MIC5210(2000) Просмотр технического описания (PDF) - Micrel
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MIC5210 Datasheet PDF : 8 Pages
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MIC5210
Micrel
Applications Information
Enable/Shutdown
Forcing EN (enable/shutdown) high (> 2V) enables the regu-
lator. 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 (CBYP) connected
from BYPB to GND quiets this reference, providing a signifi-
cant reduction in output noise. CBYP reduces the regulator
phase margin; when using CBYP, 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 re-
quiring a slow ramp-up of output voltage should consider
larger values of CBYP. Likewise, if rapid turn-on is necessary,
consider omitting CBYP.
If output noise is not a major concern, omit CBYP 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 CBYP is not
used (see Figure 2). 2.2µF minimum is recommended when
CBYP 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 ad-
equate; 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 recom-
mended 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-lead MSOP) has a thermal resis-
tance of 200°C/W when mounted on a FR4 board with
minimum trace widths and no ground plane.
PC Board
θJA
Dielectric
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:
PD(max) = (125°C – 50°C) ÷ 200°C/W
PD(max) = 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:
PD(each regulator) = (VIN – VOUT) × IOUT + (VIN × IGND)
PD(each regulator) = (6V – 5V) × 150mA + (6V × 2.5mA)
PD(each regulator) = 165mW
PD(both regulators) = 2 regulators × 165mW
PD(both regulators) = 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:
PD = (5V – 3.3V) 150 + 2.5mA (5V)
PD = 267.5mW
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 MIC5210, power dissipation can be reduced
by using the lowest possible input voltage to minimized the
input-to-output voltage drop.
February 2000
7
MIC5210
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