Micrel, Inc.
MIC33050 Evaluation Board
March 24, 2014
2
Revision 4.0
Output Voltage (Adjustable Option Only)
The output voltage of the MIC33050-AYHL is set by the
feedback resistors R2 and R3. Follow the equation and
circuit below to determine V
OUT
:
R3
R2
1
0.4V
V
OUT
Eq. 1
Figure 1. Typical Circuit for MIC33050-AYHL (V
OUT
= 1.8V)
The default output voltage for the evaluation board is set
to 1.8V (R2=348k
Ω, R3=100kΩ). A different output
voltage can be obtained by removing R2 and replacing it
with the desired resistance. The equation below can be
used to find R2:
1
0.4V
V
R3
R2
OUT
Eq. 2
Changing the output voltage to 2.5V, assuming
R3=100k
Ω, can be accomplished via the equation
below:
1
0.4V
2.5V
100k
Ω
R2
Eq. 3
The result is 523k
Ω for R2 which gives an output voltage
of 2.5V.
HyperLight Load Mode
MIC33050 uses a minimum on and off time proprietary
control loop (patented by Micrel). When the output
voltage falls below the 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. If the output voltage is
over the regulation threshold, then the error comparator
turns the PMOS off for a minimum-off-time until the
output drops below the threshold. The NMOS acts as an
ideal rectifier that conducts when the PMOS is off. Using
a NMOS switch instead of a diode allows for lower
voltage drop across the switching device when it is on.
The asynchronous switching combination between the
PMOS and the NMOS allows the control loop to work in
discontinuous mode for light load operations. In
discontinuous mode, the MIC33050 works in pulse
frequency modulation (PFM) to regulate the output. As
the output current increases, the off-time decreases,
thus provides more energy to the output. This switching
scheme improves the efficiency of MIC33050 during light
load currents by only switching when it is needed. As the
load current increases, the MIC33050 goes into
continuous conduction mode (CCM) and switches at a
frequency centered at 4MHz. The equation to calculate
the load when the MIC33050 goes into continuous
conduction mode may be approximated by the following
formula:
f
2L
D
)
V
(V
I
OUT
IN
LOAD
Eq. 4
As shown in the previous equation, the load at which
MIC33050 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). As shown in Figure 1, as the Output
Current increases, the switching frequency also
increases until the MIC33050 goes from HyperLight
Load mode to PWM mode at approximately 120mA. The
MIC33050 will switch at a relatively constant frequency
around 4MHz once the output current is over 120mA.
0.001
0.01
0.1
1
10
1
10
100
1000
10000
SW FRE
QUEN
CY
(
MHz)
OUTPUT CURRENT (mA)
SW Frequency
vs Output Current
L = 4.7µH
V
OUT
= 1.8V
C
OUT
= 4.7µF
V
IN
= 3.0V
V
IN
= 3.6V
V
IN
= 4.2V
Figure 2. SW Frequency vs. Output Current
