MIC23155 Datasheet, PDF (13/19 Pages) Micrel Semiconductor – 3MHz PWM 2A Buck Regulator with HyperLight Load and Power Good

Micrel Inc.
MIC23155
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency
%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞
× 100
There are two types of losses in switching converters;
DC losses and switching losses. DC losses are simply
the power dissipation of I²R. Power is dissipated in the
high side switch during the on cycle. Power loss is equal
to the high side MOSFET R_DSON multiplied by the switch
current squared. During the off cycle, the low side N-
channel MOSFET conducts, also dissipating power.
Device operating current also reduces efficiency. The
product of the quiescent (operating) current and the
supply voltage represents another DC loss. The current
required driving the gates on and off at a constant 3MHz
frequency and the switching transitions make up the
switching losses.
Efficiency (V_OUT = 1.8V)
100
90
80
70
V_IN =2.7V
V_IN = 4.2V
60
50
V_IN = 3.6V
V_IN = 5V
40
30
20
10
C_OUT = 4.7µF
0
1
10
100
1000 10000
OUTPUT CURRENT (mA)
Figure 3. Efficiency Under Load
The figure above shows an efficiency curve. From no
load to 100mA, efficiency losses are dominated by
quiescent current losses, gate drive and transition
losses. By using the HyperLight Load^® mode, the
MIC23155 is able to maintain high efficiency at low
output currents.
Over 100mA, efficiency loss is dominated by MOSFET
R_DSON and inductor losses. Higher input supply voltages
will increase the gate-to-source threshold on the internal
MOSFETs, thereby reducing the internal R_DSON. This
improves efficiency by reducing DC losses in the device.
All but the inductor losses are inherent to the device. In
which case, inductor selection becomes increasingly
critical in efficiency calculations. 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 x DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
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
The MIC23155 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 MIC23155 works
in HyperLight Load^® 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 MIC23155 during light load
currents by only switching when it is needed. As the load
current increases, the MIC23155 goes into continuous
conduction mode (CCM) and switches at a frequency
centered at 3MHz. The equation to calculate the load
when the MIC23155 goes into continuous conduction
mode may be approximated by the following formula:
I_LOAD
>
⎜⎜⎝⎛ ^(VIN
− V_OUT
2L × f
)
×
D
⎟⎟⎠⎞
As shown in the previous equation, the load at which the
MIC23155 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 4, as the Output
Current increases, the switching frequency also
increases until the MIC23155 goes from HyperLight
Load^® mode to PWM mode at approximately 180mA.
The MIC23155 will switch at a relatively constant
frequency around 3MHz once the output current is over
180mA.
April 2011
13
M9999-041811-A