Micrel, Inc. MIC33153 Evaluation Board
December 2010
2
M9999-121710-
Soft start Capacitor
The MIC33153 has a nominal 270kΩ resistor charging
the capacitor on the SS pin. This enables the output to
follow a controlled soft start characteristic. Setting C3 to
100pF sets the startup time to the minimum. The startup
time can be determined by:
T
SS
= 270 x 10
3
x ln(10) x C
SS
The action of the soft start capacitor is to control the rise
time of the internal reference voltage between 0% and
100% of its nominal steady state value.
Feedback Resistors (R1, R2) for Adjustable Output
The output voltage is set nominally to 1.8V. This output
can be changed by adjusting the upper resistor, R1, in
the feedback potential divider. Therefore:
R1 = R2 × V
REF
/(V
O
− V
REF
)
where V
REF
= 0.62V.
Some example values are:
V
OUT
R1 R2
1.2V 274k 294k
1.5V 316k 221k
1.8V 301k 158k
2.5V 324k 107k
3.3V 309k 71.5k
The feed forward capacitor, C4, is typically not fitted
given that transient load regulation is already very good.
However, it can be improved slightly by fitting a
capacitor at C4 to inject fast output voltage deviations
directly into the feedback comparator. This improved
load regulation occurs at the expense of slightly
increasing the amount of noise on the output at higher
loads. Values between 100pF and 1nF are
recommended to prevent instability.
Power Good (PG)
The evaluation board has a test point provided to the
right of EN for testing PG. This is an open drain
connection with an on board pull up resistor of 10k to
the output voltage. This is asserted high approximately
70µs after the output voltage passes 92% of the nominal
set voltage.
HyperLight Load™ Mode
The MIC33153 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 MIC33153
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
MIC33153 during light load currents by only switching
when it is needed. As the load current increases, the
MIC33153 goes into continuous conduction mode
(CCM) and switches at a frequency centered at 4MHz.
The equation to calculate the load when the MIC33153
goes into continuous conduction mode may be
approximated by the following formula:
I
LOAD
>
f×2L
D×VV
OUTIN
As shown in the above equation, the load at which
MIC33153 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). For example, if V
IN
= 3.6V, V
OUT
= 1.8V, D
= 0.5, f = 4MHz and the internal inductance of
MIC33153 is 0.47μH, then the device will enter
HyperLight Load™ mode or PWM mode at
approximately 200mA.
