Micrel, Inc. MIC69502
December 2006
7
M9999-121406-B
Application Information
The MIC69502 is an ultra-high performance low dropout
linear regulator designed for high current applications
requiring fast transient response. It utilizes a single input
supply and has very low dropout voltage perfect for low-
voltage DC-to-DC conversion. The MIC69502 requires a
minimum of external components. As a µCap regulator
the output is tolerant of virtually any type of capacitor
including ceramic and tantalum.
The MIC69502 regulator is fully protected from damage
due to fault conditions offering constant current limiting
and thermal shutdown.
Input Supply Voltage
V
IN
provides high current to the collector of the pass
transistor. The minimum input voltage is 1.65V allowing
conversion from low voltage supplies.
Output Capacitor
The MIC69502 requires a minimum of output
capacitance to maintain stability. However, proper
capacitor selection is important to ensure desired
transient response. The MIC69502 is specifically
designed to be stable with a wide range of capacitance
values and ESR. A 10µF ceramic chip capacitor should
satisfy most applications. See typical characteristics for
examples of load transient response.
X7R dielectric ceramic capacitors are recommended
because of their temperature performance. X7R-type
capacitors change capacitance by only 15% over their
operating temperature range and are the most stable
type of ceramic capacitors. Z5U and Y5V dielectric
capacitors change value by as much as 50% and 60%,
respectively over their operating temperature ranges. To
use a ceramic chip capacitor with Y5V dielectric the
value must be much higher than an X7R ceramic or a
tantalum capacitor to ensure the same capacitance
value over the operating temperature range. Tantalum
capacitors have a very stable dielectric (10% over their
operating temperature range) and can also be used with
this device.
Input Capacitor
An input capacitor of 1µF or greater is recommended
when the device is more than 4 inches away from the
bulk supply capacitance or when the supply is a battery.
Small, surface mount, ceramic chip capacitors can be
used for the bypassing. The capacitor should be placed
within 1" of the device for optimal performance. Larger
values will help to improve ripple rejection by bypassing
the input to the regulator further improving the integrity of
the output voltage.
Minimum Load Current
The MIC69502 regulator is specified between finite
loads. If the output current is too small, leakage currents
dominate and the output voltage rises. A 10mA minimum
load current is necessary for proper operation.
Adjustable Regulator Design
The MIC69502 adjustable version allows programming
the output voltage anywhere between 0.5V and 5.5V
with two resistors. The resistor value between V
OUT
and
the adjust pin should not exceed 10kΩ. Larger values
can cause instability. The resistor values are calculated
by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+∗= 1
R
R
0.5V
2
1
OUT
Where V
OUT
is the desired output voltage.
Enable
The MIC69502 features an active high enable input (EN)
that allows on-off control of the regulator. Current drain
reduces to near “zero” when the device is shutdown,
with only microamperes of leakage current. The EN input
has TTL/CMOS compatible thresholds for simple logic
interfacing. EN may be directly tied to V
IN
and pulled up
to the maximum supply voltage.
Thermal Design
Linear regulators are simple to use. The most
complicated design parameters to consider are thermal
characteristics. Thermal design requires the following
application-specific parameters:
• Maximum ambient temperature (T
A
)
• Output current (I
OUT
)
• Output voltage (V
OUT
)
• Input voltage (V
IN
)
• Ground current (I
GND
)
First, calculate the power dissipation of the regulator
from these numbers and the device parameters from this
data sheet.
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
I
GND
where the ground current is approximated by using
numbers from the “Electrical Characteristics” or “Typical
Characteristics” sections. The heat sink thermal
resistance is then determined with this formula:
θ
SA
= ((T
J
(max) – T
A
)/ P
D
) – (θ
JC
+ θ
CS
)
Where T
J
(max) ≤125
°
C and θ
CS
is between 0
°
C and
2
°
C/W.
