Advance
Micropower Dual 150mA CMOS RF-LDO™ Regulators with Power Supply Reset Monitor
APPLICATIONS INFORMATION - Regulators
Dual-Supply Operation
When used in dual supply systems where the regulator load is
returned to a negative supply, the output voltage must be
diode clamped to ground.
Enable/Shutdown
Forcing ENA/B (enable/shutdown) high (>2V) enables the reg-
ulator. ENA/B is compatible with CMOS logic gates. If the
enable/shutdown feature is not required, connect ENA/B to
VINA/B (supply input).
Thermal Considerations
Multilayer boards having a ground plane, wide traces near the
pads, and large supply bus lines provide better thermal con-
ductivity. The 8-lead MSOP has a thermal resistance of
200°C/W when mounted on an FR4 board with minimum trace
widths and no ground plane.
Input Capacitor
A 1µF capacitor should be placed from VINA/B to GND if there is
more than 10 inches of wire between the input and the ac filter
capacitor or if a battery is used as the input.
Thermal Evaluation Examples
For example, at 50°C ambient temperature, the maximum
package power dissipation is:
Reference Bypass Capacitor
CNOISE (reference bypass) is connected to the internal volt-
age reference of regulator A and B. A 470pF capacitor
connected from CNOISE to GND decouples the reference,
P D(max) = (125°C - 50°C)/200°C/W
P D(max) = 375mW
providing a significant reduction in output noise. CNOISE
If the intent is to operate the part from a 4V supply at the full
150mA load for both outputs in a 50°C maximum ambient
temperature, make the following calculations:
reduces the regulator phase margin; when using CNOISE
,
output capacitors of 2.2µF or greater are generally
required to maintain stability. The start-up speed of the
ILC7281 is inversely proportional to the size of the refer-
ence bypass capacitor. Applications requiring a slow
ramp-up of output voltage should consider larger values of
P D(each regulator) = (VIN - VOUT
)
IOUT + (VIN IGND)
P D(each regulator) = (4V - 3V) 150mA + (4V 0.12mA)
P D(each regulator) = 150mW
CNOISE
.
Likewise, if rapid turn-on is necessary, consider
P D(both regulators) = 2 regulators 150mW
P D(both regulators) = 300mW
omitting CNOISE
.
If output noise is not a major concern,
omit CNOISE and leave the pin open.
The actual total power dissipation of 300mW is below the
375mW package maximum; therefore the regulator can be
used. Note that both regulators cannot always be used at their
maximum current rating. For example, in a 5V input to 3.0V
output application at 50°C, if one regulator supplies 150mA,
the other regulator is limited to a much lower current. The first
regulator dissipates:
Output Capacitor
An output capacitor is required between VOUTA/B and GND to
prevent oscillation. The minimum size of the output capacitor
is dependent upon whether a reference bypass capacitor is
used. 1.0µF minimum is recommended when CNOISE is not
used (see figure 2). 2.2µF minimum is recommended when
CNOISE is 470pF (see figure 1). Larger values slow the regu-
P D = (5V - 3.0V) 150 + 0.12mA (5V)
P D = 267.5mW
lator’s transient response. The output capacitor value may be
increased without limit. The output capacitor should have an
ESR (effective series resistance) of about 5 or less and a res-
onant frequency above 1MHz. Ultra low ESR capacitors may
be used and will not cause a low-amplitude oscillation and/or
underdamped transient response which shows up in some
LDO regulators. Most ceramic capacitors are adequate so
there is no need for more expensive tantalum capacitors.
Then, the load that the remaining regulator can dissipate must
not exceed: 375mW - 267.5mW = 107.5mW. This means,
using the same 5V input and 3.3V output voltage, the second
regulator is limited to about 60mA
Taking advantage of the extremely low dropout voltage char-
acteristics of the ILC7281, power dissipation can be reduced
by using the lowest possible input voltage to minimize the
input to output voltage drop.
At lower values of output current, less output capacitance is
required for output stability. The capacitor can be reduced to
0.47µF for current below 10mA or 0.33µF for currents below
1mA
No-Load Stability
The ILC7281 will remain stable and in regulation with no load
unlike many other voltage regulators. This is expecially impor-
tant in CMOS RAM KeepAlive applications.
Impala Linear Corporation
(408) 574-3939
ILC7281 1.0
www.impalalinear.com
Nov 2000
7
IMPALA [ Impala Linear Corporation ]
