LTC1439CG datasheet(24/32 Pages) LINER

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LTC1438/LTC1439

APPLICATIONS INFORMATION

3. Are the SENSE – and SENSE + leads routed together with

minimum PC trace spacing? The filter capacitors be-

tween SENSE + 1 (SENSE + 2) and SENSE – 1 (SENSE – 2)

should be as close as possible to the LTC1438/LTC1439.

4. Do the (+) plates of CIN connect to the drains of the

topside MOSFETs as closely as possible? This capacitor

provides the AC current to the MOSFETs.

5. Is the INTVCC decoupling capacitor connected closely

betweenINTVCC and the power ground pin? This capaci-

tor carries the MOSFET driver peak currents.

6. Keep the switching nodes, SW1 (SW2), away from

sensitive small-signal nodes. Ideally the switch nodes

should be placed at the furthest point from the LTC1438/

LTC1439.

7. Use a low impedance source such as a logic gate to drive

the PLLIN pin and keep the lead as short as possible.

PC Board Layout Suggestions

Switching power supply printed circuit layouts are cer-

tainly among the most difficult analog circuits to design.

The following suggestions will help to get a reasonably

close solution on the first try.

The output circuits, including the external switching

MOSFETs, inductor, secondary windings, sense resistor,

input capacitors and output capacitors all have very large

voltage and/or current levels associated with them. These

components and the radiated fields (electrostatic and/or

electromagnetic) must be kept away from the very sensi-

tive control circuitry and loop compensation components

required for a current mode switching regulator.

The electrostatic or capacitive coupling problems can be

reduced by increasing the distance from the radiator,

typically a very large or very fast moving voltage signal.

The signal points that cause problems generally include:

the “switch” node, any secondary flyback winding voltage

and any nodes which also move with these nodes. The

switch, MOSFET gate and boost nodes move between VIN

and PGND each cycle with less than a 100ns transition

time. The secondary flyback winding output has an AC

signal component of – VIN times the turns ratio of the

transformer, and also has a similar < 100ns transition

time. The feedback control input signals need to have less

than a few millivolts of noise in order for the regulator to

perform properly. A rough calculation shows that 80dB of

isolation at 2MHz is required from the switch node for low

noise switcher operation. The situation is worse by a factor

of the turns ratio for the secondary flyback winding. Keep

these switch node related PC traces small and away from

the “quiet” side of the IC (not just above and below each

other on the opposite side of the board).

The electromagnetic or current loop induced feedback

problems can be minimized by keeping the high AC

current (transmitter) paths and the feedback circuit (re-

ceiver) path small and/or short. Maxwell’s equations are at

work here, trying to disrupt our clean flow of current and

voltage information from the output back to the controller

input. It is crucial to understand and minimize the suscep-

tibility of the control input stage as well as the more

obvious reduction of radiation from the high current

output stage(s). An inductive transmitter depends upon

the frequency, current amplitude and the size of the

current loop to determine the radiation characteristic of

the generated field. The current levels are set in the output

stage once the input voltage, output voltage and inductor

value(s) have been selected. The frequency is set by the

output stage transition times. The only parameter over

which we have some control is the size of the antenna we

create on the PC board, i.e., the loop. A loop is formed with

the input capacitance, the top MOSFET, the Schottky diode

and the path from the Schottky diode’s ground connection

and the input capacitor’s ground connection. A second

path is formed when a secondary winding is used com-

prising the secondary output capacitor, the secondary

winding and the rectifier diode or switching MOSFET (in

the case of a synchronous approach). These “loops”

should be kept as small and tightly packed as possible in

order to minimize their “far field” radiation effects. The

radiated field produced is picked up by the current com-

parator input filter circuit(s), as well as by the voltage

feedback circuit(s). The current comparator’s filter ca-

pacitor placed across the sense pins attenuates the radi-

ated current signal. It is important to place this capacitor

immediately adjacent to the IC sense pins. The voltage

sensing input(s) minimizes the inductive pickup compo-

nent by using an input capacitance filter to SGND. The

capacitors in both case serve to integrate the induced

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