IC is
recommended.
Current Limit
The RT9018A/B contains an independent current limit and
the short circuit current protection to prevent unexpected
applications. The current limit monitors and controls the
pass transistor’ s gate voltage, limiting the output current
to higher than 4.5A typical. When the output voltage is
less than 0.4V, the short circuit current protection starts
the current fold back function and maintains the loading
current 1.8A. The output can be shorted to ground
indefinitely without damaging the part.
Power Good
The power good function is an open-drain output. Connects
100kΩ pull up resistor to VOUT to obtain an output voltage.
The PGOOD pin will output high immediately after the
output voltage arrives 90% of normal output voltage. The
PGOOD pin will output high with typical 1.5ms delay time.
Thermal-Shutdown Protection
Thermal protection limits power dissipation to prevent IC
over temperature in RT9018A/B. When the operation
junction temperature exceeds 160°C, the over-temperature
protection circuit starts the thermal shutdown function
and turns the pass transistor off. The pass transistor turn
on again after the junction temperature cools by 30°C.
RT9018A/B lowers its OTP trip level from 160°C to 110°C
when output short circuit occurs (V
OUT
< 0.4V). It limits
IC case temperature under 100°C and provides maximum
safety to customer while output short circuit occurring.
Power Dissipation
For continuous operation, do not exceed absolute
maximum operation junction temperature 125°C. The
power dissipation definition in device is:
P
D
= (V
IN
− V
OUT
) x I
OUT
+ V
IN
x I
Q
The maximum power dissipation depends on the thermal
resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference between
junctions to ambient. The maximum power dissipation can
be calculated by following formula:
P
D(MAX)
= ( T
J(MAX)
− T
A
) / θ
JA
Where T
J(MAX)
is the maximum operation junction
temperature 125°C, T
A
is the ambient temperature and the
θ
JA
is the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT9018A/B, where T
J (MAX)
is the maximum junction
temperature of the die (125°C) and T
A
is the maximum
ambient temperature. The junction to ambient thermal
resistance for SOP-8 (Exposed Pad) package is 75°C/W
on the standard JEDEC 51-7 (4 layers, 2S2P) thermal
test board. The copper thickness is 2oz. The maximum
power dissipation at T
A
= 25°C can be calculated by
