MIC2811_08 Datasheet, PDF (13/17 Pages) Micrel Semiconductor – Digital Power Management IC 2MHz, 600mA DC/DC with Triple 300mA LDOs

Micrel, Inc.
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
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.
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, 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:
L _ Pd = Iout ² × DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows;
Efficiency
_
Loss
=
⎡
⎢^1−
⎣
⎜⎜⎝⎛ _VoutV×^oIuotu×_t
Iout
+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.
PGND
Power ground (PGND) is the ground path for the high
current PWM mode. The current loop area for the power
ground should be as small as possible.
SGND
Signal ground (SGND) is the ground path for the biasing
and control circuitry. The current loop for the signal
ground should be as small as possible.
BYP (MIC2811 only)
For enhanced noise and PSRR performance on LDO1 &
LDO2, the internal reference of the MIC2811 can be
bypassed with a capacitor to ground. A quick-start
feature allows for quick turn-on of the output voltage.
The recommended nominal bypass capacitor is 0.1µF,
but it can be increased, which will also result in an
increase to the start-up time.
MIC2811/2821
MIC2811 Layout Recommendations
A poor layout of the MIC2811 may cause unwanted
voltage and current spikes. This can lead to noise on DC
voltages and EMI radiating to nearby devices. The
following are recommendations for the MIC2811/21
layout. The evaluation board layout is included as an
example.
1. Place the MIC2811/21 with the pad size designated
in the “Recommended Land Patterns” page of the Micrel
website.
2. When laying out the components, keep the MIC2811,
inductor, and filter capacitors physically close to keep
traces as short as possible. The traces between these
components carry relatively high switching currents and
can affect adjacent signals.
3. The input capacitor between DVIN and PGND should
be placed right next to the MIC2811/21. This will
eliminate trace inductance effects and reduce internal
noise for the MIC2811/21 control circuitry. The trace
from the DVIN filter capacitor to the MIC2811/21 device
should not be routed through any vias. This lessens the
chance of noise coupling by the effective antenna of the
via.
4. Monitoring the path of the switching currents will help
minimize the radiated noise. In the first half of the
switching cycle, current flows from the input filter
capacitor through the high side switch within the
MIC2811, then through the inductor to the output filter
capacitor and lastly through ground. In the second half of
the switching cycle, current is pulled up from ground
through the low side synchronous switch within the
MIC2807 by the inductor, to the output filter capacitor
and then back through ground, forming a second current
loop. Route these loops to ensure the current curls in the
same direction, preventing magnetic field reversal
between the switching cycles.
5. Connect the Bypass capacitor (MIC2811 Only) to the
BYP pin and the AGND pin. AGND and PGND should be
connected close to the chip at a single point in order to
minimize undesirable behavior due to ground bounce.
Input and output filter capacitors should be connected to
PGND.
6. Connections between power components and the
MIC2811 should have wide traces. It is good practice to
use a minimum of 30mils (0.762mm) per Ampere for 1oz
copper weight.
7. Route noise sensitive traces such as Feedback (FB),
BIAS, and BYP away from the switching traces and the
inductor. Noise coupled into these pins can affect the
accuracy of the output. The Feedback pin should be
connected at point of load for an accurate load
regulation.
April 2008
13
M9999-042208-A