MIC23153_11 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – 4MHz PWM 2A Buck Regulator with HyperLight Load™ and Power Good

Micrel Inc.
HyperLight Load™ Mode
MIC23153 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 MIC23153 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
MIC23153 during light load currents by only switching
when it is needed. As the load current increases, the
MIC23153 goes into continuous conduction mode (CCM)
and switches at a frequency centered at 4MHz. The
equation to calculate the load when the MIC23153 goes
into continuous conduction mode may be approximated
by the following formula:
I_LOAD
> _{⎜⎜⎝⎛} ^(VIN
− V_OUT
2L × f
)
×
D
⎟⎟⎠⎞
MIC23153
As shown in the previous equation, the load at which the
MIC23153 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 3, as the Output
Current increases, the switching frequency also
increases until the MIC23153 goes from HyperLight
Load™ mode to PWM mode at approximately 120mA.
The MIC23153 will switch at a relatively constant
frequency around 4MHz once the output current is over
120mA.
10000
1000
Switching Frequency
vs. Load Current
L = 2.2µH
100
L = 1µH
10
1
VOUT = 1.8V
0.1
0.0001 0.001 0.01 0.1
1
10
LOAD CURRENT (A)
Figure 3. SW Frequency vs. Output Current
September 2011
13
M9999-092211-B