LTC3569_15 Datasheet, PDF (21/26 Pages) Linear Technology – Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

LTC3569
Applications Information
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.
V_IN 2.5V TO 5.5V
22µF _{SVIN PVIN SW1}
DIGITAL
CONTROL
V_IN
EN1
FB1
EN2
LTC3569
EN3
MODE
SW2
R_T
SW3
511k
FB2
PGOOD
FB3
SGND PGND
2.2µH
210k
162k
2.2µH
210k
162k
V_IN
20pF
20pF
OUT1 800mA
1.2V STANDBY
10µF ^{1.6V ACTIVE}
OUT2 800mA
1.2V STANDBY
10µF ^{1.6V ACTIVE}
3569 F08
Design Example 4: Dual 1A Bucks
V_OUT = 1.6V, Burst Mode Operation
100
90 _{VIN = 2.5V}
80
70
V_IN = 5.5V
60
50
40
0.1
1
BUCK 1
BUCK 2, 3
10 100 1000 10000
I_LOAD (mA)
3569 F08b
Figure 8. Dual Programmable Buck DC/DC Regulators
For more information www.linear.com/LTC3569
3569fe
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