MIC5209 Datasheet, PDF (8/13 Pages) Micrel Semiconductor – 500mA Low-Noise LDO Regulator

MIC5209
Applications Information
Enable/Shutdown
Enable is available only on devices in the SO-8 (M) and
TO-263-5 (U) packages.
Forcing EN (enable/shutdown) high (> 2V) enables the regu-
lator. EN is compatible with CMOS logic. If the enable/
shutdown feature is not required, connect EN to IN (supply
input).
Input Capacitor
A 1µF capacitor should be placed from IN 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.
Output Capacitor
An output capacitor is required between OUT and GND to
prevent oscillation. The minimum size of the output capacitor
is dependent upon whether a reference bypass capacitor is
used. 1µF minimum is recommended when C_BYP is not used
(see Figure 1). 2.2µF minimum is recommended when C_BYP
is 470pF (see Figure 2). Larger values improve the regulator’s
transient response.
The output capacitor should have an ESR (equivalent series
resistance) of about 5Ω and a resonant frequency above
1MHz. Ultra-low-ESR capacitors can cause a low amplitude
oscillation on the output and/or underdamped transient re-
sponse. Most tantalum or aluminum electrolytic capacitors
are adequate; film types will work, but are more expensive.
Since many aluminum electrolytics have electrolytes that
freeze at about –30°C, solid tantalums are recommended for
operation below –25°C.
At lower values of output current, less output capacitance is
needed 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 MIC5209 will remain stable and in regulation with no load
(other than the internal voltage divider) unlike many other
voltage regulators. This is especially important in CMOS
RAM keep-alive applications.
Reference Bypass Capacitor
BYP (reference bypass) is available only on devices in SO-8
and TO-263-5 packages.
BYP is connected to the internal voltage reference. A 470pF
capacitor (C_BYP) connected from BYP to GND quiets this
reference, providing a significant reduction in output noise
(ultra-low-noise performance). Because C_BYP reduces the
phase margin, the output capacitor should be increased to at
least 2.2µF to maintain stability.
The start-up speed of the MIC5209 is inversely proportional
to the size of the reference bypass capacitor. Applications
requiring a slow ramp-up of output voltage should consider
larger values of C_BYP. Likewise, if rapid turn-on is necessary,
consider omitting C_BYP.
If output noise is not critical, omit C_BYP and leave BYP open.
Micrel
Thermal Considerations
The SOT-223 has a ground tab which allows it to dissipate
more power than the SO-8. Refer to “Slot-1 Power Supply” for
details. At 25°C ambient, it will operate reliably at 2W dissipa-
tion with “worst-case” mounting (no ground plane, minimum
trace widths, and FR4 printed circuit board).
Thermal resistance values for the SO-8 represent typical
mounting on a 1"-square, copper-clad, FR4 circuit board. For
greater power dissipation, SO-8 versions of the MIC5209
feature a fused internal lead frame and die bonding arrange-
ment that reduces thermal resistance when compared to
standard SO-8 packages.
Package
SOT-223 (S)
θ_JA
50°C/W
θ_JC
8°C/W
SO-8 (M)
50°C/W
25°C/W
TO-263-5 (U)
—
2°C/W
Table 1. MIC5209 Thermal Resistance
Multilayer boards with a ground plane, wide traces near the
pads, and large supply-bus lines will have better thermal
conductivity and will also allow additional power dissipation.
For additional heat sink characteristics, please refer to Micrel
Application Hint 17, “Designing P.C. Board Heat Sinks”,
included in Micrel’s Databook. For a full discussion of heat
sinking and thermal effects on voltage regulators, refer to
Regulator Thermals section of Micrel’s Designing with Low-
Dropout Voltage Regulators handbook.
Low-Voltage Operation
The MIC5209-1.8 and MIC5209-2.5 require special consid-
eration when used in voltage-sensitive systems. They may
momentarily overshoot their nominal output voltages unless
appropriate output and bypass capacitor values are chosen.
During regulator power up, the pass transistor is fully satu-
rated for a short time, while the error amplifier and voltage
reference are being powered up more slowly from the output
(see “Block Diagram”). Selecting larger output and bypass
capacitors allows additional time for the error amplifier and
reference to turn on and prevent overshoot.
To ensure that no overshoot is present when starting up into
a light load (100µA), use a 4.7µF output capacitance and
470pF bypass capacitance. This slows the turn-on enough to
allow the regulator to react and keep the output voltage from
exceeding its nominal value. At heavier loads, use a 10µF
output capacitance and 470pF bypass capacitance. Lower
values of output and bypass capacitance can be used,
depending on the sensitivity of the system.
Applications that can withstand some overshoot on the
output of the regulator can reduce the output capacitor and/
or reduce or eliminate the bypass capacitor. Applications that
are not sensitive to overshoot due to power-on reset delays
can use normal output and bypass capacitor configurations.
Please note the junction temperature range of the regulator
at 1.8V output (fixed and adjustable) is 0˚C to +125˚C.
MIC5209
8
August 2, 2000