Applications Information: continued

11

,

where:

Period =

“Droop” Resistor for Adaptive Voltage Positioning

Adaptive voltage positioning is used to reduce output volt-

age excursions during abrupt changes in load current.

Regulator output voltage is offset +40mV when the regula-

tor is unloaded, and -40mV at full load. This results in

increased margin before encountering minimum and maxi-

mum transient voltage limits, allowing use of less capaci-

tance on the regulator output (see Figure 7).

To implement adaptive voltage positioning, a “droop”

resistor must be connected between the output inductor

and output capacitors and load. This is normally imple-

mented by a PC board trace of the following value:

Adaptive voltage positioning can be disabled for improved

using a separate, non-load current carrying circuit trace.

Input and Output Capacitors

These components must be selected and placed carefully to

yield optimal results. Capacitors should be chosen to pro-

vide acceptable ripple on the input supply lines and regula-

tor output voltage. Key specifications for input capacitors

are their ripple rating, while ESR is important for output

capacitors. For best transient response, a combination of

low value/high frequency and bulk capacitors placed close

to the load will be required.

Output Inductor

The inductor should be selected based on its inductance,

current capability, and DC resistance. Increasing the induc-

tor value will decrease output voltage ripple, but degrade

transient response.

Thermal Considerations for Power MOSFETs and Diodes

In order to maintain good reliability, the junction tempera-

ture of the semiconductor components should be kept to a

maximum of 150°C or lower. The thermal impedance (junc-

tion to ambient) required to meet this requirement can be

calculated as follows:

Thermal Impedance =

A heatsink may be added to TO-220 components to reduce

their thermal impedance. A number of PC board layout

techniques such as thermal vias and additional copper foil

area can be used to improve the power handling capability

of surface mount components.

EMI Management

As a consequence of large currents being turned on and off

at high frequency, switching regulators generate noise as a

consequence of their normal operation. When designing for

compliance with EMI/EMC regulations, additional com-

ponents may be added to reduce noise emissions. These

components are not required for regulator operation and

experimental results may allow them to be eliminated. The

input filter inductor may not be required because bulk filter

and bypass capacitors, as well as other loads located on the

board will tend to reduce regulator di/dt effects on the cir-

cuit board and input power supply. Placement of the

power component to minimize routing distance will also

help to reduce emissions.

Figure 16: Filter components

Figure 17: Input Filter

Layout Guidelines

1. Place 12V filter capacitor next to the IC and connect

capacitor ground to pin 11 (PGnd).

2. Connect pin 11 (PGnd) with a separate trace to the

ground terminals of the 5V input capacitors.

and connect its ground terminal with a separate, wide trace

directly to pin 14 (LGnd).

4. Connect the ground terminals of the Compensation

capacitor directly to the ground of the fast feedback filter

capacitor to prevent common mode noise from effecting

the PWM comparator.

5. Place the output filter capacitor(s) as close to the load as

possible and connect the ground terminal to pin 14 (LGnd).

6. To implement adaptive voltage positioning, connect

the regulator output right at the inductor terminal. Connect

inductor to the output capacitors via a trace with the fol-

lowing resistance:

+

2µH

1200µF x 3/16V

33Ω

1000pF

2µH

Power

Thermal Management

80mV

1

switching frequency

Period × (1 - duty cycle)

4848.5