MIC23031 Datasheet, PDF (12/17 Pages) Micrel Semiconductor – 4MHz PWM 400mA Buck Regulator with Hyper Light Load™

Micrel Inc.
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 4MHz
frequency and the switching transitions make up the
switching losses.
Efficiency VOUT = 2.5V
100
= 3.0V
90
80
= 4.2V
= 3.6V
70
60
L
501
10
100
1000
OUTPUT CURRENT (mA)
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 Hyper Light Load™ mode, the
MIC23031 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:
L Pd = I_OUT2 × DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
Efficiency
Loss
=
⎡
⎢¹
⎢⎣
−
⎜⎜⎝⎛
V_OUT ×
V_OUT × I_OUT
I
OUT
+L
_
P_D
⎟⎟⎠⎞⎥⎥⎦⎤
× 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.
Hyper Light Load™ Mode
MIC23031 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
MIC23031
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 MIC23031 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
MIC23031 during light load currents by only switching
when it is needed. As the load current increases, the
MIC23031 goes into continuous conduction mode (CCM)
and switches at a frequency centered at 4MHz. The
equation to calculate the load when the MIC23031 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
MIC23031 transitions from Hyper Light 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. Since
the inductance range of MIC23031 is from 0.47µH to
4.7µH, the device may then be tailored to enter Hyper
Light Load™ mode or PWM mode at a specific load
current by selecting the appropriate inductance. For
example, in the graph below, when the inductance is
4.7µH the MIC23031 will transition into PWM mode at a
load of approximately 4mA. Under the same condition,
when the inductance is 1µH, the MIC23031 will transition
into PWM mode at approximately 70mA.
SW Frequency
vs Output Current
10
=
1
=
0.1
=
=
0.01
0.0011
10
100
1000
OUTPUT CURRENT (mA)
May 2008
12
M9999-051608-A