MIC5245 Micrel
MIC5245 8 August 2002
Applications Information
Enable/Shutdown
The MIC5245 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 MIC5245 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 MIC5245 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.
X7R dielectric ceramic capacitors are recommended be-
cause of their temperature performance. X7R-type capaci-
tors change capacitance by 15% over their operating tem-
perature range and are the most stable type of ceramic
capacitors. Z5U and Y5V dielectric capacitors change value
by as much 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 minimum
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.
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.
Transient Response
The MIC5245 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 (100µA).
Active Shutdown
The MIC5245 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 MIC5245 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
dissipation of the package, use the junction-to-ambient ther-
mal resistance of the device and the following basic equation:
P
TT
D(max)
J(max) A
JA
=
−
θ
T
J(max)
is the maximum junction temperature of the die,
125°C, and T
A
is the ambient operating temperature. θ
JA
is
layout dependent; Table 1 shows examples of junction-to-
ambient thermal resistance for the MIC5245.
Package
θθ
θθ
θ
JA
Recommended
θθ
θθ
θ
JA
1" Square
θθ
θθ
θ
JC
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:
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
I
GND
Substituting P
D(max)
for P
D
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 MIC5245-3.3BM5 at 50°C with
a minimum footprint layout, the maximum input voltage for a
set output current can be determined as follows:
P
125 C 50 C
235 C/W
D(max)
=
°− °
°
P
D(max)
= 315mW
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 3.3V and an output current of 150mA,
the maximum input voltage can be determined. Because this
device is CMOS and the ground current is typically 87µA over
the load range, the power dissipation contributed by the
ground current is < 1% and can be ignored for this calculation.
315mW = (V
IN
– 3.3V) 150mA
315mW = V
IN
× 150mA – 495mW
810mW = V
IN
× 150mA
V
IN(max)
= 5.4V
Therefore, a 3.3V application at 150mA of output current can
accept a maximum input voltage of 5.4V in a SOT-23-5
package. For a full discussion of heat sinking and thermal
