LT1432CS8 datasheet(20/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

boards, their maximum rated current must also be consid-

ered. For currents greater than 1A, multiple vias may have

to be used.

4. The catch diode has large square wave currents

flowing in it. Connect the anode directly to the ground

plane and the cathode directly to the IC ground pin.

5. The ground pin of the LT1432 is the reference point

for output voltage. It should be routed separately to power

ground as near to the load as is reasonable.

Transient Response

Load transient response may be important in portable

applications where parts of the system are switched on

and off to save power. There are two types of problems

that differ by time scale. The first occurs very rapidly and

is caused by the surge current created in charging the

supply bypass capacitors on the switched load. This can

be a very serious problem if large (>0.1

µF) capacitors

must be charged. No regulator can respond fast enough to

handle the surge if the load switch on-resistance is low and

it is driven quickly. The solution here is to limit the rise time

of the switch drive so that the load rise time is limited to

approximately 25

× CLOAD. A 1µF load capacitor would

require a 25

µs load rise time, etc. This limits surge to

about 200mA. This time frame is still too quick for a

switching regulator to adjust to, but the surge is limited to

a low enough value that the output capacitor will attenuate

the surge voltage to an acceptable level.

A second problem is the change in DC load current.

Switching regulators take many switching cycles to re-

spond to sudden output load changes. During this time,

the output shifts by an amount equal to

∆load (ESR + t/C),

where ESR is the series resistance of the output capacitor,

t is the time for the regulator to shift output current, and C

is the output capacitor value. For example, if the load

change is 0.5A, ESR is 0.1

Ω, t is 30µs, and C = 390µF, the

shift in output voltage would be:

∆Ω

= 0.5A

30 s

390 F

0.088V

OUT









100mV output ripple on the main capacitor will be reduced

to (0.083)(100) = 8.3mV at the output of the filter.

Layout Considerations

Although buck converters are fairly tolerant with regard to

layout issues, there are still several important things to

keep in mind. Most of these revolve around spikes created

by switching high currents at high speeds. If 3A of current

is switched in 30ns, the rate of change of current is 10E8

A/S. Voltage generated across wires will be equal to this

rate multiplied by the approximate 20nH per inch of wire.

This calculates to 2V per inch of wire or trace!! Needless

to say, connections should be kept short if the circuitry

connected to these lines is sensitive to narrow spikes.

1. The input bypass capacitor must be kept as close to

the switcher IC as possible, and its ground return must go

directly to the ground plane with no other component

grounds tied to it. The output capacitor should also

connect directly to the ground plane.

2. The frequency compensation components shown in

Figure 1 (R1 + C4, and C5) and the feedback pin bypass

capacitor (C6) are shown connected to the floating ground

pin of the IC switcher. This ground pin is also the high

current path for the switch. To avoid differential spikes

being coupled into the VC and FB pins, these components

must tie together and then be connected through a direct

trace to the IC switcher ground pin. No other components

should be connected anywhere on this trace and the trace

area should be minimized. A separate wide trace must be

used to connect the IC ground pin to the catch diode and

inductor. Smaller traces can be used to connect the

floating supply capacitor (C3) and the diode pin of the

LT1432 to the wide trace reasonably close to the IC ground

pin.

3. Traces which carry high current must be sized

correctly. To limit temperature rise to 20

°C, using 1oz

copper, the trace width must be 20 mils for each ampere

of current. 1/2oz copper requires 30 mils/A. These high

current paths include the IC switcher ground pin and

switch pin, the inductor, the catch diode, the current limit

sense resistor, and the input bypass capacitor. If vias are

used to connect these components on multiple layer

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