MIC23250_10 Datasheet, PDF (14/20 Pages) Micrel Semiconductor – 4MHz Dual 400mA Synchronous Buck Regulator with HyperLight Load

Micrel, Inc.
MIC23250
HyperLight 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 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). 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 HyperLight
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
5mA. 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
L = 1µH
0.1
0.0111
L = 2.2µH
V_IN = 3.6V
V_OUT = 1.8V
C_OUT = 4.7µF
0
100
1000
OUTPUT CURRENT (mA)
June 2010
14
M9999-061110-E