LTC3569
21
3569fe
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applicaTions inForMaTion
Figure 8. Dual Programmable Buck DC/DC Regulators
Design Example 4: Dual 1A Bucks
V
OUT
= 1.6V, Burst Mode Operation
I
LOAD
(mA)
EFFICIENCY (%)
3569 F08b
100
90
80
70
60
50
40
0.1 1 1000 1000010010
BUCK 1
BUCK 2, 3
V
IN
= 2.5V
V
IN
= 5.5V
3569 F08
SV
IN
PV
IN
SW1
PGNDSGND
SW2
SW3
PGOOD
FB1
EN1
EN2
EN3
MODE
R
T
FB2
FB3
22µF
V
IN
2.5V TO 5.5V
2.2µH
10µF20pF
20pF
10µF
2.2µH
210k
162k
210k
162k
OUT1 800mA
1.2V STANDBY
1.6V ACTIVE
OUT2 800mA
1.2V STANDBY
1.6V ACTIVE
LTC3569
511k
V
IN
V
IN
DIGITAL
CONTROL
Design Example 4: Dual Programmable Bucks
In this example consider two buck regulators operating
from a 2.5V to 5.5V unregulated supply that are required
to generate two independently programmable supplies that
must step from 1.2V in standby up to 1.6V when active,
with a maximum load current of 0.8A when active and 1mA
in standby. Furthermore, when switching between active
and standby, the load voltage should not droop.
Buck 1 is selected for the first regulator, and buck 3 is
configured as a slave power stage in parallel with buck 2
by pulling FB3 up to V
IN
to obtain the required current level
for the second regulator. Burst Mode operation is selected
to achieve high efficiency during standby operation. The
internal 2.25MHz clock frequency is selected, as it satisfies
the minimum on-time requirement. Next, two reference
voltages are selected to match the ratio of the active to
standby voltages: 1.6V/1.2V = 1.3333. There are three
reference value ratios that match this ratio: 0.8V and 0.6V,
0.7V and 0.525V, and 0.6V and 0.45V. As the load cannot
tolerate a voltage droop when switching from standby to
active, the 0.7V and 0.525V references are selected to match
the ratio of output voltages. With this ratio, the buck does
not need to be shutdown as it would if the full-scale 0.8V
reference level was chosen. The resistors shown in Figure 8
are selected to obtain the nearest feedback ratio from
standard 1% resistors. Calculating the inductor values
for 30% ripple current at maximum SV
IN
:
L = 1.6V/(2.25MHz•240mA)•(1–1.6V/5.5V) ≈ 2.2µH.
The output capacitor values are selected as the nearest
standard value to obtain 5% voltage droop at maximum
load current step.
C
OUT
= 2.5•800mA/(2.25MHz•90mV) ≈ 10µF.
Select an output capacitor with an ESR of less than 50mΩ
to obtain an output voltage ripple of less than 30mV.
Finally select an input capacitor rated for the worst-case
short-circuit ripple current of 2 I
PK
/√3 ≈ 2.5A, when both
outputs are shorted to GND.
