MIC5280 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – 25mA, 120V, Low IQ, High PSRR LDO

Micrel, Inc.
Application Information
MIC5280 is a linear regulator with a wide operating
range of input voltage 4.5V to 120V. The input can go
down to –24V without any damage to the device. This
feature gives the advantage to use in harsh environment
applications such as remote keyless entry and telecom
applications.
Thermal Protection
MIC5280 has internal thermal shutdown to protect from
over dissipation and failure. When the junction
temperature exceeds +160°C, an internal thermal sensor
signals the shutdown logic to turn off the pass transistor
and allows the IC to cool. The thermal sensor turns the
pass transistor on again after the junction temperature
cools by 10°C. This results in a cycled output during
continuous thermal overload conditions. The recovery
from thermal shutdown is smooth without any overshoot.
For continuous operation, do not exceed the maximum
junction temperature rating of +125°C.
Current Limit
MIC5280 has a current limit protection to protect it from
over loads. The recovery from current limit protection is
smooth without any overshoots. The output can be
shorted to GND for an indefinite period of time without
damage to the device. During a short circuit, the power
dissipated across the pass transistor can quickly heat
the device. When the die temperature reaches +160°C,
the MIC5280 shut down and automatically restart after
the die temperature cools by 10°C. This results in a
pulsed output operation.
CBYP Output when Enable is Low
When the Enable is low and the MIC5280 is disabled,
while V_IN voltage is present, the CBYP pin voltage is
typically 4.5V, even for V_IN as high as 120V. The Bypass
regulator although maintaining the Bypass voltage,
requires no current from V_IN other than leakage current.
Bypass Capacitor
A 10µF capacitor should be connected from bypass pin
to ground to maintain stability of the bypass regulator.
The voltage rating of the capacitor must be 8V or higher.
Low ESR ceramic capacitors work well. Although a 1µF
capacitor maintains good AC power supply rejection,
higher capacitance values will enhance AC power supply
rejection.
MIC5280
PSRR
Power Supply Rejection Ratio is the measure of a V_OUT
signal caused by a signal on the V_IN pin. The ratio of
V_IN/V_OUT in dB is PSRR. The MIC5280’s PSRR from 0Hz
to 1kHz exceeds 80dB.
Output Voltage Setting
For this adjustable regulator, V_OUT is programmable from
1.22V to 5.0V. A 2.2µF or larger capacitor must be
connected to V_OUT pin-to-ground to maintain stability.
This capacitor value can be increased without bound.
The V_OUT capacitor can have an ESR of 0Ω. When V_IN is
lower than V_OUT, V_OUT is not pulled low by V_IN. A small
current, typically 4µA, will sink into V_OUT pin.
The output voltage is controlled by the feedback
resistors (R1 and R2) and can be calculated as follows:
V_OUT
= V_REF
⋅ ⎜⎛ ^R1 + 1⎟⎞
⎝ ^R2 ⎠
V_REF = 1.219V
The evaluation board is initially adjusted to 5V, but can
easily be modified by removing R1 and replacing it with
the value that yields the desired output voltage.
R1
=
R2
⋅
⎜⎜⎝⎛
V_OUT
V_REF
− 1_{⎟⎟⎠⎞}
Ensure the output voltage selected does not exceed 5V.
Enable Pin
The voltage on enable pin EN of MIC5280 could vary
from –1V to 120V. So enable pin EN of MIC5280 can be
used in a different configuration as the application
dictates.
If the MIC5280 needs to be always enabled then simply
connect the EN pin to CBYP pin, as shown in Figure 1.
Then whenever V_IN has a supply voltage output is
developed. Input can swing from –24V to 120V.
One can also connect EN pin through 100kΩ resistor to
input as shown in Figure 2 to achieve the same result.
January 2009
10
M9999-012209-A