MIC23451 Datasheet, PDF (14/20 Pages) Micrel Semiconductor – 3MHz, 2A Triple Synchronous Buck Regulator

Micrel, Inc.
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 does not vary.
Therefore, it is important to note that if L is large enough,
the HLL transition level will not be triggered.
That inductor is:
L _MAX
=
V_OUT × 135ns
2 × 50mA
Eq. 3
Compensation
The MIC23451 is designed to be stable with a 0.47µH to
2.2µH inductor with a 4.7µF ceramic (X5R) output
capacitor.
Duty Cycle
The typical maximum duty cycle of the MIC23451 is 80%.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency
%
=

V_OUT
V_IN
× I_OUT
× I_IN

× 100
Eq. 4
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 current consumption for
battery-powered applications. Reduced current draw from
a battery increases the device’s operating time and is
critical in hand-held devices.
There are two types of losses in switching converters: DC
losses and switching losses. DC losses are 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 to drive
the gates on and off at a constant 4MHz frequency, and
the switching transitions, make up the switching losses.
MIC23451
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 HyperLight Load mode, the MIC23451 can 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 voltage 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.
Because of this, inductor selection becomes increasingly
critical in efficiency calculations. As the inductors are
reduced in size, the DC resistance (DCR) can become
very significant. The DCR losses can be calculated as
shown in Equation 5.
PDCR = IOUT ² × DCR
Eq. 5
From that, the loss in efficiency caused by inductor
resistance can be calculated as shown in Equation 6.
Efficiency
Loss
=

¹

−

V_OUT × I_OUT
V_OUT × I_OUT + P_DCR
 ^{× 100}
Eq. 6
Efficiency loss caused by 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.
November 5, 2013
14
Revision 1.2