MIC5247(2000) 查看數據表（PDF） - Micrel
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MIC5247 Datasheet PDF : 10 Pages
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MIC5247
Applications Information
Enable/Shutdown
The MIC5247 comes with an active-high enable pin that
allows the regulator to be disabled. Forcing the enable pin low
disables the regulator and sends it into a “zero” off-mode-
current state. In this state, current consumed by the regulator
goes nearly to zero. Forcing the enable pin high enables the
output voltage. This part is CMOS and the enable pin cannot
be left floating; a floating enable pin may cause an indetermi-
nate state on the output.
Input Capacitor
An input capacitor is not required for stability. A 1µF input
capacitor is recommended when the bulk ac supply capaci-
tance is more than 10 inches away from the device, or when
the supply is a battery.
Output Capacitor
The MIC5247 requires an output capacitor for stability. The
design requires 1µF or greater on the output to maintain
stability. The capacitor can be a low-ESR ceramic chip
capacitor. The MIC5247 has been designed to work specifi-
cally with the low-cost, small chip capacitors. Tantalum
capacitors can also be used for improved capacitance over
temperature. The value of the capacitor can be increased
without bound.
Bypass Capacitor
A capacitor can be placed from the noise bypass pin to
ground to reduce output voltage noise. The capacitor by-
passes the internal reference. A 0.01µF capacitor is recom-
mended for applications that require low-noise outputs.
The bypass capacitor can be increased without bound,
further reducing noise and improving PSRR. Turn-on time
remains constant with respect to bypass capacitance. Refer
to the Typical Characteristics section for a graph of turn-on
time vs. bypass capacitor.
Transient Response
The MIC5247 implements a unique output stage to dramati-
cally improve transient response recovery time. The output is
a totem-pole configuration with a P-channel MOSFET pass
device and an N-channel MOSFET clamp. The N-channel
clamp is a significantly smaller device that prevents the
output voltage from overshooting when a heavy load is
removed. This feature helps to speed up the transient re-
sponse by significantly decreasing transient response recov-
ery time during the transition from heavy load (100mA) to light
load (85µA).
Active Shutdown
The MIC5247 also features an active shutdown clamp, which
is an N-channel MOSFET that turns on when the device is
disabled. This allows the output capacitor and load to dis-
charge, de-energizing the load.
Thermal Considerations
The MIC5247 is designed to provide 150mA of continuous
current in a very small package. Maximum power dissipation
can be calculated based on the output current and the voltage
drop across the part. To determine the maximum power
Micrel
dissipation of the package, use the junction-to-ambient ther-
mal resistance of the device and the following basic equation:
PD(max)
=
TJ(max) −
θJA
TA
TJ(max) is the maximum junction temperature of the die,
125°C, and TA is the ambient operating temperature. θJA is
layout dependent; Table 1 shows examples of junction-to-
ambient thermal resistance for the MIC5247.
Package
θJA Recommended θJA 1" Square θJC
Minimum Footprint Copper Clad
SOT-23-5 (M5)
235°C/W
185°C/W 145°C/W
Table 1. SOT-23-5 Thermal Resistance
The actual power dissipation of the regulator circuit can be
determined using the equation:
PD = (VIN – VOUT) IOUT + VIN IGND
Substituting PD(max) for PD and solving for the operating
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit. For
example, when operating the MIC5247-2.4BM5 at room
temperature with a minimum footprint layout, the maximum
input voltage for a set output current can be determined as
follows:
PD(max)
=
125°C − 25°C
235°C/W
PD(max) = 425mW
The junction-to-ambient thermal resistance for the minimum
footprint is 235°C/W, from Table 1. The maximum power
dissipation must not be exceeded for proper operation. Using
the output voltage of 2.4V and an output current of 150mA,
the maximum input voltage can be determined. Because this
device is CMOS and the ground current is typically 100µA
over the load range, the power dissipation contributed by the
ground current is < 1% and can be ignored for this calculation.
425mW = (VIN – 2.4V) 150mA
425mW = VIN·150mA – 360mW
785mW = VIN·150mA
VIN(max) = 5.2V
Therefore, a 2.4V application at 150mA of output current can
accept a maximum input voltage of 5.2V in a SOT-23-5
package. For a full discussion of heat sinking and thermal
effects on voltage regulators, refer to the Regulator Thermals
section of Micrel’s Designing with Low-Dropout Voltage Regu-
lators handbook.
Fixed Regulator Applications
VIN
MIC5247-x.xBM5
1
5
2
3
4
VOUT
1µF
0.01µF
Figure 1. Ultra-Low-Noise Fixed Voltage Application
MIC5247
8
November 6, 2000
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