Micrel, Inc.
MIC23030 Evaluation Board
August 2008
2
M9999-072208-A
Output Voltage (Adjustable Option Only)
The output voltage of the MIC23030-AYMT regulator is
determined by feedback resistors R1 and R2 whose
values are calculated as follows:
⎟
⎠
⎞
⎜
⎝
⎛
+
×
=
1
R2
R1
0.62V
V
OUT
The output on the evaluation board is by default set to
1.8V (R1=383k
Ω
, R2=200k
Ω
), but can easily be modified
by removing R1 and replacing it with the value that yields
the desired output voltage.
R2
1
0.62V
V
R1
OUT
×
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
For optimum transient response performance, R2 should
be in the range of 180k
Ω
to 220k
Ω
.
HyperLight Load™ Mode
MIC23030 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 the current is sourced from ground. In
discontinuous mode, the MIC23030 works in pulse
frequency modulation (PFM) to regulate the output. As the
output current increases, the off-time decreases, more
energy is delivered to the output. This switching scheme
improves the efficiency of MIC23030 during light load
currents by activating the power FETs only as needed. As
the load current increases, the MIC23030 goes into
continuous conduction mode (CCM) and switches at a
frequency centered at 8MHz. The equation to calculate the
load where the MIC23030 goes into continuous conduction
mode is approximated by the following formula:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
×
×
−
>
f
L
D
V
V
I
OUT
IN
LOAD
2
)
(
As shown in the previous equation, the load at which
MIC23030 transitions from HyperLight Load™ mode to
PWM mode is a function of the input voltage (V
IN
), output
voltage (V
OUT
), duty cycle (D), inductance (L) and
frequency (f). This is illustrated in the graph below.
Suitable inductors for the MIC23030 range from 0.47µH to
2.2µH. 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 2.2µH
inductor is selected in a 3.6V
IN
to 1.8V
OUT
application, the
MIC23030 will transition into PWM mode at a load of
approximately 30mA. If under the same condition a 0.47µH
inductor is chosen, the MIC23030 will transition into PWM
mode at approximately 120mA.
SW Frequency vs. Inductance
