LTC3545/LTC3545-1

15

35451fb

Similar situations can occur when all three channels are

operating at maximum loads at high ambient temperature.

As an example, consider a channel supplying 800mA at

1.8V output and 85% efficiency. The dissipated power can

be calculated using

Loss P

E

E

WW

O

= ⎛

⎝⎜

⎞

⎠⎟

==

1

14

017 0 25

–

.• .

.

In this case the temperature rise is 17°C, similar to the

dropout scenario described above. Whereas one channel

operating at these levels will safely fall within the tem-

perature limitations of the part, three channels operating

simultaneously at these levels will place limits on the peak

ambient temperature.

Note that at higher supply voltages, the junction tempera-

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

equal to (

for overshoot or ringing that would indicate a stability

problem. For a detailed explanation of switching control

loop theory, see Application Note 76.

A second, more severe transient is caused by switching

in loads with large (>1μF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in paral-

can deliver enough current to prevent this problem if the

load switch resistance is low and it is driven quickly. The

only solution is to limit the rise time of the switch drive

so that the load rise time is limited to approximately (25

require a 250μs rise time, limiting the charging current

to about 130mA.

APPLICATIONS INFORMATION

Design Example

As a design example, consider using the LTC3545/LTC3545-

1 in a portable application with a Li-Ion battery. The battery

one channel at 2.5V is 600mA. Using this channel as an

example, first calculate the inductor value for 40% ripple

a form of Equation 1:

L

V

MHz

mA

V

V

1

25

2 25

240

1

25

36

14

)

⎛

⎝⎜

⎞

⎠⎟

=

.

.

–

.

.

.1

1µH

Use the closest standard value of 1.5μH. For low ripple

applications, 10μF is a good choice for the output capacitor.

A smaller output capacitor will shorten transient response

settling time, but also increase the load transient ripple. A

value for C5 = 4.7μF should suffice as the source imped-

ance of a Li-Ion battery is very low. C5 and C1 both provide

switching current to the output power switches. They

should be placed as close a possible to the chip between

are the supply and return power paths for both channels

2 and 3, so a value of 10μF for C1 is appropriate. The

feedback resistors program the output voltage. Minimiz-

ing the current in these resistors will maximize efficiency

at very light loads, but totals on the order of 200k are a

good compromise between efficiency and immunity to

any adverse effects of PCB parasitic capacitance on the

feedback pins. Choosing 10μA as the feedback current with

0.6V feedback voltage makes R4 = 60k. A close standard

1% resistor is 60.4k. Using:

R

V

V

Rk

3

25

06

1

4 191 1

= ⎛

⎝⎜

⎞

⎠⎟

=

.

.

–•

.

The closest standard 1% resistor is 191k. A 20pF feed-

forward capacitor is recommended to improve transient

response. The component values for the other channels

are chosen in a similar fashion. Figure 4 shows the com-

plete schematic for this example, along with the efficiency

curve and burst mode ripple at an output current for the

2.5V output.