MIC33153 Datasheet, PDF (12/17 Pages) Micrel Semiconductor – 4MHz PWM 1.2A Internal Inductor Buck Regulator with HyperLight Load and Power Good

Micrel Inc.
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:
P_DCR = I_OUT2 x DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
Efficiency
Loss
=
⎡
⎢^1−
⎢⎣
⎜⎜⎝⎛
V_OUT ×I_OUT
V_OUT ×I_OUT + P_DCR
⎟⎟⎠⎞⎥⎥⎦⎤ ^×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.
The effect of MOSFET voltage drops and DCR losses in
conjunction with the maximum duty cycle combine to
limit maximum output voltage for a given input voltage.
The following graph shows this relationship based on the
typical resistive losses in the MIC33153:
V_OUTMAX vs. V_IN
5
4.5
100mA
4
400mA
3.5
3
1.2A
2.5
2
800mA
1.5
1
0.5
0
2.5 3 3.5 4 4.5 5 5.5
INPUT VOLTAGE (V)
MIC33153
HyperLight Load™ Mode
MIC33153 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 MIC33153 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
MIC33153 during light load currents by only switching
when it is needed. As the load current increases, the
MIC33153 goes into continuous conduction mode (CCM)
and switches at a frequency centered at 4MHz. The
equation to calculate the load when the MIC33153 goes
into continuous conduction mode may be approximated
by the following formula:
I_LOAD
> _{⎜⎜⎝⎛} ^(VIN
− V_OUT
2L × f
)
×
D
⎟⎟⎠⎞
As shown in the above equation, the load at which
MIC33153 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). For example, if V_IN = 3.6V, V_OUT=1.8V,
D=0.5, f=4MHz and the internal inductance of MIC33153
is 0.47μH, then the device will enter HyperLight Load™
mode or PWM mode at approximately 200mA.
September 2010
12
M9999-092910-A