Micrel, Inc.
MIC23031 Evaluation Board
Output Voltage (Adjustable Option Only)
The output voltage of the MIC23031-AYMT regulator is
determined by feedback resistors R1 and R2 whose
values are calculated in Equation 1:
+
×
=
1
2
R
1
R
V
62
.
0
V
OUT
Eq. 1
The output on the evaluation board is by default set to
1.8V (R1 = 383kΩ, R2 = 200kΩ), but can easily be
modified by removing R1 and replacing it with the value
that yields the desired output voltage.
2
R
1
V
62
.
0
V
1
R
OUT
×
−
=
Eq. 2
For optimum transient response performance, R2 should
be in the range of 180k
Ω to 220kΩ.
HyperLight Load
MIC23031 uses a patented minimum on and off time
control loop at light loads. When the output voltage falls
below the lower regulation threshold, the error comparator
begins a switching cycle that turns the PMOS on and
keeps it on for the duration of the minimum on-time. This
increases the output voltage. Once the output voltage rises
to the higher regulation threshold, the error comparator
turns the PMOS off for a minimum off-time until the output
drops down to the lower threshold. The NMOS acts as an
ideal rectifier that conducts when the PMOS is off. Using a
NMOS switch instead of a diode reduces power dissipation
as current is sourced from ground. In discontinuous mode,
the MIC23031 works in PFM to regulate the output. As the
output current increases, the off-time decreases, therefore
more energy is delivered to the output. This switching
scheme improves the efficiency of the MIC23031 during
light load currents by activating the power FETs only as
needed. As the load current increases, the MIC23031
goes into continuous conduction mode (CCM) and
switches at a frequency centered at 4MHz. Equation 3
calculates the load where the MIC23031 goes into
continuous-conduction mode:
(
)
×
×
−
>
f
L
2
D
V
V
I
OUT
IN
LOAD
Eq. 3
As shown in Equation 3, the load at which the MIC23031
transitions from HyperLight Load mode to PWM mode is a
function of the input voltage (VIN), output voltage (VOUT),
duty cycle (D), inductance (L), and frequency (f) (
Figure 1
).
Suitable inductors for the MIC23031 range from 0.47µH to
4.7µH. The device may be tailored to enter HyperLight
Load mode or PWM mode at a specific load current by
selecting the appropriate inductor. For example, if a 4.7µH
inductor is selected in a 3.6VIN to 1.8VOUT application,
the MIC23031 will transition into PWM mode at a load of
approximately 5mA. If, under the same conditions, a 1µH
inductor is chosen, the MIC23031 will transition into PWM
mode at approximately 70mA.
Figure 1. MIC23031 SW Frequency vs. Output Current
June 17, 2014
2
Revision 2.0
