MIC2230 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – Dual Synchronous DC/DC Regulator

Micrel, Inc.
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
Current². 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 2.5MHz frequency and the
switching transitions make up the switching losses.
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 forcing the MIC2230 into Trickle Mode™
(/FPWM=High), the buck regulator significantly reduces
the required switching current by entering into a PFM
(Pulse Frequency Modulation) mode. This significantly
increases efficiency at low output currents.
Over 100mA, efficiency loss is dominated by MOSFET
RDSON and inductor losses. Higher input supply
voltages will increase the Gate-to-Source threshold on
the internal MOSFETs, reducing the internal RDSON.
This improves efficiency by reducing DC losses in the
MIC2230
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;
L_Pd = Iout ² × 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 + L_Pd
⎟⎟⎠⎞⎥⎥⎦⎤
× 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.
Trickle Mode™ Operation
Trickle Mode™ operation is achieved by clamping the
minimum peak current to approximately 150mA. This
forces a PFM mode by comparing the output voltage to
the internal reference. If the voltage is less than 0.8V,
the MIC2230 turns on the high side until the peak
inductor current reaches approximately 150mA. A
separate comparator then monitors the output voltage. If
the feedback voltage is greater than 0.8V, the high side
switch is then used as a 10µA current source, never
turning off completely. This creates a highly efficient light
load mode by increasing the time it takes for the output
capacitor to discharge, delaying the amount of switching
required and increasing light load efficiency. When the
load current is greater than approximately 100mA, the
MIC2230 automatically switches to PWM mode.
FPWM Operation
In forced PWM Mode (/FPWM=LOW) the MIC2230 is
forced to provides constant switching at 2.5MHz with
synchronous internal MOSFETs throughout the load
current. In FPWM Mode, the output ripple can be as low
as 7mV.
April 2010
13
M9999-040810-C