MIC23250_10 Datasheet, PDF (13/20 Pages) Micrel Semiconductor – 4MHz Dual 400mA Synchronous Buck Regulator with HyperLight Load

Micrel, Inc.
MIC23250
V_OUT (V)
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
3.3
R_TOP (kΩ)
49
111
172
233
295
356
417
479
540
602
663
724
786
847
909
970
1031
1093
1154
1216
1277
1338
1400
1461
1522
1584
R_BOTTOM (kΩ)
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
442
CFF (pF)
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
Table 1. Recommended Feedback Component Values
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
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 devices 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 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 is
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.
Efficiency V _OUT = 1.8V
100
V_IN = 2.7V
80
V_IN = 3.6V
60
V_IN = 3.3V
40
20
V_OUT = 1.8V
L = 1µH
00.1
11
0 100 1000
LOAD (mA)
The Figure above 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 MIC23250 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
follows:
DCR Loss = I_OUT2 × DCR
From that, the loss in efficiency due to inductor resistance
can be calculated as follows:
Efficiency
Loss
=
⎡
⎢^{1 −}
⎣
⎜⎜⎝⎛
V_OUT ×
V_OUT × I _OUT
I _OUT
+L
_
P_D
⎟⎟⎠⎞⎥⎦⎤
×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.
June 2010
13
M9999-061110-E