MIC23450 Datasheet, PDF (16/22 Pages) Micrel Semiconductor – 3MHz, PWM, 2A Triple Buck Regulator with HyperLight Load® and Power Good

Micrel, Inc.
Now we have a function describing P_DISS in terms of I_OUT,
we can substitute P_DISS with Equation 7 to form the
function of maximum output current I_OUTMAX vs. ambient
temperature T_AMB (Equation 10):
I_OUTMAX =
T_JMAX − T_AMB
Rθ_{(J− A)}
R_{DSON_P} ×
V_OUT
V_IN
+ R_{DSON_N} × _{}1−
V_OUT
V_IN

Eq. 10
The curves shown in the characteristic curves section
are plots of this function adjusted to account for 1, 2 or 3
regulators running simultaneously.
HyperLight Load Mode
Each regulator in the MIC23450 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 MIC23450 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 MIC23450 during light load currents by
only switching when it is needed. As the load current
increases, the MIC23450 goes into continuous
conduction mode (CCM) and switches at a frequency
centered at 3MHz. The equation to calculate the load
when the MIC23450 goes into continuous conduction
mode may be approximated in Equation 11:
I_LOAD
> _{} ^(VIN

− V_OUT
2L × f
)
×
D

Eq. 11
MIC23450
As shown in Equation 11, the load at which the
MIC23450 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 MIC23450 goes from HyperLight
Load mode to PWM mode at approximately 120mA. The
MIC23450 will switch at a relatively constant frequency
around 3MHz once the output current is over 120mA.
10000
Switching Frequency
vs. Load Current
1000
100
10
V_IN = 3V
V_IN = 3.6V
V_IN = 5V
1
V_OUT = 1.8V
0.1
0.0001 0.001 0.01
0.1
1
10
OUTPUT CURRENT (A)
Figure 4. SW Frequency vs. Output Current
Multiple Sources
The MIC23450 provides all the pins necessary to
operate the 3 regulators from independent sources. This
can be useful in partitioning power within a multi rail
system. For example, it is possible that within a system,
two supplies are available; 3.3V and 5V. The MIC23450
can be connected to use the 3.3V supply to provide two,
low voltage outputs (e.g. 1.2V and 1.8V) and use the 5V
rail to provide a higher output (e.g. 2.5V), resulting in the
power blocks shown in Figure 5.
December 2012
Figure 5. Multi-Source Power Block Diagram
16
M9999-120712-A