ADP1147-3.3/ADP1147-5

–9–

REV. 0

The Schottky diode is in conduction during the MOSFET off-

tion on for the diode. During this time it must be capable of

tion below is used to calculate the average current conducted by

the diode under normal load conditions.

× I

LOAD

To guard against increased power dissipation due to undesired

ringing, it is extremely important to adhere to the following:

1. Use proper grounding techniques.

2. Keep all track lengths as short as possible, especially connec-

tions made to the diode (refer to PCB Layout Considerations

section).

The allowable forward voltage drop of the diode is determined

by the maximum short circuit current and power dissipation.

determined by the system efficiency and thermal requirements

(refer to Efficiency Section).

During the continuous mode of operation the current drawn

from the source is a square wave with a duty cycle equal to

capacitor with a low ESR value and capable of handling the

maximum rms current should be selected. The formula below

is used to determine the required maximum rms capacitor

current:

0.5

× I

design margin. Manufacturers of capacitors typically base the

current ratings of their caps on a 2000-hour life. This requires a

prudent designer to use capacitors that are derated or rated at a

higher temperature. The use of multiple capacitors in parallel

may also be used to meet design requirements. The capacitor

manufacturer should be consulted for questions regarding spe-

cific capacitor selection.

In addition, for high frequency decoupling a 0.1

µF to 1.0 µF

ceramic capacitor should be placed and connected as close to

The minimum required ESR value is the primary consideration

ESR is the primary concern. Proper circuit operation mandates

Chemicon, Nichicon and Sprague are three manufacturers of

high grade capacitors. Sprague offers a capacitor that uses an

OS-CON semiconductor dielectric. This style capacitor pro-

vides the lowest amount of ESR for its size, but at a higher cost.

will usually meet or exceed the rms current requirements. The

specifications for the selected capacitor should be consulted.

Surface mount applications may require the use of multiple

capacitors in parallel to meet the ESR or rms current require-

ments. If dry tantalum capacitors are used it is critical that they

be surge tested and recommended by the manufacturer for use

in switching power supplies such as Type 593D from Sprague.

AVX offers the TPS series of capacitors with various heights

from 2 mm to 4 mm. The manufacturer should be consulted

for the latest information, specifications and recommendations

concerning specific capacitors. When operating with low supply

voltages, a minimum output capacitance will be required to

prevent the device from operating in a low frequency mode (see

Figure 5). The output ripple also increases at low frequencies if

Transient Response

The response of the regulator loop can be verified by monitoring

the transient load response. Several cycles may be required for a

switching regulator circuit to respond to a step change in the dc

load current (resistive load). When a step in the load current

overshoot, undershoot or ringing, which may indicate a stability

problem. The circuit shown in Figure 1 contains external com-

ponents that should provide sufficient compensation for most

applications. The most demanding form of a transient that can

be placed on a switching regulator is the hot switching in of

loads that contain bypass or other sources of capacitance greater

than 1

µF. When a discharged capacitor is placed on the load it

regulators are not capable of supplying enough instantaneous

current to prevent this from occurring. Therefore, the inrush

current to the load capacitors should be held below the current

limit of the design.

Efficiency

Efficiency is one of the most important reasons for choosing a

switching regulator. The percentile efficiency of a regulator can

be determined by dividing the output power of the device by the

input power and then multiplying the results by 100. Efficiency

losses can occur at any point in a circuit and it is important to

analyze the individual losses to determine changes that would

yield the most improvement. The efficiency of a circuit can be

expressed as:

% efficiency = 100% – (% L1 + % L2 + % L3 . . . etc.)

L1, L2, L3, etc., are the individual losses as a percentage of the

input power. In high efficiency circuits small errors result when

expressing losses as a percentage of the output power.