MIC2811_08 datasheet(13/17 Pages) MICREL

13 / 17 page

Micrel, Inc.

MIC2811/2821

April 2008

M9999-042208-A

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 I

2R. Power is dissipated in the

high side switch during the on cycle. Power loss is equal

to the high side MOSFET RDSON multiplied by the Switch

Current

2. 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

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 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

Iout

From that, the loss in efficiency due to inductor

resistance can be calculated as follows;

100

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

Iout

Vout

Iout

Vout

Loss

Efficiency

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 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.

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