MIC23163 Datasheet, PDF (14/19 Pages) Micrel Semiconductor – 4MHz, 2A, 100% Duty Cycle Buck Regulator with HyperLight Load® and Power Good

Micrel, Inc.
In HLL mode, the inductor is charged with a fixed Ton
pulse on the high-side switch (HSD). After this, the LSD
is switched on and current falls at a rate V_OUT/L. The
controller remains in HLL mode while the inductor falling
current is detected to cross approximately −50mA. When
the LSD (or T_OFF) time reaches its minimum and the
inductor falling current is no longer able to reach this
−50mA threshold, the part is in CCM mode and switching
at a virtually constant frequency.
Once in CCM mode, the T_OFF time will not vary.
Compensation
The MIC23163/4 is designed to be stable with a 0.47µH
inductor with a 10µF ceramic (X5R) output capacitor. A
feed-forward capacitor in the range of 15pF to 68pF is
essential across the top feedback resistor.
Duty Cycle
The maximum duty cycle of the MIC23163/4 is 100%,
allowing operation in dropout to extend battery life.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied, as
shown in Equation 3:
Efficiency
%
=

V_OUT
V_IN
× I_OUT
× I_IN
 ^{× 100}
Eq. 3
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
considerations and it reduces consumption of current for
battery-powered applications. Reduced current draw from
a battery increases the device’s operating time and is
critical in handheld devices.
There are two types of losses in switching converters; DC
losses and switching losses. DC losses are simply the
power dissipation of I²R. Power is dissipated in the high
side switch during the on cycle. Power loss is equal to the
high side MOSFET R_DSON multiplied by the switch current
squared. During the off cycle, the low side N-channel
MOSFET conducts, also dissipating power. Device
operating current also reduces efficiency. The product of
the quiescent (operating) current and the supply voltage
represents another DC loss. The current required driving
the gates on and off at a constant 4MHz frequency and
the switching transitions make up the switching losses.
MIC23163/4
Efficiency vs. Output Current
V_OUT = 1.8V @ 25°C
95
V_IN = 3V
90
85
80
V_IN = 3.6V
V_IN = 5V
75
70
65
60
55
50
1
10
100
1000
OUTPUT CURRENT (mA)
10000
Figure 3. Efficiency under Load
Figure 3 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 HLL mode, the MIC23163/4 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 in
Equation 4:
P_DCR = I_OUT ² × DCR
Eq. 4
From that, the loss in efficiency due to inductor resistance
can be calculated as in Equation 5:
Efficiency
Loss
=

¹

−

V_OUT × I_OUT
V_OUT × I_OUT + P_DCR
 ^{× 100}
Eq. 5
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.
July 29, 2013
14
Revision 2.0