MIC23303 Datasheet, PDF (14/21 Pages) Micrel Semiconductor – 4MHz PWM 3A Buck Regulator with HyperLight Load™ and Power Good

Micrel, Inc.
As the inductors are reduced in size, the DC resistance
(DCR) can become quite significant. The DCR losses can
be calculated as follows:
P_DCR = I_OUT2 × DCR
From that, the loss in efficiency due to inductor resistance
can be calculated as follows:
MIC23303
As shown in the previous equation, the load at which the
MIC23303 transitions from HyperLight Load mode to PWM
mode is a function of the input voltage (V_IN), output voltage
(V_OUT), duty cycle (D), efficiency (η), inductance (L) and
frequency (f). As shown in Figure 4, as the output current
increases, the switching frequency also increases until the
MIC23303 goes from HyperLight Load mode to PWM
mode at approximately 300mA. The MIC23303 will switch
at a relatively constant frequency around 4MHz once the
output current is over 300mA.
Efficiency
Loss
=

^{1 −}


V_OUT × I_OUT
V_OUT × I_OUT + P_DCR
 ^{× 100}
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and size
in this case.
HyperLight Load Mode
MIC23303 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 MIC23303 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 MIC23303 during light
load currents by only switching when it is needed. As the
load current increases, the MIC23303 goes into
continuous conduction mode (CCM) and switches at a
frequency centered at 4MHz. The equation to calculate the
load when the MIC23303 goes into continuous conduction
mode may be approximated by the following formula:
Switching Frequency
vs. Load Current
10000
1000
100
10
V_IN=5V
VIN=3.3V
1
0.1
0.0001
0.001
VOUT = 1.8V
L = 0.33µH
COUT = 44µF
0.01
0.1
1
10
LOAD CURRENT (A)
Figure 4. SW Frequency vs. Output Current
Power Dissipation Considerations
As with all power devices, the ultimate current rating of the
output is limited by the thermal properties of the package
and the PCB it is mounted on. There is a simple, Ohm’s
law type relationship between thermal resistance, power
dissipation and temperature which are analogous to an
electrical circuit:
I_LOAD
>
 ^(VIN
−
V_OUT
2L ×
)
f
×
D
×
η

Figure 5. Ohm’s Law Description
September 6, 2013
14
090613-2.0