LTC3545_15 Datasheet, PDF (15/20 Pages) Linear Technology – Triple 800mA Synchronous Step-Down Regulator–2.25MHz

LTC3545/LTC3545-1
APPLICATIONS INFORMATION
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_O
⎛
⎝⎜
1– E
E
⎞
⎠⎟
=
1.4W
•
0.17
=
0.25W
where P_O is the output power and E is the efficiency.
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-
ture is lower due to reduced switch resistance R_DS(ON).
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
a load step occurs, V_OUT immediately shifts by an amount
equal to (ΔI_LOAD • ESR), where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT, which generates a feedback error signal. The
regulator loop then acts to return V_OUT to its steady-state
value. During this recovery time V_OUT can be monitored
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-
lel with C_OUT, causing a rapid drop in V_OUT. No regulator
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
• C_LOAD). Thus, a 10μF capacitor charging to 3.3V would
require a 250μs rise time, limiting the charging current
to about 130mA.
Design Example
As a design example, consider using the LTC3545/LTC3545-
1 in a portable application with a Li-Ion battery. The battery
provides V_IN ranging from 2.8V to 4.2V. The demand on
one channel at 2.5V is 600mA. Using this channel as an
example, first calculate the inductor value for 40% ripple
current (240mA in this example) at maximum V_IN. Using
a form of Equation 1:
L1=
2.5V
(2.25MHz)(240mA
)
⎛
⎝⎜
1–
2.5V
3.6V
⎞
⎠⎟
=
1.41µH
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
V_IN/GNDA and PV_IN/PGND respectively. PV_IN and PGND
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:
R3
=
⎛
⎝⎜
2.5V
0.6V
– 1^{⎞⎠⎟}
• R4
=
191.1k
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.
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