Micrel, Inc. MIC5210
September 2006
9
M9999-090806
Application Information
Enable/Shutdown
Forcing EN (enable/shutdown) high (> 2V) enables the
regulator. EN is compatible with CMOS logic gates.
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.
Reference Bypass Capacitor
BYPB (reference bypass) is connected to the internal
voltage reference of regulator B. A 470pF capacitor
(C
BYP
) connected from BYPB to GND quiets this
reference, providing a significant reduction in output
noise. C
BYP
reduces the regulator phase margin; when
using C
BYP
, output capacitors of 2.2µF or greater are
generally required to maintain stability.
The start-up speed of the MIC5210 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 a major concern, omit C
BYP
and
leave BYPB open.
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. 1.0µF minimum is recommended
when C
BYP
is not used (see Figure 2). 2.2µF minimum is
recommended when C
BYP
is 470pF (see Figure 1).
Larger values improve the regulator’s transient
response. The output capacitor value may be increased
without limit.
The output capacitor should have an ESR (effective
series resistance) of about 5Ω or less and a resonant
frequency above 1MHz. Ultralow-ESR capacitors may
cause a low-amplitude oscillation and/or underdamped
transient response. Most tantalum or aluminum
electrolytic capacitors are adequate; film types will work,
but are more expensive. Since many aluminum
electrolytic capacitors have electrolytes that freeze at
about –30°C, solid tantalum capacitors are
recommended for operation below –25°C.
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 MIC5210 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.
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.
Thermal Considerations
Multilayer boards having a ground plane, wide traces
near the pads, and large supply bus lines provide better
thermal conductivity.
The MIC5210-xxBMM (8-pin MSOP) has a thermal
resistance of 200°C/W when mounted on a FR4 board
with minimum trace widths and no ground plane.
PC Board
Dielectric
θ
JA
FR4
200
°C/W
MSOP Thermal Characteristics
For additional heat sink characteristics, please refer to
Micrel Application Hint 17, “Calculating P.C. Board Heat
Sink Area For Surface Mount Packages”.
Thermal Evaluation Examples
For example, at 50°C ambient temperature, the
maximum package power dissipation is:
P
D(max)
= (125°C – 50°C) / 200°C/W
= 375mW
If the intent is to operate the 5V version from a 6V supply
at the full 150mA load for both outputs in a 50°C
maximum ambient temperature, make the following
calculation:
P
D(each regulator)
= (V
IN
– V
OUT
) × I
OUT
+ (V
IN
× I
GND
)
= (6V – 5V) × 150mA + (6V × 2.5mA)
= 165mW
P
D(both regulators)
= 2 regulators × 165mW
= 330mW
The actual total power dissipation of 330mW 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
3.3V output application at 50°C, if one regulator supplies
150mA, the other regulator is limited to a much lower
current. The first regulator dissipates:
P
D
= (5V – 3.3V) 150 + 2.5mA (5V)
P
D
= 267.5mW
