MIC23250 Datasheet, PDF (12/15 Pages) Micrel Semiconductor – 4MHz Dual 400mA Synchronous Buck Regulator with HYPER LIGHT LOAD™

Micrel, Inc.
MIC23250
Efficiency V_OUT = 1.8V
100
V_IN = 2.7V
80
V_IN = 3.6V
60
V_IN = 3.3V
40
20
V_OUT = 1.8V
L = 1µH
00.1
1
10 100 1000
LOAD (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 MIC23250 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™
The MIC23250 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 MIC23250 works in pulse
frequency modulation (PFM) to regulate the output. As the
output current increases, the off-time decreases, thus
providing more energy to the output. This switching
scheme improves the efficiency of MIC23250 during light
load currents by only switching when it is needed. As the
load current increases, the MIC23250 goes into
continuous conduction mode (CCM) and switches at a
frequency centered at 4MHz. The equation to calculate the
load when the MIC23250 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
MIC23250 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 MIC23250 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 MIC23250 will transition into PWM mode at a
load of approximately 4mA. Under the same condition,
when the inductance is 1µH, the MIC23250 will transition
into PWM mode at approximately 70mA.
Switching Frequency
vs. Output Current
10
L = 4.7µH
4MHz
1
0.1
0.011
L = 1µH
L = 2.2µH
V_IN = 3.6V
V_OUT = 1.8V
C_OUT = 4.7µF
10
100
1000
OUTPUT CURRENT (mA)
December 2007
12
M9999-121707-A